Moparts

Comp height on 68-70 383 pistons UPDATE

Posted By: fastmark

Comp height on 68-70 383 pistons UPDATE - 02/21/20 03:09 PM

I starting a 383 build for a bud and I’m having trouble finding the right piston to get decent compression ratio. He did not check anything when he built the motor that lasted about an hour before it failed, so I’m taking over. It had a sad 7.4 CR with a 1.908 C.H. and deck height of -.068. Cast pistons are 1.848 and even lower CR. Speed pro forged are better at 1.920 and gives me 7.85. I was thinking that the oem pistons for the Hp 383 for 68-70 was a positive deck height. I’m not sure that C.H. is listed in the service manual. Anyone have a better source of pistons besides KB? They were in his first motor build and failed. They had a 7.42 CR. They are not an option for me. Thanks.
Btw. Custom pistons are not an option budget wise for him.
Posted By: lewtot184

Re: Comp height on 68-70 383 pistons - 02/21/20 03:23 PM

I think your compression ratio calculations are off. the speed-pro 2315 should give you at least 9:1 with a .020" steel shim gasket.
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/21/20 03:47 PM

Using the stock block height spec of 9.98 along with a piston ch of 1.920(L2315F, no valve pockets),at .030 over, .040 gasket, 84cc heads........ I get 9.14:1.

797- swept
84- chamber
2.8 - deck
10- gasket
1- area above rings
————-
894.8 total
97.8 total minus swept
————-
9.14:1

Imo, that’s the easy button for a street 383.
Deck block for zero deck gets you to 9.38:1.
Swap to 78cc closed chamber heads and you’re at 9.95:1 with good quench.


A 1.908ch piston should be sitting .024” down the hole of an uncut block.
The 1.848ch piston would be down the hole .084”.

The current 1.908ch KB162 piston, besides sitting down the hole .024” on a stock block....... also has 8cc of valve pockets.

With the 1.848ch pistons (no dish, no valve pocket) at + .030 and 90cc heads with a shim gasket:

797- swept
90- chamber
19.8- deck
5- shim gasket
1- area above rings
————-
912.8- total
115.8 - total minus swept
————-
7.88:1

Mill the heads to 84cc and you’re at 8.25:1.
Posted By: cudaman1969

Re: Comp height on 68-70 383 pistons - 02/21/20 04:23 PM

My 383 built many years ago have Speed Pro pop ups. The flat part of piston is level with deck. ( could find a set and mill the dome off) Look for a NHRA Stock approved piston, these are allowed .020 above deck. If needed look on “class racer” website go to specs and you’ll find the piston makes.
Posted By: lewtot184

Re: Comp height on 68-70 383 pistons - 02/21/20 06:21 PM

Originally Posted by fast68plymouth
Using the stock block height spec of 9.98 along with a piston ch of 1.920(L2315F, no valve pockets),at .030 over, .040 gasket, 84cc heads........ I get 9.14:1.

797- swept
84- chamber
2.8 - deck
10- gasket
1- area above rings
————-
894.8 total
97.8 total minus swept
————-
9.14:1

Imo, that’s the easy button for a street 383.
Deck block for zero deck gets you to 9.38:1.
Swap to 78cc closed chamber heads and you’re at 9.95:1 with good quench.


A 1.908ch piston should be sitting .024” down the hole of an uncut block.
The 1.848ch piston would be down the hole .084”.

With the 1.848ch pistons (no dish, no valve pocket) at + .030 and 90cc heads with a shim gasket:

797- swept
90- chamber
19.8- deck
5- shim gasket
1- area above rings
————-
912.8- total
115.8 - total minus swept
————-
7.88:1

Mill the heads to 84cc and you’re at 8.25:1.
the 383 I did a few years back with .030 over 2315 pistons and '906 heads (with .015" off them) was around 9.24:1 with a .039" head gasket. the 2315 piston is an easy no brainer.
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/21/20 09:27 PM

I’m headed to the shop later today so I’ll re checked my calculations. Unfortunately, this a a budget build and I’m limited to my parts choices.
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/21/20 10:37 PM

Nuthin fancy req’d for a 9:1 383.
Posted By: BSB67

Re: Comp height on 68-70 383 pistons - 02/22/20 12:24 AM

Factory '68/'69 - 383 Piston: 1.932" CH, no valve relief.

Reasonable choices:

Speed Pro 2315: 1.920" CH, flat top, no valve relief

Icon IC687: 1.912" CH, small dome and valve relief, positive 4.5 cc, 674 grams

Diamond 51900: 1.916" CH, flat top w/ relief, negative 3.0 cc., 630 grams
Posted By: calrobb2000

Re: Comp height on 68-70 383 pistons - 02/22/20 03:24 AM


hi
what do you mean when you said the k b pistons failed .

i have done several with them and no problem ? no power adders .
Posted By: Twostick

Re: Comp height on 68-70 383 pistons - 02/22/20 04:14 AM

Originally Posted by calrobb2000

hi
what do you mean when you said the k b pistons failed .

i have done several with them and no problem ? no power adders .





I suspect they ignored all the installation instructions and specs regarding hyper eutectic pistons or detonated it or both.

Kevin
Posted By: BSB67

Re: Comp height on 68-70 383 pistons - 02/22/20 11:21 AM

Originally Posted by fastmark
I’m headed to the shop later today so I’ll re checked my calculations. Unfortunately, this a a budget build and I’m limited to my parts choices.


Is this now your motor, or still your "bud's" motor that you're working on? If it's your bud's how did the limited budget work out for him the first time? Sometimes you just need to pay the man.

What's weight of the KB pistons in it now?
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/22/20 03:27 PM

Well. Without going into great detail, this bud is a very good friend, to not just, me but lots of mopar guys. He works the hardest for the club, never complains, aways has a smile and a good thing to say. He is always the first to volunteer to help you out. He only has one project and it takes a back seat to his family, friends, his home and even work. He could easily win the “Stu Harmon” award. He has modest means to pay for this project. Just retiring, he was hit with a bill expense on his house that was unexpected and a must do ASAP. He does not have all the correct tools to measure the critical measurements envolved with a precision engine build. He took his stock block to the local “Gomers” machine shop. I doubt they had a torque plate to hone this block properly. Gomers did not clearance the kB 162 pistons properly. The clearance is very specific and it’s critical that you follow it. They are not very forgiving. One hole was way too tight and galled the skirt. This is the same motor that flattened the cam because it was idled too long and somehow bent a pushrod. It may have been because the ball on the plus rod broke off. It was laying in the pan when the lifter spit and oil pressure went away. The cam was way too big, I have not checked the spring pressure yet.
However, next he took the motor to a different machine shop. Well call them “Goobers “ machine. You young guys may have to think on that for awhile 😁. They just honed the motor out and called it good. That’s where me and his friends stepped in and insisted that I take over. So now I discover that the cly that failed only had .0015 clearance at the skirt AFTER they had honed it. No telling what it was before. In order to get the wear marks out at the top of the ring travel, they honed it to where it now has a .0015 TAPER in the cly from top to bottom and all the rest of the cly are too big for the .030 KB162 to have proper clearance. I doubt if Goobers have a torque plate either. The finish on the walls look like it was done with a 20 grit stone.
On top of all this, he bought some 452 heads fromm Indy Cly Heads years ago. They were milled flat, had bigger valves installed and had hardened exhaust seats installed. No telling what he paid for them many years ago. I pulled them apart to find bigger valves but not opened up to even benefit from them. They have cheap guides and stock seals and the seat were cut by a caveman with a square stone. They were obviously cut with an old stone valve bringing machine and I think they dressed the sones at a different angle that the valves. The seat was so narrow, it was barely sealing. Worst valve job I ever saw.
So, yes. This is not going to be a easy build. I’m taking off time from paying customers to build this for free and our friends are “helping out” some as well. We are limited to what we can do for the above and several other reason. Now, my friends and I are cheerfully doing this, but do not want him knowing all the details of our contributions. He does not look at this board so if you put together who this is, please don’t tell him.
So., I’ll be checking some more measurements today. I loaned out my good cc burette so I may need to get another one to make sure of my measurements. I use the diamond piston and Wallace racing CR websites so I’ll post my numbers and you guys and tell me how you figure CR.
Posted By: old_racer

Re: Comp height on 68-70 383 pistons - 02/22/20 03:57 PM

you're a stand up guy to help out a deserving friend ! nice to see in this day and age.
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/22/20 04:02 PM

Originally Posted by old_racer
you're a stand up guy to help out a deserving friend ! nice to see in this day and age.


Well, when you have multiple guys that standup for someone, that just shows how good he is.
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/22/20 05:09 PM

Quote
On top of all this, he bought some 452 heads fromm Indy Cly Heads years ago. They were milled flat, had bigger valves installed and had hardened exhaust seats installed. No telling what he paid for them many years ago. I pulled them apart to find bigger valves but not opened up to even benefit from them. They have cheap guides and stock seals and the seat were cut by a caveman with a square stone. They were obviously cut with an old stone valve bringing machine and I think they dressed the sones at a different angle that the valves. The seat was so narrow, it was barely sealing. Worst valve job I ever saw.


I had someone send me a set of those to go thru a few years ago.
The way the bowl cut was done, they actually flowed worse than a stock head with a stock valve and a very basic valve job.

They really “need” to have someone do a quick bowl blend before you put them back on.

Edit: found the notes on those.
Aerohead 452’s, 2.14/1.81, no blending, big bowl cut:
4.375 bore, 28” test pressure:

Lift——- In/Ex
.100—- 67/53
.200—-116/95
.300—-168/127
.400—-196/139
.500—-217/143
.600—-222/143

Might be the worst I’ve ever tested with big valves.
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/22/20 06:06 PM

That’s sounds like these heads. They are awful. However back to the piston discussion. I found my mistake. I bought brand new magnetic deck bridge with two dial indicators. I zeroed it on a straight edge, ran the piston up and read the dial indicator, and it was .068. I was supposed to read the from zero BACK which would have been .032. Or just subtract from .100. I started to feel really stupid about a rookie mistake then I remembered that I made that calculation last Saturday morning. I had been up since about 1am that morning with no sleep. My wife and I had taken in her Dad to our house from the Alzhiemers care facility at the middle of January. He’s 93 with congestive Heart failure AND Alzhiemers. He got really bad on Thursday and fought off all his meds and family. Friday was even worse. That night about 1:00am was so terrible, I would not wake my wife for her to see him struggle to breath. His wife had spent the night with him for two days and thankfully had gone home to rest one night. He passed away at 2:25am Saturday morning with only me watching him. It was tough. We called in Hospice and the Family so I did not go back to sleep. I had to come to the shop to get my mind off of things while the family took care of arrangements. So, I guess I was intitled to a little mistake that day. I’ll refigure my CR my corrected data. Thanks for the help.
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/22/20 08:09 PM

So I’ve double checked my deck height with a clear head and the measured height is .031 below the deck. This is with the kB 162 and their 1.908 listed CH. My head gasket is .039 fel pro with a 4.490 bore. My biggest problem is with the heads I am stuck with. They are 452 with a 92cc chamber. The best piston I can find is the speed pro forged L2315F with a 1.920 CH. that should make my deck height a .019 now. My phone is almost dead so I can’t run these through the website calculators right now. I may calculate by hand later tonight. the 70 service manual list two different CR of 8.7 and 9.5. I bet they don’t make the 9.5 piston anymore.
Posted By: BSB67

Re: Comp height on 68-70 383 pistons - 02/22/20 08:46 PM

With a shim head gasket:

If you use the 2315, you're at 9.2:1

If you use the Icon 687, you're at 9.6:1

I don't know if I'd run 9.6:1 CR with an non-quench iron head.

What's the rest of the build, and what is the goal?
Posted By: lewtot184

Re: Comp height on 68-70 383 pistons - 02/22/20 10:01 PM

Originally Posted by fastmark
So I’ve double checked my deck height with a clear head and the measured height is .031 below the deck. This is with the kB 162 and their 1.908 listed CH. My head gasket is .039 fel pro with a 4.490 bore. My biggest problem is with the heads I am stuck with. They are 452 with a 92cc chamber. The best piston I can find is the speed pro forged L2315F with a 1.920 CH. that should make my deck height a .019 now. My phone is almost dead so I can’t run these through the website calculators right now. I may calculate by hand later tonight. the 70 service manual list two different CR of 8.7 and 9.5. I bet they don’t make the 9.5 piston anymore.
going to a steel shim gasket will take out 4cc's. milling .030" off the heads will take out another 5-6cc's. real issue may be pump gas or gas blends. camshaft will dictate some of this. for pump gas driver i'd stay around 9-9.25:1 compression ratio. kb says 8.6:1 with those pistons, 90cc chambers, and .040 gasket. I wouldn't change the pistons. 9:1 is easy to get to. just make sure the end gaps are correct and work with what you have.
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/22/20 10:32 PM

Sound advice from Lew.

If most or all of the pistons aren’t reusable...... then the 2315’s may be a good replacement option..... unless the crank was balanced for the 162’s, which are way lighter than the OE replacement stuff.
If the crank was balanced to the 162’s, I’d stick with them.

If the crank wasn’t balanced for the 162’s, then I’d just go to the 2315’s, since they are close to OE weight.
Posted By: BSB67

Re: Comp height on 68-70 383 pistons - 02/22/20 11:35 PM

That's why I asked what the KB162's weigh. Looks like 738 gr. The 2315's are 786 gr. about the same as OEM.

Looks like balancing or machining heads if you're looking to bump the CR, or going back with what it was before if the budget is tight.

Do we know how big the cam is?
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/23/20 01:42 AM

The crank was not balanced to the KB and the goobers that did the last one job, ruined the cly for these KB162 and made them too rough, too big, put a .0015 taper in the cly and did not use a plate. We have to have pistons. They are taken care of and we are going .040 over. Have not chosen a cam yet.
Posted By: lewtot184

Re: Comp height on 68-70 383 pistons - 02/23/20 01:48 AM

i'm going to take a stab at piston/pin weights. a 2315 will be very close to 1000grms. a stock is around 1000grms. kb162 about 930grms. the last 383 I did had 8:1 cast in it and they were replaced with 2315's. I don't know the weight of those 8:1 pistons. we didn't re-balance, which I would have preferred, but there absolutely was no money so it went together. I was dumbfounded by how smooth the engine ran. actually noticeably smoother than my balanced "properly put together" 440.
Posted By: TrackPack

Re: Comp height on 68-70 383 pistons - 02/23/20 02:15 AM

So, 2315's in a 4.290 bore sounds like the best solution without blowing the budget. As mentioned, these pistons came out as a forged replacement for stock pistons, so balancing wasn't an issue supposedly. Be nice to find a set of heads without the huge combustion chamber, but that can be changed in the future. Good luck and hope it all works out.
Posted By: BSB67

Re: Comp height on 68-70 383 pistons - 02/23/20 02:59 AM

Originally Posted by lewtot184
i'm going to take a stab at piston/pin weights. a 2315 will be very close to 1000grms. a stock is around 1000grms. kb162 about 930grms. the last 383 I did had 8:1 cast in it and they were replaced with 2315's. I don't know the weight of those 8:1 pistons. we didn't re-balance, which I would have preferred, but there absolutely was no money so it went together. I was dumbfounded by how smooth the engine ran. actually noticeably smoother than my balanced "properly put together" 440.


This.

The 2315 and the factory pistons were the same within their respective tolerances, probably +/- 10 grams. The 2315 comes with a factory-like pin as well, total right at Lew's 1000 gr.
Posted By: lewtot184

Re: Comp height on 68-70 383 pistons - 02/23/20 03:29 AM

Originally Posted by BSB67
Originally Posted by lewtot184
i'm going to take a stab at piston/pin weights. a 2315 will be very close to 1000grms. a stock is around 1000grms. kb162 about 930grms. the last 383 I did had 8:1 cast in it and they were replaced with 2315's. I don't know the weight of those 8:1 pistons. we didn't re-balance, which I would have preferred, but there absolutely was no money so it went together. I was dumbfounded by how smooth the engine ran. actually noticeably smoother than my balanced "properly put together" 440.


This.

The 2315 and the factory pistons were the same within their respective tolerances, probably +/- 10 grams. The 2315 comes with a factory-like pin as well, total right at Lew's 1000 gr.
factory piston is 770grms. .040" 2315 will be close to 790. I wasn't trying to get the OP to spend $400 on new pistons. balancing for the kb162's is cheaper. it's my understanding balancing is not an exact science. I was just trying to layout some differences and how some perceptions fool us. either way it will work.
Posted By: Cab_Burge

Re: Comp height on 68-70 383 pistons - 02/23/20 07:23 AM

Mopar made some embellished "factory blue print" information to NHRA back in the late 1960s and early 1970s, the 1970 383 motors being one of them, No stock 1970 383 motors made had anywhere close to +.005 piston tops above the decks, none of them tsk The standard stamp steel head gaskets for the 361,383 413 and all of the 426W where all .022 thick scope work I use to keep dozens of them in stock until you couldn't buy them anymore whiney
Ford said that the 1968 428 Cobra Jet motors had 335 HP, just like the 1966 and 1967 390 HP had work Hence the Mopar response in 1969 for the 1970 model yearwhistlingshruggy
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/24/20 02:31 AM

We can’t use the kB now. We’re getting the 2315f. I just have to figure out the heads. Now that I think about it, I have a original 70 383 hp short block at the shop. I’ll measure the C.H. of a piston.
Posted By: Cab_Burge

Re: Comp height on 68-70 383 pistons - 02/24/20 05:27 AM

Do you mean deck height of thosestock pistons?
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/24/20 12:27 PM

Originally Posted by Cab_Burge
Do you mean deck height of thosestock pistons?

I can measure the compression height and figure out what the deck height would be. I could install it but it’s a std bore piston in a .030 hole. I don’t know if I can get the old motor to turn over easily.
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/24/20 02:38 PM

I’d just forge ahead on repairing the current motor, instead of starting all over again.

If nothing else...... the heads should be good cores with new parts.

It really shouldn’t be that expensive to fix them up.
Posted By: bee1971

Re: Comp height on 68-70 383 pistons - 02/25/20 01:05 AM

Originally Posted by lewtot184
Originally Posted by fast68plymouth
Using the stock block height spec of 9.98 along with a piston ch of 1.920(L2315F, no valve pockets),at .030 over, .040 gasket, 84cc heads........ I get 9.14:1.

797- swept
84- chamber
2.8 - deck
10- gasket
1- area above rings
————-
894.8 total
97.8 total minus swept
————-
9.14:1

Imo, that’s the easy button for a street 383.
Deck block for zero deck gets you to 9.38:1.
Swap to 78cc closed chamber heads and you’re at 9.95:1 with good quench.


A 1.908ch piston should be sitting .024” down the hole of an uncut block.
The 1.848ch piston would be down the hole .084”.

With the 1.848ch pistons (no dish, no valve pocket) at + .030 and 90cc heads with a shim gasket:

797- swept
90- chamber
19.8- deck
5- shim gasket
1- area above rings
————-
912.8- total
115.8 - total minus swept
————-
7.88:1

Mill the heads to 84cc and you’re at 8.25:1.
the 383 I did a few years back with .030 over 2315 pistons and '906 heads (with .015" off them) was around 9.24:1 with a .039" head gasket. the 2315 piston is an easy no brainer.


I ran the 2315 pistons .030 over - 906 heads - Mopar 284/484 cam - For 20 years in my numbers block with zero issues


Then I got greedy , and installed the E Street 75cc heads on that same setup a few years back Looking for more

Motor ran awesome , 6,000 Rpms came on really really fast on the highway running 3:91 gears if you weren’t paying attention

Right up until I kissed two intake valves , took out the pushrods , lifters , and camshaft - Again greedy

Then I decided to build my 432 Stroker

So watch your camshaft selection and valve clearance - Only disadvantage , no valve reliefs
Posted By: lewtot184

Re: Comp height on 68-70 383 pistons - 02/25/20 02:24 AM

Originally Posted by bee1971
Originally Posted by lewtot184
Originally Posted by fast68plymouth
Using the stock block height spec of 9.98 along with a piston ch of 1.920(L2315F, no valve pockets),at .030 over, .040 gasket, 84cc heads........ I get 9.14:1.

797- swept
84- chamber
2.8 - deck
10- gasket
1- area above rings
————-
894.8 total
97.8 total minus swept
————-
9.14:1

Imo, that’s the easy button for a street 383.
Deck block for zero deck gets you to 9.38:1.
Swap to 78cc closed chamber heads and you’re at 9.95:1 with good quench.


A 1.908ch piston should be sitting .024” down the hole of an uncut block.
The 1.848ch piston would be down the hole .084”.

With the 1.848ch pistons (no dish, no valve pocket) at + .030 and 90cc heads with a shim gasket:

797- swept
90- chamber
19.8- deck
5- shim gasket
1- area above rings
————-
912.8- total
115.8 - total minus swept
————-
7.88:1

Mill the heads to 84cc and you’re at 8.25:1.
the 383 I did a few years back with .030 over 2315 pistons and '906 heads (with .015" off them) was around 9.24:1 with a .039" head gasket. the 2315 piston is an easy no brainer.


I ran the 2315 pistons .030 over - 906 heads - Mopar 284/484 cam - For 20 years in my numbers block with zero issues


Then I got greedy , and installed the E Street 75cc heads on that same setup a few years back Looking for more

Motor ran awesome , 6,000 Rpms came on really really fast on the highway running 3:91 gears if you weren’t paying attention

Right up until I kissed two intake valves , took out the pushrods , lifters , and camshaft - Again greedy

Then I decided to build my 432 Stroker

So watch your camshaft selection and valve clearance - Only disadvantage , no valve reliefs

performance 383's need valve reliefs in the pistons. I've busted a lot of parts with flat tops, no reliefs, 4spd and 383.
Posted By: Scully

Re: Comp height on 68-70 383 pistons - 02/25/20 03:30 AM

I've built plenty of budget street .070" 383s using these or similar for my customers, the deck height is almost identical to the 68-69 383. At one time I had a barrel full of stock standard 72 440 style pistons and found a good use for them. https://www.summitracing.com/parts/slp-424np?rrec=true a .040" over makes a budget .020" 400.
Posted By: fastmark

Re: Comp height on 68-70 383 pistons - 02/25/20 02:31 PM

Looking in my service manuals and another source I have, the 383 CR has varied.
62-65. 10.0 closed ch
66-67. 10.0 and 9.2 closed ch
68. 10.1 and 9.2 open ch
69. 10.0 and 9.2
70. 9.5 and 8.7
71. 8.5

I assume all of these CR changes were made by pistons changes. As far as I know, mopar only used .023 shim steel gaskets from the factory. I have three 383s at the shop. One bone stock oem 69 non hp that’s too nasty to turn over and check. I’m sure it’s a pass car 9.2 CR. One 71 hp with oem pistons and one 70 with aftermarket .040 cast pistons of unknown CR. Both of these later 383 have had non factory cams installed at one time because both of them have valve marks on the pistons from hitting valves. So that is a major concern for sure on these 383 pistons with no valve reliefs. I will have a cam with less than 230 degree of duration @.050 because I need my power brakes to work. I may have to go even less. At least his first motor did not have that problem with KBs. He had big cam with 241@.050 but the KBs have hugh valve reliefs. My other issue is all the cost of rebuilding the 452 is fasting approaching new stealth head price. Cleaning, mag testing, good three angle VJ is $400. $100 for cutting cutting for better seal and seals. Then new springs and setup pressures and were getting close to new stealths. All the cheap crappy machine shops have vanished from poor work work like we got on these heads. These heads he has at least have new hardened ex seats and new valves but they appear milled some and still have 92 cc chambers. I’m going to measure some more heads I have and make sure it’s not my measurements. I’m going to order a new burette.
Posted By: lewtot184

Re: Comp height on 68-70 383 pistons - 02/25/20 03:36 PM

the difference in compression ratios were different chambers and 2bbl vs 4bbl pistons. using stock hydraulic cam valvetrains may be the biggest culprits to piston to valve collision. when the springs would float the tappets would pump up (keep in mind .100+" pre-load on the tappets) and the intake valve is into the piston. tight lobe separations, added duration, aggressive lobes, increased lift just makes matters worse. keep in mind that the intake valve is chasing the piston on it's downward motion and the valve is moving faster than the piston. valve reliefs are the only true answer to the problem. a stock magnum cam with slower ramps and wider lobe separation is safer but like all hydraulics needs pre-load reduced. aftermarket rockers usually have higher than advertised ratios so using them may not be the cure.

the 2315's are easy to use but can set you up for piston to valve collision when revving the engine up. the kb162 is safer and lighter. another thing; big valves will give you a little less piston to valve clearance. advancing the cam will reduce clearance, AND, don't over cam a 383.
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/25/20 04:08 PM

FWIW, I don’t pay any attention at all to what the factory CR specs are.
Pretty much useless info for the real world.

Measure everything...... and iiwii, regardless of what any spec in any book says.

And yes......... it’s easy to spend Stealth money on fixing old heads...... even refurbished ones, if they’re screwed up enough.

Posted By: cudaman1969

Re: Comp height on 68-70 383 pistons - 02/26/20 03:56 PM

Originally Posted by lewtot184
the difference in compression ratios were different chambers and 2bbl vs 4bbl pistons. using stock hydraulic cam valvetrains may be the biggest culprits to piston to valve collision. when the springs would float the tappets would pump up (keep in mind .100+" pre-load on the tappets) and the intake valve is into the piston. tight lobe separations, added duration, aggressive lobes, increased lift just makes matters worse. keep in mind that the intake valve is chasing the piston on it's downward motion and the valve is moving faster than the piston. valve reliefs are the only true answer to the problem. a stock magnum cam with slower ramps and wider lobe separation is safer but like all hydraulics needs pre-load reduced. aftermarket rockers usually have higher than advertised ratios so using them may not be the cure.

the 2315's are easy to use but can set you up for piston to valve collision when revving the engine up. the kb162 is safer and lighter. another thing; big valves will give you a little less piston to valve clearance. advancing the cam will reduce clearance, AND, don't over cam a 383.

Zero valve lash and triple springs, no valve float or kissed pistons, shifted my 383 @6800 all day long, MP 509
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/26/20 05:00 PM

I really don’t think V/P clearance will be an issue at all for the proposed build if an “appropriately” sized cam is used, and the heads/block are not excessively milled.

2315’s with the heads/block/gasket situation set up for low 9’s CR shouldn’t present any problems with a suitable street cam.

The stock eliminator guys get it done without valve pockets(pistons have to be stock configuration) and much bigger cams than what this build will get.
Posted By: lewtot184

Re: Comp height on 68-70 383 pistons - 02/26/20 06:29 PM

Originally Posted by fast68plymouth
I really don’t think V/P clearance will be an issue at all for the proposed build if an “appropriately” sized cam is used, and the heads/block are not excessively milled.

2315’s with the heads/block/gasket situation set up for low 9’s CR shouldn’t present any problems with a suitable street cam.

The stock eliminator guys get it done without valve pockets(pistons have to be stock configuration) and much bigger cams than what this build will get.
stocker guys don't run .100" preload on their tappets. ain't apples to apples.
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/26/20 06:38 PM

Well...... that’s true....... but I don’t think I’d be running .100 preload either.

Just use the Lunati micro-trol lifters that use the tru-arcs to retain the plunger and run them at zero preload.
Posted By: cudaman1969

Re: Comp height on 68-70 383 pistons - 02/26/20 09:17 PM

Originally Posted by fast68plymouth
Well...... that’s true....... but I don’t think I’d be running .100 preload either.

Just use the Lunati micro-trol lifters that use the tru-arcs to retain the plunger and run them at zero preload.

And only stock .450 ish Lift, but they do cut it real close to that piston, they pushed for notches awhile back, nhra said no, the 383 would dominate the classes if they let them because that’s the biggest downside of running a 383, drop the piston or smaller cam. We can run solids lifters now on the hyd cams so that helps. And the LD340 intake for my ride, About 50 pounds lighter.
Posted By: BSB67

Re: Comp height on 68-70 383 pistons - 02/26/20 09:53 PM

My old machinist had a SS/H or I 383 Dart running out of his shop. It had valve notches, but they looked like piston valve contact impressions, so he said. whistling
Posted By: fast68plymouth

Re: Comp height on 68-70 383 pistons - 02/26/20 10:06 PM

Valve notches are legal in SS....... but they probably weren’t always.

In Stock, at one point it was pretty common to hammer some reliefs into stock pistons with some junk heads and valves.

Then, NHRA made a rule against it.
No valve contact witness marks allowed on the pistons.
Posted By: BSB67

Re: Comp height on 68-70 383 pistons - 02/26/20 10:31 PM

Yup, that's generally it.

This was in the early 1990s. I think at the time the class was dominated by small block Camaros. I think his belief was that the small AVS (verse Q-Jet) and carb to hood clearance was a hindrance for the 383.
Posted By: Scully

Re: Comp height on 68-70 383 pistons - 02/27/20 02:00 AM

Originally Posted by cudaman1969
Originally Posted by fast68plymouth
Well...... that’s true....... but I don’t think I’d be running .100 preload either.

Just use the Lunati micro-trol lifters that use the tru-arcs to retain the plunger and run them at zero preload.

And only stock .450 ish Lift, but they do cut it real close to that piston, they pushed for notches awhile back, nhra said no, the 383 would dominate the classes if they let them because that’s the biggest downside of running a 383, drop the piston or smaller cam. We can run solids lifters now on the hyd cams so that helps. And the LD340 intake for my ride, About 50 pounds lighter.

The LD's not legal anymore.
Posted By: cudaman1969

Re: Comp height on 68-70 383 pistons - 02/27/20 05:10 PM

Originally Posted by Scully
Originally Posted by cudaman1969
Originally Posted by fast68plymouth
Well...... that’s true....... but I don’t think I’d be running .100 preload either.

Just use the Lunati micro-trol lifters that use the tru-arcs to retain the plunger and run them at zero preload.

And only stock .450 ish Lift, but they do cut it real close to that piston, they pushed for notches awhile back, nhra said no, the 383 would dominate the classes if they let them because that’s the biggest downside of running a 383, drop the piston or smaller cam. We can run solids lifters now on the hyd cams so that helps. And the LD340 intake for my ride, About 50 pounds lighter.

The LD's not legal anymore.

Easy come easy go I guess, $300 in the trash. That Pontiac guy was screaming blood murder to the upper honchos over nothing.
Posted By: Scully

Re: Comp height on 68-70 383 pistons - 02/27/20 05:42 PM

Originally Posted by cudaman1969
Originally Posted by Scully
Originally Posted by cudaman1969
Originally Posted by fast68plymouth
Well...... that’s true....... but I don’t think I’d be running .100 preload either.

Just use the Lunati micro-trol lifters that use the tru-arcs to retain the plunger and run them at zero preload.

And only stock .450 ish Lift, but they do cut it real close to that piston, they pushed for notches awhile back, nhra said no, the 383 would dominate the classes if they let them because that’s the biggest downside of running a 383, drop the piston or smaller cam. We can run solids lifters now on the hyd cams so that helps. And the LD340 intake for my ride, About 50 pounds lighter.

The LD's not legal anymore.

Easy come easy go I guess, $300 in the trash. That Pontiac guy was screaming blood murder to the upper honchos over nothing.

Yeah, that Pontiac guy is one of a kind, they took away the LD for the 273 too.
Posted By: 69dart

Re: Comp height on 68-70 383 pistons - 02/28/20 01:47 PM

I keep an eye out for the old TRW 2293's at swap meets or ebay and buy them up.

I think I have 3 or 4 sets - I could use .060 over set though for my next rebuild.

My engine builder calls them boat anchors but they work perfect.
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