21 study cylinder head differences

Adam/Mill Valley CA

Adam/Mill Valley CA

Senior Member
Can anyone tell me the functional difference between these 2 types of 21 stud cylinder heads for flat top pistons? Both are cast iron, the one with the more tapered combustion chamber (Ford script) looks like the "A1" cylinder head in the '35-36 EFV8 book, the other with the more truncated chamber looks like the later "A2" type. Both are replacement heads date-coded from the 1940's. Is one higher compression than the other, or better in some other way?
Thanks,
Adam
 

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The short explanation? The one with the larger chamber is for dome top pistons, the other is for flat top pistons.
 
From my side of the monitor, I can't really see a dome shape chamber on either of them. Is it my eyes?
 
Both are for flat top pistons. From just looking at the pictures, I'd say the on the right (or on the top in the above post) has the smallest chamber, so highest comp. ratio
 
Thanks you guys. I stand corrected.

Mart, it is not your eyes. Rather, it is mine. Yikes.
 
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For me, the one at the top of Pete picture, looks like the smaller chamber, because the valve pocket, looks shallower. The extension to the cylinder, is longer than the bottom one but, it is also narrower.

You need to find someone that can CC them, to know for sure but, a quick test, would be to level them and use a measuring cup and see if there is any difference when eye balled, to the top of the chamber.
 
The A2 head configuration follows the shape of the head gasket, and actually resembles, apart from the transition area, the profile of the 37 and later combustion chamber design.
The earlier design is narrower but deeper and longer in the transition area. I 'think' compression ratios between the two is much the same...I actually currently have an engine installed in my 35 with one of each style of head fitted. I know this, going by the exterior shape of the heads around the spark plug recesses... [check the above photos] I've not done a compression test, but, having driven this thing for several hundred miles, it appears to be a very smooth running engine, so I don't think there is a lot of difference. Flow rates between the two could possibly be different, but, as I say, I cannot detect any irregularities
 
Is there a part number on the unpainted head? I can see there may be additional numbers cast into the head near the very top by the water pump surface. What is the number? Is there a date on the unpainted head? The -B is the typical indication the head is cast iron.
 
frnkeore said:
For me, the one at the top of Pete picture, looks like the smaller chamber, because the valve pocket, looks shallower. The extension to the cylinder, is longer than the bottom one but, it is also narrower.

You need to find someone that can CC them, to know for sure but, a quick test, would be to level them and use a measuring cup and see if there is any difference when eye balled, to the top of the chamber.
Click to expand...

I would encourage anyone interested in 'cc's', to get the very inexpensive stuff to do it yourself. I would suggest that a measuring cup, using water, will not give sufficient accuracy, and will only confuse the issue. Water 'grows', that is, it does not 'lay flat' with the surface, and undetermined amounts of water above the surface will only confuse, and drive you nuts trying to understand. That's why a flat piece of plexiglass is used to hold the water flat to the head surface. Another way is to use something besides water, like solvent. It does not 'grow' above the surface. And, if no plexiglass, then you will learn quickly that the head is not level, and generally no amount of fiddling will get it level enough. Another vote for plexiglass.
In my case, I have a piece of plexiglass with a hole in it (to insert water or solvent), 'sealed' to the head using a thin layer of grease (or oil), and a medical item to administer water or solvent. (burette, or big syringe)
Works for me, but still some fiddling to get repeatable results. I will suggest that anyone quoting 75.4 cc's, is pulling your chain. I'm lucky if I can get 75 to repeat.
 
Brian, that's interesting that you would mix and match the two types of cylinder heads (if I'm reading your post correctly), was that by design?

Terry, there is an additional number there which appears to be a date-11/13/41, which I take to mean this was perhaps a service part of later manufacture?
bobH I know I should get off my duff and check the cc's as you suggest, I just have to overcome some inertia
5851 it does look like the spark plugs are in a different position, I have no idea if that is significant or not


Adam
 
Adam, The engine I referred to is a complete, running engine I bought out of a restored 35 coupe. The owner is going to fit a 284 etc....anyway, I took the intake manifold off and lo and behold; a set of adjustable lifters and nice clean valley....hmmm someone's done some work on this engine...so I fitted it into my 35 just to see how it runs. And very nicely it runs too. But I noticed it's got those two different heads on it. And yes, they are replacement heads cast in the 40's by the dates cast on them.
 
If you are making a cheap replacement head in the later 30s use the combustion chamber for the dome pistons and just make it flat for the early flat top pistons
 
Bob, I only suggested the measuring cup as a way to see if there was a obvious difference. Not as a exact measurement.

If you have a good level and plane the head surface at 3 points, with jack screws, you can get at least one chamber VERY level.
 
Kurt in NJ said:
If you are making a cheap replacement head in the later 30s use the combustion chamber for the dome pistons and just make it flat for the early flat top pistons
Click to expand...

That makes a lot of sense, Kurt.
Then just cut the reverse domes for the domed engines. Same casting.
 
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