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.

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.

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.

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.

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’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.

Nuthin fancy req’d for a 9:1 383.

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

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

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?

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.

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.

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.

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.

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.

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?

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.

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.