Piston to head clearance?

B

BobM

Senior Member
My Offy heads have been milled .020" and when I set them on the block with no gasket the head lifts up when piston hits TDC; is this to be expected? The only thing preventing catastrophic failure is the head gasket? When I clay with gasket & head torqued to 50lbs. I get .040" at the piston edge, does this make sense? I know the head is coming off the block with no gasket almost .040". Nervous & confused
 
You get .040 at the edge, what do you have in the center of the piston? Was this engine running before and all you are doing is changing heads? If fresh build, do you have piston, rod, crank combination right? Are the heads new? or have they been milled before?
 
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Something doesn't add up. If you have .040 clearance with a torqued gasket, there must be some area that is tighter that you haven't measured. Are you sure it is lifting .040?
 
flatjack9 said:
Something doesn't add up. If you have .040 clearance with a torqued gasket, there must be some area that is tighter that you haven't measured. Are you sure it is lifting .040?
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Here are my measurements: .040" piston edge .043" at midpoint between edge and center of piston and .085" at center of piston with gasket & head torqued to 50lbs. Actual lift off block is about .038" with head & no gasket at TDC. This engine has never been fired, Merc crank, full floating rods because crank had to be turned way down and Ross pistons. With .085" at piston center I think all the parts are matched correctly, could milling the head .020" be the reason head raises up?
 
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Your measurements are they with gasket i presume. .040" .043" .085".

You only get lift with no gasket? With gasket you have clearances.
.050" is desired but you "may" get away with .040"!! Some gurus will chime in.Whose gasket are you using?

R
 
What Offy numbers are the heads? When it's at TDC was it on compression stroke? Are you sure it is the piston lifting the head and not a valve? If the heads are 400 or 375 and they were milled .020" plus a high lift cam the valve may be lifting it.

Vergil
 
Vergil said:
What Offy numbers are the heads? When it's at TDC was it on compression stroke? Are you sure it is the piston lifting the head and not a valve? If the heads are 400 or 375 and they were milled .020" plus a high lift cam the valve may be lifting it.

Vergil
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Vergil, the heads are 425 Offy, valve pockets cut to depth of .445". Both valves are closed on top of power stroke and then I rotate 720 degrees just to make sure valves open fully. I can see the exhaust valve at full lift through plug whole and is not making contact with head, intake comes out .002" further than exhaust with a net lift of .354", when I clay intake valve I get .081" Maybe I should clay intake valve at different locations; I clayed on outside edge of valve with alumna peas. How do you change font size on this site? Thanks for your input. Bob
 
When you post go to adavnced and pick your font size by the Verdana font name, this is set to 3.

Vergil
 
Obviously something is lifting the head. The places you have checked are not the culprit. You've got to check different spots. Put some blueing on the valves and piston, crank it over with no gasket and see where you get a mark.
 
I use alum balls stuck to the piston at several places to determine clearance. Do the same with valves, but not at the same time.
 
Ronnie said:
Your measurements are they with gasket i presume. .040" .043" .085".

You only get lift with no gasket? With gasket you have clearances.
.050" is desired but you "may" get away with .040"!! Some gurus will chime in.Whose gasket are you using?

R
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Only lifts with gasket off, am using Best graphite gasket. Yeah, I think .040" is a little scary but I have seasoned rods and hope they don't stretch at upper RPM. I just don't understand how it got so tight even with milling the head .020", have heard of guys who milled .050" off a head but maybe they weren't Offy heads. Yes, those measurements are with gasket and torqued to 50lbs.
 
BobM said:
How do I go to advanced?
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63622
 

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Ol'Ron, your post couldn't have come at a better time, just got done re-claying the piston at different locations and got a .037" reading, made me real nervous, feel better now. I need to get address for your book. Thanks, Bob
 
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