What head is this?

1

1stWoody

Member
Can anyone tell me about this?
It appears to be a 5.2:1 head but it does not have a "B" cast into it.
 

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Is a high comp head! Made in the good ole USA. Clean it up good inside and out and use it!
 
As Vince Falter's most excellent web page notes (see Stickshift's post #3) there are a lot of aftermarket heads. I have a "Model A" aftermarket head on my '31, which apparently came in both 4.2 and 5.2 configuration. I pulled the head off last week to change the head gasket and sure enough, I have the "high compression" version. I put that in quotes because a 5.2 compression ratio ought to put cylinder pressure somewhere in the 70's psi. Well, I have 80s and 92 in one cylinder! I suspect that the block and head have been decked sometime in the past, the engine is 0.080" overbore so it likely has been rebuilt at least twice and the block possibly decked each time. The head appears flat, I used a 0.003" feeler gauge and a straightedge and could not get it in anywhere, either lengthwise or crosswise. So I'm going to put it back together (after I get the broken stud out, of course...) and run with it.

JayJay
 

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JayJay said:
As Vince Falter's most excellent web page notes (see Stickshift's post #3) there are a lot of aftermarket heads. I have a "Model A" aftermarket head on my '31, which apparently came in both 4.2 and 5.2 configuration. I pulled the head off last week to change the head gasket and sure enough, I have the "high compression" version. I put that in quotes because a 5.2 compression ratio ought to put cylinder pressure somewhere in the 70's psi. Well, I have 80s and 92 in one cylinder! I suspect that the block and head have been decked sometime in the past, the engine is 0.080" overbore so it likely has been rebuilt at least twice and the block possibly decked each time. The head appears flat, I used a 0.003" feeler gauge and a straightedge and could not get it in anywhere, either lengthwise or crosswise. So I'm going to put it back together (after I get the broken stud out, of course...) and run with it.

JayJay
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I have a Brumfield. So how do you measure these things for compression ratio?
 
Y-Blockhead said:
Cylinder Displacement + Combustion Chamber Volume / Combustion Chamber Volume = Theoretical Compression Ratio.
Click to expand...
Y-B has given the definition of compression ratio. Practically speaking, most folks simplify it as

*measured compression test pressure divided by 14.7 = compression ratio.

​
So the "standard" psi of say 62 psi would be 62/14.7 = 4.22 (round to 4.2). And a 5.2 head would be around 75 psi on a compression test.

JayJay
 
JayJay said:
Y-B has given the definition of compression ratio. Practically speaking, most folks simplify it as

*measured compression test pressure divided by 14.7 = compression ratio.

​
So the "standard" psi of say 62 psi would be 62/14.7 = 4.22 (round to 4.2). And a 5.2 head would be around 75 psi on a compression test.

JayJay
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No, what I gave him is the equation to find his Theoretical Compression Ratio.

Too many variables in way you suggest.
 
Y-Blockhead said:

Too many variables in way you suggest.
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I agree completely. I'm not suggesting to use it, but it's simple and what a lot of folks can relate to easily. That method doesn't account for the difference between absolute (the 14.7) and gauge (what you measure) pressures, for example.

And the 14.7 only applies at sea level.

JayJay
 
So after leaving the head in a rust911 solution for a day and cleaning off I think I know why it was sitting on the shelf. It has cracks across the top of the head and I don't trust it enough to try it out.
 

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Looks like freeze cracks to me. If there is no other cracks I would solder them up and give it a try.
 
I thought about grinding out the cracks a little and using some JB but I'm worried about the crack near the #2 spark plug especially with the higher compression.
 
5.2? How about a Winfield red head? 7.5 to 1…she’ll pull your hat clean off your head.
 

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JayJay said:
I agree completely. I'm not suggesting to use it, but it's simple and what a lot of folks can relate to easily. That method doesn't account for the difference between absolute (the 14.7) and gauge (what you measure) pressures, for example.

And the 14.7 only applies at sea level.

JayJay
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Plus, cranking speed varies, ring seal may not be very good, valve timing varies (affected by lash), ambient temperature and coolant temperature vary, etc., so the compression gauge method is a waste of time!
 
40 Deluxe said:
Plus, cranking speed varies, ring seal may not be very good, valve timing varies (affected by lash), ambient temperature and coolant temperature vary, etc., so the compression gauge method is a waste of time!
Click to expand...
And let's add - overbore increases cylinder displacement relative to combustion chamber volume.

All in all, a lot of variables. That many degrees of freedom makes the engineer in me cringe. Having said all that, it's an attractively simple formula. And it's at least qualitatively correct - compression ratio and cylinder pressure do co-vary.

I think you, Y-B and I all violently agree.

JayJay
 
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