Valve-to-cylinder wall clearance

[troyt]

I'm finally starting my initial racemotor mockup. I'm pretty tight on the exhaust valve to cylinder wall clearance, even after eyebrowing. I'm under 1/32" clearance, about .025, at max valve lift. Here's the numbers I've got: an early E88 head, milled .030, using .025 cam tower shims, intake and exhaust valves at 44mm and 35mm, L24 block at .040 over, .535 lift in/exh cam, comp head gasket measuring .056 before crush and assuming .047 after. Cylinder eyebrowing can't go any deeper and keep enough room above the top piston ring land. Wider eyebrowing is not an issue, depth is. Current eyebrowing has already dropped CR from 12:1 to 11.7:1. Exh valve clearance at rocker/cam set at .010, intake at .008.

 

Questions - Has anyone gone this tight on a valve-to-cylinder wall clearance before? If so, what was the outcome? How much can you decrease exh valve clearance at the rocker/cam before you run into problems? How does a manufacturer regrind a new cam to take a little lift off? (Isky L1)

 

I'd prefer not to use a thicker gasket and loose more CR. Some options are a little less cam (down to .510 or so), another set of pistons with a lower top ring (more $$ and less CR), or ??? In retrospect I would have stayed with the stock exhaust valve, but now that I've finally got the cylinder head the way I want it, and I don't want to re-do it.

 

Thanks, Troy

[troyt]

Is this the wrong forum for this type of question?

Troy

[defrag010]

I haven't found many answers on my questions on here either, but I will take a stab at yours.

 

The resolutions you're describing (less cam lift, thicker hg, etc) are remedies for piston to valve clearance. If you are having valve to cylinder wall clearance issues, the only way you can realistically cure it is to run a bigger bore. It's not causing a physical problem, though.. rather it just causes bad valve shrouding. If your valve guides are bloose enough, you can have a valve contact the wall.. but as long as your valves are good, you shouldn't worry about any physical contact since the valves in a L series aren't canted.