Piston to head clearance

[inline6]

What limits the minimum piston to head clearance you can run on an typical gasoline engine. lets limit to iron block, steel rod, alloy piston, alloy head.
my understanding is the following (in no particular order)
- clearances between main bearing/BE bearing and pin (~ 0.1 mm combined)
- piston rock (mostly an issue when engine is not warmed up though not sure on the running clearances)
- rod stretch (dependent on stiffness and piston mass)
- differential thermal expansion (probably small as cylinder is close to coolant temp and rod at oil temp which often are close to each other)

but which is the more dominant? rod stretch seems to be the one touted but is it really the case?

 

[MikeHalloran]

I'm guessing that the sum of associated tolerances would dominate.


Mike Halloran
Pembroke Pines, FL, USA

 

[romke]

I would guess that in a gasoline engine the real limiting factor is the combustion process. if you want knockfree operation, you have to limit the compression ratio to a value that is compatible with the fuel used.

 

[CWB1]

Thermal expansion, inertia, and to a lesser extent stackup. Its not uncommon to see 0.010-0.015" PTHC in a modern engine.

 

[RodRico]

My engine design consultant told me to keep the minimum clearance at 0.010" or greater for all the reasons you mentioned.

 

[inline6]

the recommendations i've seen are for 0.040" though these are not minimums and seem to be more for mixture motion. ive never heard of anything less than around 0.025" being used atleast on a "performance" type engine

 

[PackardV8]

It depends:

Bore diameter
Length and construction of piston skirt
Piston-to-wall clearance
Rod modulus of elasticity
Piston weight
RPM
Rod material
Block material

Some very high performance engines built with very carefully controlled tolerances actually leave cylinder head witness marks in the carbon on top of the piston. At max RPM, the piston-to-head clearance is zero.

jack vines