Variance in piston diameter from top of piston to the skirt

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Rocket

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Curious what to expect if you measure the top of a new 340 or 360 forged piston and how much the diameter changed from above the top ring land versus mid way down the skirt perpendicular to the wrist pin. What should one expect?
 
Depends on application and who designed the piston and the ring pack.

They all have their own ideas.


But...IIRC it's like .060 or something. Dam I can't remember.
 
So for example on a piston that is suppose to be 4.065 in diameter at the skirt, it would not be uncommon for the top of the piston to be smaller say 0.030 smaller at the top above the first ring land for difference in expansion rates?
 
Every manufacturer will tell you where to measure their piston for the true diameter. It's normally 1/2" up from the lower edge of the skirt. Beyond that measurement, the other dimensions are much less important. Somepistons may have more dead volume above the top ring. Some may have the ring higher. Some will have more diameter up there to reduce the dead volume, but because it rocks you don;t want to have it the same diameter as the skirt. It will be round as that's the best way to fully support the rings for the best ring seal.
 
Rocket said:
Curious what to expect if you measure the top of a new 340 or 360 forged piston and how much the diameter changed from above the top ring land versus mid way down the skirt perpendicular to the wrist pin. What should one expect?
Click to expand...

The top better be much smaller than the skirt--this is because the top of the piston expands a huge amount due to be close to the most heat from combustion. It's usually about .040" smaller than the skirt if you want a number. I actually have a wild story involving incorrectly machined pistons from Speed-Pro on this very subject. J.Rob
 
This helps me make sure I am reading the dial indicators and calipers I am using correctly. Did not think I was reading them wrong. Just wanted confirmation that the top is indeed less than the advertised size of the piston. I was measuring 0.040 smaller up top on a 4.07 piston so this provides me assurance I am not going crazy.
Thanks all.
 
Rocket said:
This helps me make sure I am reading the dial indicators and calipers I am using correctly. Did not think I was reading them wrong. Just wanted confirmation that the top is indeed less than the advertised size of the piston. I was measuring 0.040 smaller up top on a 4.07 piston so this provides me assurance I am not going crazy.
Thanks all.
Click to expand...


Yes the top has to be smaller. It runs hotter. Always measure for clearance perpendicular to the pin bore at about the pin bore.

And you should use a mic and not a dial caliper to measure stuff like that.




I too would love to hear the story about the piston made incorrectly. You never know who may learn something from it.
 
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