Factory head casting thickness
 

Does anyone have experience on how thick factory head castings are above the piston?

Background:
Terry (caddilac512) recently made a tool to re-cut the dome in factory heads and showed his work in this thread. That got me to thinking, which can be dangerous. I have a Mercury crankshaft that has been offset ground to 4 1/8 stroke. The only pistons commonly available for that stroke are forged construction and for a large overbore. Cast pistons in a moderate overbore would require ordering custom pistons. I’m not excited about the idea of custom ordering pistons (I’m cheap). I don’t have a car that needs an engine at the moment, so I have time to plan. I also have a set of NOS +.030 Mercury pistons, a good set of 21A rods, and a worn standard bore 3 3/16" block that with luck will have no fatal cracks.

Pondering:
The re-doming thread got me pondering on the possibility of running 4” stroke pistons in a 4 1/8 stroke motor for 1/16 pop up (.0625”), and cutting the dome in a factory set of heads for .040 piston to head clearance. Does anyone know if there is enough material in a factory set of heads to pull this off?

Re: Factory head casting thickness
 

Take a junk [scrap] similar head and cut it open. I have some on the scrap pile- what head? The pros will know also. Newc

Re: Factory head casting thickness
 

Heads would be 81A castings, the 1938 specific versions with the part number in the face of the head. I doubt the casting thickness would be much different than the 1939 through 1942 version 81A heads, but can't verify. I don't have a pile of junk heads to destructively test otherwise I would follow that great suggestion.

Re: Factory head casting thickness
 

I don't know the thickness of the cast iron heads. This might give you some direction a friend decided to use stock iron cast heads on his engine when he was running a small blower. He discovered the heads would not handle the added cylinder pressure from the blower he found cracks developed in the dome area crating a water leak. He was hoping he could get away with the iron heads trying to save some money not wanting to buy aluminum heads.
He ended up with aluminum heads thankfully he didn't damage the engine.
Ronnieroadster

Re: Factory head casting thickness
 

Hey Ronnie: Do you think it possible he suffered from detonation? It is hard to imagine the iron heads cracking due to compression pressure . . . but one never knows?

Re: Factory head casting thickness
 

I don't have any early heads to sacrifice, but I do have a set of VERY rusty (unusable) EAB heads I can drill and check if you'd like. You'd be cutting only .020" more than I did, and maybe less than that depending on your pistons' compression height. Could work!


Terry

Re: Factory head casting thickness
 

I just drilled a scrap 59AB in the dimple that is in the center of the combustion dome- looks like 1/8th" + more like 3/16" thick. I did find a pair of 81 heads in the 'good' pile tho. Interesting project for the Foundation, a set of cutaway engine parts. Newc

Re: Factory head casting thickness
 

I drilled the EAB. It's .260-.275 thick in the domes. Let's see if I can get the pic up...

Re: Factory head casting thickness
 

Re: Factory head casting thickness
 

Thank you, this is great information.


I'm guessing that the dome should have as much or more thickness than the cylinder walls, which typically I read should be .120+, preferably .150+ thick. Sounds like I could machine out heads and be ok if I'm careful.

Thanks again!

Re: Factory head casting thickness
 

You're welcome, and it sure looks to me like it's plenty safe to do what you propose.


Terry

Re: Factory head casting thickness
 

those 81 heads in my 'good' pile are cheap if you need them. Newc

Re: Factory head casting thickness
 

38 coupe,
good idea to machine heads. I have used this technic many times when building Harley flathead strokers. easier with flat top pistons ofcource.

Sonic test is the way to go. and do a test for cracks also.


I do not believe comp pressure will destroy iron heads, that's likely because overheat some point past their life.

Or thin casting, core shift, rust damage. sonic test before to be sure. I have seen a few iron heads with dome cracks caused by overheat.




I dont have test data about those stock heads but here is my 81AS head. dimensons are mm.

these are prewar heads but please measure yours.

Re: Factory head casting thickness
 

Excellent! Thank you for posting that-very good information. It appears the early heads are the same thickness as the later.


Terry

Re: Factory head casting thickness
 

https://www.fordbarn.com/forum/attac...1&d=1681105175

Re: Factory head casting thickness
 

21stud, that is great information. Thank you for the effort and picture.

Re: Factory head casting thickness
 

Now you have to measure the exact amount you'll have to remove. Mt thiughts are just about .100". I say this because I didn something like this for a Bivlle engine to get mor compression and still keep the transfer area, by using stock stroke pistons on a 4" crank.
Yes the Cr will increase quite abit
Gramps

Re: Factory head casting thickness
 

.100 seems about right for Ford pistons on a Merc crank, but the set up the O/P is referring to is only a 1/8" increase in stroke, not 1/4". This means that the relief would be half of that, or .050". I would be a bit more comfortable with that.

Re: Factory head casting thickness
 

Thanks, guess my eyesight iz getting worse. Glad to see the help here.

Re: Factory head casting thickness
 

This is interesting. If I recall correctly, the 81AS part number denotes them as being high compression factory heads. I wondering if these are thinner than a "normal" compression head?