Factory head casting thickness

38 coupe

38 coupe

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

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Newc said:
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. Newc
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cadillac512 said:
I drilled the EAB. It's .260-.275 thick in the domes.
Click to expand...

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!
 
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.
 

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21stud said:
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.
Click to expand...


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


Terry
 
21stud said:
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.
Click to expand...


513591
 
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
 
Ol' Ron said:
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
Click to expand...

.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.
 
21stud said:
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.
Click to expand...

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?
 
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