mixing heads - any major problems?

[Diplomat360]

Hi Everyone!

Since I broke into a water port on my last 915J small block head while attempting to port it I decided to give the other set a try. That other set is not a 915J casting but a later '76 casting (sorry, don't have the number in front of me). As it turned out one of the heads is cracked. This leaves me with nothing else to go on.

I have heard and read however that there WERE no port configuration changes in small block heads until the swirl port stuff came out. Would mixing the 915J head with the other one cause any problems? Assuming of course that the heads are matched in terms of combustion chamber volue and the intake/exhaust runner volumes, and the valve size of course!

Thanks for your opinions...

[Christopher]

As long as EVERYTHING is the same,it will work ok.

[Diplomat360]

Hi Everyone!

I checked the heads and they are the 596 casting...so the question is: will mixing the 915J with 596 be a problem?

[Belvedere]

I would have both heads cc'd to check for a difference in compresion ratios. The valves should be the same size and you might do the old play dough on the piston trick to make sure you have valve clearance close to the same. I'm sure different port configurations would have some effect in horse power, but its hard to say how much. I have a cousin with a Shoverolet powered pulling truck and it is really scary to see some of the stuff he puts together. I've seen one high compression head and one low. Header on one side, stock exhaust on the other. He even bored one cylinder .060 over and left the rest at .030. He didn't want to buy more pistons and had a partial set of .060's. Just about anything will run. How well it runs and how much power is up to you.

[SUBLIME 73 CHARGER]

I WOULD HAVE A REPUTABLE MACHINE SHOP CHECK THEM TO SEE HOW MUCH DIFFERENCE THERE IS. YOU MAY BE ABLE TO HAVE THEM BOTH MACHINED TO MATCHING SPECS.