Offenhauser Heads 375/400/425
 

Hi All,

Looking for some advice on upgrading my currently standard 8CM Flathead.

I'd like to get some Offenhauser cylinder heads, and have seen they come in 400/425 new, or sometimes some older 375s come up.

What would be best for a stock 8CM engine?

I've got hold of the Flathead compression chart by James Leis that does the rounds on various forums.

It looks like on my 255ci engine the compression ratios would be 8.15 / 9.28 / 8.9 respectively for 425 / 400 / 375.

Interestingly these are different to those listed in the Offenhauser catalogue, these being 8 / 8.5 / 9.

I'm also planning on fitting a matching Offenhauser dual intake and 2 97s of this makes a difference to selection?

Re: Offenhauser Heads 375/400/425
 

Either the 400 or 425 would be fine, but with a stock 8CM engine you do not need the additional valve lift clearance provided by the 425. So prefer the 400 for a little higher compression ratio. With 255 cubic inches, two 97's would be plenty and look great. The heads are irrelevant to that choice. If it were up to me, I would adapt a Holly 360 four barrel for the automatic choke and reliability.

Re: Offenhauser Heads 375/400/425
 

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.

Re: Offenhauser Heads 375/400/425
 

Thanks! If I wanted to go to 275ci or 284ci in the future would the 400's still be ok, or would compression be too high?

You didn't mention the 376's, is that because compression is too high on aerc flathead?

Re: Offenhauser Heads 375/400/425
 

Thanks Tubman! You always provide great advice.

Re: Offenhauser Heads 375/400/425
 

As Drolston mentioned, those numbers are referring to the amount of can lift the heads will take. Meaning, 400 will accept a .400" lift cam, 425 will accept a .425" lift cam.

So if you have the stock cam in there, 375's would also work. If you plan to change to the cam to a different grind, you'll just need to be mindful of that.

Re: Offenhauser Heads 375/400/425
 

It's hard to get a very high compression ratio in a stock engine. Having the volume of the cylinder and combustin chamber, doesn't do it. the only way is to bring the piston up th 2/4" from the top. add fluid ito the chamber and keep track of it. Keep moving the piston up untill it just comes out the spark lug hole. Now you know the chancer volumn and actual CT. at 8.5/9.0 you need good controle of the timing advance. one of Charley's distributor and a 2Gc carg work well/ When I do this I am looking for the best fuel economy
Gramps

Re: Offenhauser Heads 375/400/425
 

I am running the 425 heads on a 284 inch engine and can still advance the spark by three degrees (one and a half marks) on the crab distributor without getting pinging (spark knock, pre-ignition) or regular 87 octane with 10% ethanol. With my Schneider 348F cam valve lift of 0.350" I could have run any Offy head, but chose the lower compression 425 because it is strictly a street engine for which I did not want to get into advance curves and the like to get the most out of the higher compression

I did not mention the 375's because they will be old and used and hard to find. On a big inch engine you might have to go to premium fuel with the 375's.
I am suspicious of old used aluminum heads based on one bad experience many years ago. But I am sure there are good ones out there.