by panic » Sat Dec 12, 2020 9:27 am
Flathead engines (H-D, Indian, Ford, Triumph, Pontiac) REQUIRE very tight piston to head clearance, about .035" minimum @ TDC.
K, KH, KR, WR and all Scout and Chief Bonneville engines have pistons that rise above the deck @ TDC and enter a recess in the head.
The same clearance is necessary in all cases. No sidevalve engines were built without this design feature in the last 100+ years (although the production tolerances tend to be a bit looser).
This has nothing to do with the compression ratio, it's anti-knock protection.
Hemispherical chambers (H-D single-cam big twin, iron Sportster), Triumph, Chrysler) have almost no quench surfaces and do reasonably well with very loose clearances. It's still a good idea.
What happens if I use a zero or slightly negative deck piston (such as a 45, Scout, Atlas) with a head that requires a dome (K, KH, KR, WR)?
Overheating, knocking, low power and eventual piston failure.
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