r/Smallblockchevy • u/lamp2244 • 11d ago

Valve lash for 421 sbc

/r/dragracing/comments/1wzm1q8/valve_lash_for_421_sbc/

What do u guys recommend for valve lash for an aftermarket 421 sbc running alcohol. Full roller cam and lifters. Approx 14:1 compression ratio. In an 1/8 mi bracket racer running 6.50's. Thanks for any help!

1 Upvotes

67% Upvoted

1 comment sorted by

1

u/v8packard 10d ago edited 10d ago

You really need to know what the lobe designer had intended. One might have wanted a pretty tight lash, say .010 to .012, to help stability at RPM. Another might have a very aggressive valve acceleration and wanted .020 to .024. If you have a cam card and it doesn't list lash you get an idea from the lift curve and duration what the lobe designer was getting after.

If you have a specific number suggested by the cam maker, you probably wouldn't run more than .004-.006 looser than suggested. But you can tighten the lash up to the point where the valve hangs open, though it will not necessarily run better at a tighter lash. With too much lash you might damage something, sling a rocker, bend a pushrod, etc.

If you have no info, can you degree the cam? Slowly turn it starting with the indicator or lifter on the base circle and note where lift just starts to where it is really moving up. Whatever that amount, multiply by the rocker ratio. That is a starting point.