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I mocked up the cylinders and pistons on the UH motor I’m building and the piston to head clearance seems excessive.
Total clearance at TDC is .0115” (.070” piston to head + .020” base gasket + .025” head gasket)
I’m using stock grind Andrews cams with stock repop pistons and VT Cycles aluminum 6.5:1 “ULR” heads, the bike will be used as a around town cruiser.
I need some opinions on what the ideal clearance should be, I don’t mind having the cylinders decked a little but I can’t go
too much because the valve seats are already done and ready fo final lapping.
Using any engine for that spec, thats not excessive and it's almost irrelevant if its even more. What you want to verify is what your actual compression ratio is the way it is now by cc'ing the heads and calculating the rest. I never trust a piston manufacturer and if you need it .100" down the bore at tdc (compression height) to get the right ratio in a rebuild then so be it.
Yes I meant .115”, good catch.
The measurement I’m trying to calculate is the quench, I had read somewhere that .050”
total with gaskets was ideal for the big Flathead street motor tighter if it’s a race engine.
It’s really important when building a hot rod V8, you normally deck the block to achieve the desired clearance which is much tighter then the .070” this motor has.
Here’s a pic
I understand completely. Im building a 454 right now and I set the compression height to .010" to get the ratio I want, not thinking about quench because the chamber is outdated and has plenty.
The Ferrari engines I rebuild start with a .079" compression height from the factory for whatever reason and its a hemi head so clearance is large. I lower the pin when I have pistons made to raise the compression height to .010" and I raise the ring pack up as well to reduce quench and gain more potential ratio doing my best to reduce the dome. Aiming for a deck height just because without knowing the actual ratio is a backwards approach and worrying about quench on a flathead engine is like worrying about fuel economy on your BB chev.
Yes it is, BBi though. Just finished a 365BB resto, spent the last two weeks road testing and tuning.
Sounds like a pretty good way to spend a couple weeks.
I was thinking of cleaning up the inside of the ports, they’re pretty rough castings.
This is the first Harley motor I’ve ever built so I’m trying to learn all I can reading everything I can find and asking you guys
lots of questions, you and John have been really helpful.
I’m not really sure how to relieve the deck, in my limited knowledge of flatheads I was thinking that the “improved” combustion chamber of the VT Cycle heads would be sufficient.
My goal on every motor I build is to adjust all the tolerances to ideal spec, I’m no expert, I try new things because I dig motors.
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Piston to Head Clearance
