Piston to valve Clearance?

[gvr4]

Putting together my 6-bolt with evo-9 pistons and its looking like piston to valve clearance is pretty tight, I'm talking something like .03 intake and .05" exhaust. (I have to bring my indicator home from work and get some real numbers).

Its almost common knowledge that a pushrod engine needs .08" intake and .100" exhaust, but I haven't even seen any recommendations for an OHC engine...

Any engine builders out there have a rule of thumb for piston to valve clearance? My build is nothing fancy and won't be spun higher than 6500 on purpose.

[sleepydsm]

Wait.... 0.030" and 0.050" sounds like a lot to me. I as long as it's not zero does it really matter? lol.

I'm a bit biased because I work with things in 0.0001"-0.00001" range :)

[gvr4]

Do you deal with metals at those tolerances? If so, I'm sure you are aware of the changes that will occur over the temperature range seen by a component like a valve. 0 clearance at 70 degrees means -.025" or so at 600-700 degrees

There's also a phenomenon called valve float that really tends to make a mess out of things if there's too little clearance. It's easy to see how a pushrod engine would end up with quite a bit of valve float because of all the extra mass in the valve train. But valve springs in an overhead cam engine have a lot less inertia to contend with, and I'm wondering if this translates to tighter acceptable clearances.

[gvr4]

Called DSM Graveyard and they said they've run them with as little as .025" without issue. Just figured I'd bump this in case someone comes across this thread in a search. Actual numbers on this topic are pretty scarce.

One other thing to consider; if head and block have been milled enough to cause concern with piston to valve clearance, the cams should degreed in with adjustable cam gears since at that point the timing belt geometry has changed significantly.

[Halon]

I've never actually measured any of that. These are interference engines so if you have valve float then no matter the tolerance, you will run the risk of valves touching pistons. You shouldn't be seeing valve float at 6500rpms though, unless you have some insanely old wore out springs, and/or some very aggressive cams.

Unless both the block and head have been milled several times, or you're running some sort of unique or crazy setup, I guess I wouldn't be overly concerned because it is what it is.

[gvr4]

I guess I'm just a little anal when I build my engines. I'd rather check it now than find out the hard way O.o

[turbotalon1g]

I'd talk to guys that build motors, MAP, DB, Gosh, a few users on here could chime in too.
This place is bolt-on central these days though.

[gvr4]

Well... The executive decision has been made! f**k it, run it.

.027 measured P2V.
.052" uncompressed HG will compress to .051"
Leaving .026" PTV clearance.

Its about time I finally get to finish putting this thing together.

[Halon]

Being that you're anal, what's your squish pad measure in at out of curiosity?

[gvr4]

Haha don't worry bout it.....

















.055" in the hole + hg thickness :(