Need more P/V clearance in a BB mopar
 

Having trouble finding enough P/V clearance. Hope someone here can give some advice. It’s a 70 dodge 383 for stock. Iron heads. Pistons are flat top without valve reliefs. Short block is finished, cam installed. I'm using a spare cylinder head to make measurements so I can give directions to the cylinder head builder so he can finish the heads.


I’m shooting for at a minimum .025/.060 when done. Right now I have .040/.057. To get that number, I’m comparing valve drop numbers from my heads to the new one at the cylinder guys shop. I'm setting valves at hot lash setting of .008” , solid lifters. The new heads haven’t been decked yet, although the valve job is done. Heads are at 88 cc now, min. spec is 79.5. It should take 0.040 to get the chambers to min. spec. I’ve moved the cam around to find the sweet spot for the intake clearance, it’s at 4 degrees retarded now. I‘m thinking of decking the heads 0.020 and sinking the valves 0.010/0.025. It seems to be a juggling act between sinking valves, how much to cut the cylinder deck, and or change gaskets. I’d like to keep the current spec gasket, which is a steel shim .021”


I hope I've explained that clearly enough, this is my first time building a stocker engine. I guess the question is what kills power more, sinking valves or losing compression ?

Re: Need more P/V clearance in a BB mopar
 

Degree the cam where the engine will like it to be. Then live with the loss in compression to get desired piston to valve cleaance. Ie don't mill heads as much.

Re: Need more P/V clearance in a BB mopar
 

Maybe sink the valves a little bit and or angle mill the heads.
FelPro 1009 Gasket will give you a extra .020 to .022.

I prefer to make heads near spec and use deck and gasket
on Big Block Mopar stuff to make them more universal for different shortblocks.

Re: Need more P/V clearance in a BB mopar
 

What method did you use to figure out your clearance ? Clay with full valve springs , or dial indicator with a light checking spring ?

Re: Need more P/V clearance in a BB mopar
 

Use more deck clearance and keep the gasket thin. You will need about .035 '' total to keep pistons from contacting head. Have P/V clearance through the range of adjustment, so you can roll the cam forward or backwards without bending stuff. Exhaust valve will like to make contact in split overlap.

Re: Need more P/V clearance in a BB mopar
 

Having the correct cam timing is more important then losing a half a point of compression.

Re: Need more P/V clearance in a BB mopar
 

Checking springs, with an indicator on the retainer

I've already moved the cam around to give me the most intake clearance, which has been the biggest problem, and still have around twice that amount of exhaust clearance.

Piston is .010 in the hole, spec is .004. I'm leaning on leaving the valves alone and using whatever gasket gives me the clearance needed. Those 2 together should give me the .035 Larry mentioned.

In hindsight, I should have gotten a smaller cam so I could keep some compression........

Re: Need more P/V clearance in a BB mopar
 

Cut the valve face a few thousands.
Also the cam could be ground to twink the duration, you won't lose compression and horsepower loss is negligible.

Re: Need more P/V clearance in a BB mopar
 

Indicator method is good , but usually gives the closest value . When the real springs are used the clearance will be greater . Would be nice to be able to run the cam advanced , but that would be based on dyno testing .

Re: Need more P/V clearance in a BB mopar
 

Done it both ways, more compression and less than ideal cam timing and vice versa. Cam timing always more important than compression.