Oversized Head Gasket

T

TMB

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My pistons extend about 0.032 above the engine block at the piston edges. Les Andrews' book mentions using an oversized head gasket (.100) if pistons are above the block but does not specify how much. I purchased the A-6051-C copper head gasket from Snyder's (Best 509 stamped on gasket) before starting my project. Do I need a different "oversized" gasket or will the one I purchased do? Where would I obtain an oversize gasket if needed? I do not see "oversized" gaskets listed in Snyder's, Mike's or Bratton's websites or catalogs.
 

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Besides clearing the piston diameter, make sure that there is sufficient clearance at the pockets that are machined into the head. Measure the depth of the pockets, add the compressed thickness of the head gasket (to be on the safe side figure they compress about 20%) then subtract the 0.032 that the pistons stick up above the block. The ideal clearance is 1/16 inch.
 
A short time ago, a friend asked for my help replacing a cracked he on his car. I gave him one I had laying about but before fitting it, I had it made flat again. I could see by looking at the under side of the water pump boss that a lot of material had been removed over the years. To verify that I had clearance, I sat the head on the block with no gasket and turned the engine over by hand. The pistons hit the head so I took a few pieces of the old compressed head gasket and laid them under the head in a few places and checked again. No contact - bolt it down. Running well now.
 
Besides clearing the piston diameter, make sure that there is sufficient clearance at the pockets that are machined into the head.
nkaminar - what are the pockets you are referring to? I am not familiar with that terminology. Are you referring to the combustion chambers in the head?
 
Besides the combustion chambers there are recesses cut into the head that reside above the pistons so that the tops of the pistons do not hit the head. These are circular recesses that can be cast or machined into the head. I called them pockets but there is probably a correct name that I don't remember. When the head is decked (machined to clean it up) the clearance pockets are re machined.
 
There is an old school test to check the piston to cylinder head clearance. Put some clay or silly putty on top of each piston, install the gasket & head, torque the head nuts, crank the engine over & then remove the head. Measure the clay / putty thicknesses.
 
Nearly all the head gaskets that are available now (the copper clad ones at least) are already oversized in bore width. If I'm not mistaken, I believe that all Model A pistons extend above the block a little. But I like Bob Bidonde's advice to use clay to check head to piston clearance.
 
Here's the math:
head “pocket” clearance = 0.083”
Gasket = 0.077” – 20% (0.0154) = 0.0616”
Piston above block = 0.032"
Head pocket + compressed gasket = 0.083 + 0.0616 = 0.1446"
Piston to pocket clearance = 0.1446 – 0.032 = 0.1126”
Does this sound reasonable?

If I were to do the clay-on-piston method suggested by Bob Bidonde, I assume it is acceptable to compress then reuse the same new gasket correct? Thanks again for all the advice.
 
That will work just fine. It is a little more than what is recommended. The 1/16 inch was found to be ideal when the research was done in the late 1920's because it will provide more velocity to the air for more turbulence. But 0.113 will work almost as well. You will not notice any difference.
 
When new, the pistons protruded (from memory) about .015" above the block. Your block must have been "decked" a few times? Allow at least .060" clearance between the pistons and the head, rods stretch a bit. You may need to run 2 head gaskets?
 
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