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Adequate valve to piston clearance? #2465393
03/12/18 08:28 PM
03/12/18 08:28 PM
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Minnesota
peabodyracing Offline OP
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Started lining things up to reassemble my 426 wedge after a very long hibernation, and am now concerned.

I bought a rebuilt set of 906 heads some time back, with new valves in place. Comparing these to my old set of 906's I noticed the new valves sit 'higher' in the combustion chamber than the old ones ( which I'd reground at least once years ago)

Since my engine has domed pistons, I assembled things with head gasket, using the clay on top of piston routine. Set valve clearance according to cam specs and turned the engine over twice in the normal rotation direction.

The first time I did this I was somewhat shocked by how close the intake valve appeared, so I started over and did it all a second time.

It looks like I've about 18 thousandths clearance between intake valve and the piston valve relief. My assumption is this is too tight.

I had run this cam years ago in the engine, long enough to break it in, but had not made it to the track with the car before putting it away. At that time the old heads were installed, with the valves sitting deeper in the seats. There's got to be 25+ thousandths difference in valve height on the seat, between old and new heads.

Cam is not really radical, with .550 lift. I'd not anticipated this concern till today.

Opinions please? Thanks in advance.


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Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465397
03/12/18 08:40 PM
03/12/18 08:40 PM
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Bend,OR USA
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Cab_Burge Offline
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You can have the valves ground down to get them to sit in the seats a little further or buy or rent the tools needed to cut the piston valve reliefs deeper.
Are you checking all eight cylinders for P to V clearances? If not do that now before finding out later that you should have twocents
I raced several different NHRA legal 1963 426 M. W. motors, both 415 and 425 HP and the stock rods are not all the exact same length, same as any Mopar OEM rod sets from the factory scope
My last 425 HP short block had .009 difference in rod lengths puke scope Same thing on rocker arm ratio,, they are not all the same shruggy
Good luck, take the time now to get all parts to fit correctly up
I have ran the intake to piston valve clearances as close as .029 on my original 415 HP short block due to having the heads cut to reduce the CC from 86.0 CC + to 81.5 CC based on checking ONLY one chamber in each head and not rechecking the P to V clearances realcrazy I got lucky on that on, not so on other parts changing without doing all the clearance checking whiney
Never ASSUME anything on any motors, ever twocents


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465425
03/12/18 09:34 PM
03/12/18 09:34 PM
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Washington
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madscientist Offline
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Did you measure the clearance with clay?


Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465534
03/12/18 11:21 PM
03/12/18 11:21 PM
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Fancy Farm Ky
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wyoming Offline
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IMHO Id cut the pistons, or go a bit smaller on the cam, sinking the valves will hurt flow, thats my experience anyway. Any chance you were using the steel shim gasket for mock up?

Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465539
03/12/18 11:28 PM
03/12/18 11:28 PM
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wyoming Offline
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IMHO Id cut the pistons, or go a bit smaller on the cam, sinking the valves will hurt flow, thats my experience anyway. Any chance you were using the steel shim gasket for mock up?

Re: Adequate valve to piston clearance? [Re: madscientist] #2465570
03/13/18 12:27 AM
03/13/18 12:27 AM
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WHITEDART Offline
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Originally Posted By madscientist
Did you measure the clearance with clay?
this is what I would do or use the valve drop method.. are you able to mock it up with a set of checking Springs.... you need room you can face the valve and shrink the margin.. I think this is better then sinking the valve..

Last edited by WHITEDART; 03/13/18 12:29 AM.

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Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465582
03/13/18 12:53 AM
03/13/18 12:53 AM
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Australia
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ozymaxwedge Offline
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OP said he has 18thou clearance, he needs another 100th to be safe. Is that about right ?


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Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465599
03/13/18 01:28 AM
03/13/18 01:28 AM
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Cotati, CA
Dave Hall Offline
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You could retard the cam some. A couple of degrees should get it above .050 and it will be fine.

Re: Adequate valve to piston clearance? [Re: ozymaxwedge] #2465600
03/13/18 01:29 AM
03/13/18 01:29 AM
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Jeremiah Offline
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I think it depends on intended use, etc. That's very little in my world. We faced the exhaust valves .020 on my last motor to open the clearance up but that was a 7500rpm stick shift package. Is there a general rule of thumb and if so, what is it based on?

Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465606
03/13/18 01:40 AM
03/13/18 01:40 AM
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Dave Hall Offline
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Big compression engines with big cams run tighter than the general rule of .080. They sometimes leave a witness mark on the pistons. IMO .040 to .050 will be fine with that cam as it probably won't see 7,000. Was the clay checked at zero lash? If so .020 on the lash makes it .038 right?

Re: Adequate valve to piston clearance? [Re: Dave Hall] #2465621
03/13/18 02:18 AM
03/13/18 02:18 AM
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pittsburghracer Offline
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I can’t afford putting my stuff together praying that all goes right under the bare minimum clearances so I take time and make it right the first time. Good luck.


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Re: Adequate valve to piston clearance? [Re: Dave Hall] #2465627
03/13/18 03:12 AM
03/13/18 03:12 AM
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Originally Posted By Dave Hall
Big compression engines with big cams run tighter than the general rule of .080. They sometimes leave a witness mark on the pistons. IMO .040 to .050 will be fine with that cam as it probably won't see 7,000. Was the clay checked at zero lash? If so .020 on the lash makes it .038 right?
iagree On the intake valves .050 at the closest will work fine with good parts and don't float the valves, .060+ on the exhaust with good valves and springs, rockers and pushrods work well also twocents
I've had them close enough that there was no carbon on the piston tops under the valves and I couldn't feel any contact on the pistons or valves and none of the valves where bent shruggy work
It's not that I am recommending that though whistling grin


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Adequate valve to piston clearance? [Re: Dave Hall] #2465631
03/13/18 04:01 AM
03/13/18 04:01 AM
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Originally Posted By Dave Hall
You could retard the cam some. A couple of degrees should get it above .050 and it will be fine.


This is what I would do first. You didn't say what the exhaust clearance was. If it's more than enough then moving the cam timing should get you where you need to be. Don't worry about changing the power band either. It's something like 4 degrees of cam timing changes the power band 200 RPM.


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Re: Adequate valve to piston clearance? [Re: Dave Hall] #2465652
03/13/18 09:20 AM
03/13/18 09:20 AM
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Minnesota
peabodyracing Offline OP
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Thanks much for all the good input!

I did measure using clay. I use a composition head gasket. The heads have the regular valve springs on them; didn't have a set of lightweight springs for this purpose.

I didn't measure at zero lash. I'd set valve lash per cam mfr's info. There was plenty of clearance on the exhaust side.

This used to be a 4 speed car, and 7000 RPM was not unheard of. It's now an automatic and I've switched a number of other things to try and avoid twisting it that hard.

Having said all that, I'd not thought of retarding the cam. That would seem a good option at this point, particularly since I appear to be flush with clearance on the exhaust side. Been too many years. I'll pursue that and recheck.

Cab, I will check all cylinders. Your point on rod length was another one I'd not considered.

Thank you again guys for all your wisdom! It's great to be able to come on here and run this sort of stuff by you all.


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Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465887
03/13/18 05:29 PM
03/13/18 05:29 PM
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Romeo MI
MR_P_BODY Offline
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A few questions for ya.. is this a street car..
(assuming its a street/strip set up) you can
put the timing back to give you more clearance..
what head gaskets thickness.. can you give the
open/close specs on the can(this will help tell
you if you can move the cam.. I just cut a set
of pistons yesterday to lower my compression..
you can also cut the valve pockets to help if you
dont want to move the cam.. ( I always use a zero
lash to get my setting.. just saying)
wave

Last edited by MR_P_BODY; 03/13/18 05:32 PM.
Re: Adequate valve to piston clearance? [Re: MR_P_BODY] #2465979
03/13/18 08:01 PM
03/13/18 08:01 PM
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Posts: 2,212
Minnesota
peabodyracing Offline OP
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Originally Posted By MR_P_BODY
A few questions for ya.. is this a street car..
(assuming its a street/strip set up) you can
put the timing back to give you more clearance..
what head gaskets thickness.. can you give the
open/close specs on the can(this will help tell
you if you can move the cam.. I just cut a set
of pistons yesterday to lower my compression..
you can also cut the valve pockets to help if you
dont want to move the cam.. ( I always use a zero
lash to get my setting.. just saying)
wave


Thanks for your questions. I use a FelPro composition head gasket that measures right around .040" thick.

I measured the amount the intake valves protrude into the combustion chamber and get just shy of .080" which causes me to wonder if there's something amiss on the heads/valves. This is even more than I guessed at just looking at them.

Anyway, cam specs:

Duration @ .050 261
Lobe lift 368
lobe separation 103

seat duration @ .020 295
gross valve lift .552
lash (hot) .026 .028

I'd not driven the car on the street for many years. Trying to avoid taking it completely apart to cut pistons, etc. My intent was to get the car going again (after 20 years of sitting) and make some passes with it for old time's sake. I've had the car since the early 70's.

Mid 90's I'd switched the car from 4 speed to automatic, and at that time bought this cam after consulting with the supplier. Put the cam in, broke it in and took the car out on the road to try out the 727 only to run into a bunch of problems. Fought with it for a few weeks and ultimately just put it away for another day. Life then got in the way.

Over the 20 years I had someone reputable go back through the 727, bought a new converter, changed the gears from 4:89 to 4:56 and welded up a new set of fenderwell headers for it, but that was the extent of progress.

This winter I've gone through all the brakes, fuel system, the carburetor, and so on.

So my preference is to find a solution that will allow me to bolt the top end back together and go from there. If it all works, great. If not, who knows. I gotta admit I burned out on drag racing back then, so still not sure how deep I want to get financially into the whole deal.

Anyway, thanks in advance for your thoughts. I'll shut up now.


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Re: Adequate valve to piston clearance? [Re: peabodyracing] #2465983
03/13/18 08:12 PM
03/13/18 08:12 PM
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Romeo MI
MR_P_BODY Offline
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From the sound of things the easiest and cheapest
thing to do would retard the cam 4* then check it
(thats a pretty tight LSA so retarding it will help
you.. sounds like a old cam)
wave

Re: Adequate valve to piston clearance? [Re: MR_P_BODY] #2466016
03/13/18 08:59 PM
03/13/18 08:59 PM
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Posts: 2,212
Minnesota
peabodyracing Offline OP
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Thanks for your input. Doubly so now that I see you're recovering from a stroke. Hope you're back to 100% very soon.

Again, thanks and best wishes.


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