Big block exhaust valve/cylinder wall clearance

Happy New Year to all! I have a 4.350" bore early iron mega block. I've been triple checking all clearances and fit of all parts before final assembly.I have measured (actual) clearance at gross valve lift, between the cylinder wall and the exhaust valve and come up with .030" at the closest point.I am fully aware of the shrouding effect it presents.My concern is this an acceptable operational clearance for a 7-7,500 rpm race only moto??. Relieving the top of the cylinder bore is out of the question as it is too close to the top ring territory.It was exactly why I didn't go with a 2.250 intake valve upgrade with Victor heads on this relatively small bore motor. Figured that the shrouding efffect of the bigger valve in that bore size would be an expensive cost with very minimal or no gain.All feedback welcome here. Seems acceptable to me but Imay be missing something here . Thanks in advance. Bill

.030” radial clearance from the exhaust valve to the bore wouldn’t bother me.

Since the valve and bore are not on parallel lines...... and the direction that the valve is moving vs where it is in the bore...... as the lift increases, the clearance decreases.

it is too close to the top ring territory

How close?
Even a 1/16" radius will help some.

I've ran less with no bent valves or witness marks on the valves or block, you should check all 8 exhaust valves in the head your going to use on the side of the block your going to run them on to make sure what you have on all of them

.030" was the absolute minimum radial clearance measured at full gross valve lift.I left the .023" valve lash as a cushion. With this I would have a few thousands more radial clearance. Top ring land is .250" down on the piston,and the piston is .020"in the hole.Not a lot of options here as the closest point of contact is not at the very top of the bore but near top ring territory making a die grinder taper not very effective at the point needed the most.

Is that with a 2.19 intake valve ? Ron

Ron, I am using the as supplied 2.20" int. & 1.81"exh. valves that came with the Edelbrock Victor heads w/ an extra half thou. guide clearance added per recommendation by Kris at Nelson Competition.Most race time will be during the the hot climate in Florida.My thoughts are that radial cyl. wall/exhaust valve clearance can be tighter than piston to valve radial clearance due to the fact that piston and aluminum rod growth don't enter the equation. I am "all good" with piston to valve clearance.Has anyone else used Edelbrock Victor heads on a big block with a 4.350 bore and .690" lift cam? Thanks, Bill

I've often wondered about the notching of the top of the cylinder to match the contours of the cylinder head and the possible effects on flow related to this unshrouding. I would think there should be some video or technical papers of the flow changes by researchers somewhere.There were masses of research as far back as WW1 in the aircraft engine research files.

Yes I'm running 4.350 bore with victor mw port head .720 lift with no problems don't know valve to cylinder wall clearance 3400 with driver 6.00 in the 1/8

Thanks all for the input.I know it is close but with the info at hand ,I believe I wil be good. Gonna roll with it. Bill

How much margin do you have to play with. You may be able to reduce the valve OD a smidge if you can’t sleep over the current clearance.

Re-installed heads w/only all the exhaust valves w/checking springs installed. No pushrods, and mechanically opened each valve to the maximimum valve lift that this camshaft has. This allowed me to turn the crankshaft get the throws out of the way for realworld accurate measurements between the cylinder wall and the exhaust valve. Sounds corny but used a measured paper clip as a wire gauge (.029") and each valve had very close to the same clearance allowing the paper clip to slide easily through between the wall and the valve. Increase the valve lift and the clearance goes away quickly.Not high tech but pretty darn accurate.I used .690" as my lift number. Ithink a.750- .800 lift cam (actual) would have trouble here in this 4.350" bore. I talked with the tech dept at Hughes engines,(where the heads were purchased and ported) and they said the've had no complaints at that bore.
Just doesn't look like any camshaft upgrades at this bore. Thanks for all the input here. Bill

any chance you can move the heads up the block ? offset dowel pins .015-.030 " reduce the intake manifold with a side cut.... ? maybe move the valves // angles..

I believe that can be an option, but would also need to recheck the radial clearance in the piston valve reliefs as this would be reduced by moving the heads upward. I am comfortable with the .030" clearance after a lot of research. Not a lot offered on this subject. I spent a lot of time on it.Glad, now that I didn't go to a much higher lift cam in that small bore Thanks all. Bill