Valve to piston clearance question

[ksquared]

How much clearance? Running a 428 CJ with solid roller & FElony heads. Won't be going over 6500 and only on occasion. I've read somewhere 0.080 for intakes and 0.125 for exhausts. Good numbers?

[blykins]

You can pretty much get away with anything you want on the intake side.  The intake valve chases the piston down the bore.

The exhaust side is the one you need to watch, as the piston chases the valve back down to the seat.  If there's any valve bounce or float at all, it could tag the piston. 

With that being said, I have ran .070" on both before.  But that was with a *really* rigid valvetrain, with Jesel rockers, 7/16" pushrods, etc., and I didn't just check #1 piston...

I really don't see issues with most combinations and modern pistons.  The only time I see issues is if the piston manufacturer messed up, or the heads have been cut a mile. 

Also, remember that lift isn't the only thing that plays into the clearance.  Camshaft overlap is a very big component.  On some combinations, valve to cylinder wall clearance can also ruin your day. 

[Mark Bliss]

If .125 was the exh. limit, I'd be in BIG trouble!
I've run the crap out of engines tighter than that many times with no issue.

I have checked a few where the piston relief radius was worthy of closer attention, but that is actually a fairly rare combination of parts in my experience.

That valve to cylinder wall thing though...
It may be uncommon, but when your "unusual" combination gets close...
You may excuse the flow/shrouding issues, but here's my current conundrum- You build a combination of parts for an "effort" and discover later you may be under-cammed to approach target values. And the engine is built "tightly around" a specific cam.
Mistakes have been made, people will be blamed.

[Hemi Joel]

How about radial clearance to the valve relief edges?

[blykins]

How about radial clearance to the valve relief edges?

I’ve ran .025-.030” a lot, so I know you can get that close.

[Barry_R]

I've been down at those tight clearances Brent mentioned - - and I have pulled engines down later and seen some witness marks from the valves and pistons getting together just a "little bit".  Nothing catastrophic - but not very confidence inspiring.  For confidence sake I would add .010" to the low end of those numbers and run it.

[jayb]

On my big SOHC I ran .050" and still saw those witness marks.  Bigger is better IMO...

[ksquared]

Thanks all for your very appreciated input! Regarding actual measurement, I'm familiar with the clay test but I recently read of a method where an assembled engine is simply checked with a feeler gauge set to desired clearance between valve stem & rocker tip. Then rotate the crank 2 revs. If there is no bind, all is well. Good method? BTW to round out the build description, Harland Sharp rockers/Smith Bros 3/8" push rods/Survival heads with 2.20 intakes/.030 over old school forged TRW pistons/Chet Herbert CG6PJ solid roller cam.

[Barry_R]

Cheap and quick? 
Assemble one side without a head gasket. 
Rotate it around. 
If it does not touch or bind you know you have at least a "gasket's worth" of clearance.
If it hits somewhere you can paint the piston with a black Sharpie & re-test - it will leave a mark showing where you need to add some more room.

[Falcon67]

I typically use the Barry method.  It's quick and dirty - I use Felpro gaskets that spec at .041 compressed.  I'm in the .05x intake range on the 351C I'm building.  And what Brent said - intake chases the piston to no worries there.