Offenhauser Heads 375/400/425

[Matt6494]

Originally Posted by tubman

I seem to be a broken record about this, but if you're going go add a set of aftermarket heads, do it right and make sure you get the "quench" right. In short, shoot for .045-.050" clearance over the piston crown. This will optimize the combustion in the cylinder and I believe it provides the biggest "bang for the buck" you can do on a flathead, increasing both power and economy.

When I built my first "real" flathead almost 30 years ago, I was unaware of this step and just bolted a set of brand new Offenhauser heads on a fresh 276" Merc with an Isky MAX-1 cam and an MSD "Ready to Run" distributor; all the best, most expensive stuff. I was never happy with that engine and fooled with it for several years before I sold the car it was in. In the intervening years, I met and talked to a lot of people and gained a bunch of knowledge along the way. (I now believe the problems with that engine were due to inadequate "quench" in the heads and an incorrect advance curve in the distributor.)

I did a 255" Merc for my '51 Ford coupe, retaining the Merc 1CM cam and adding a Mallory dual point, a Rochester 2G, and a set of massaged Edmunds finned aluminum heads. I carefully custom fitted the heads to get to a nominal .050" over the pistons. Surprisingly, this took milling one head .020" and the other .025" to get them close and then touching them up with a die grinder (these were used heads and still needed milling).

It takes a little time and effort to get them just right, but it is very satisfying and produces positive results. At least check the piston/head clearance with foil balls or clay so you have some idea what your "quench" is; you might be surprised.

Thanks Tubman! You always provide great advice.