Valve to Piston Clearance

[Jon Miller]

I'm getting ready to bolt the heads on my motor and I've run into a situation that I thought I would avoid. I've got a cam with .536 valve lift, and with the piston at TDC, and no head gasket, I've got about .300 before the valve hits the piston. The pistons have valve reliefs cut into them, so I had hoped that my motor would be "non-interfearance". I've lost a couple of timing chains over the years on different cars and luckily the motors were not zero clearance, so no damage was done.

My next step is to check clearance with rotation (Valve Lift vs Piston position) and see what I've got. What I don't know is how much to allow for the head gasket, and how much clearance is enough.

Here are the vitals:
428 SCJ
TRW 2303 pistons
Edelbrock Heads
Hydralic Cam
INT. Lift .510
EXT. Lift .536
FEL-PRO gaskets

Thanks in advance!
-Jon

[SFfiredog]

Hi Jon,
The greatest chance of valve to piston interference happens with the intake valve on overlap at about 20 to 50 degrees before top dead center. You can look down the spark plug hole and see it all happen. You should install some very light checking springs on #1 intake and exhaust valves, install the valvetrain, and rotate the motor to the point of minimum clearance. With a dial indicator on the spring retainer, push down by hand on the valve until it contacts the piston. Read the dial indicator and that will be your v to p clearance. Another way to check v to p is to put a piece of clay on top of the piston and rotate the motor thru one complete combustion cycle. Since you are using Edelbrock heads this might be the prefered method because E-brock moved the valves slightly when they designed their FE head. I ran across this concern on a 428 that I'm finishing up right now. The piston valve relief was deep enough, it just wasn't in the correct location for the valve. I ended up flycutting the intake valve relief and moving it over and up by .040". THe minimum v to p clearance you want is .100". If you check without the head gasket you can add .039 (if you are useing a Felpro #1020) to your total figure. (Felpro #1020 is, IIRC, .039") I really don't think you will have a clearance problem because your cam is smaller than the one I used. (.589 intake and exhaust lift w/248 deg. duration @.050)\
I think you will be fine but it alwayss pays to double-check.
See the enclosed photo for an idea of what the valve reliefs I cut look like.
Good luck,
--Mike

[speed220mph]

Jon: There have been a lot of publications that show in detail how to check piston-to-valve clearance. I do it in my "Engine Builder's Handbook", but there are many other publications that do the same thing.

The most common way of checking valve clearance is to put a thin slab of clay--1/8 in thick will do--over the valve relief areas. Smear a little oil on the valve heads, then install a head with an old gasket on that bank and the valve train for that cylinder. Rotate the crankshaft carefully, but stop if there's ANY resistance and investigate.

After you've turned the crank at least on revolution, remove the head and check clay thickness where the valves contacted it with the depth-gauge end of your vernier calipers. This will be valve-to-piston clearance.

There are other methods of doing this check, but the clay method is the simplest and easiest to understand.

Hope that helps.

Good luck, Tom