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Old 06-21-2021, 11:43 AM   #1
Cydways
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Default 21 stud domed vs flat top pistons.

I have recently acquired a '37 short block with flat top pistons and have been looking for a set of heads. That search has brought me to flat top or domed heads but I can not seem to figure out if that was based on year or did they have both as a option on all the years? Did the compression stay the same with both setups? Any info to clear this up for me would be greatly appreciated

-Douglas
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Old 06-21-2021, 12:57 PM   #2
flatjack9
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Default Re: 21 stud domed vs flat top pistons.

I am not aware that 37's ever had flat top pistons. Someone may have used earlier heads on that engine.
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Old 06-21-2021, 02:33 PM   #3
Terry,OH
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Default Re: 21 stud domed vs flat top pistons.

The 37 should have domed top pistons. The last flat top piston head for production was 40-6049-A2 and 40-6050-A2 these or slightly earlier style 40-6049-A1 and 40-6050-A1 should work for your 21 stud engine. You will also have to figure out what you want to do with the cooling system since the above heads have provision for water pumps bolted to the heads.

Last edited by Terry,OH; 06-21-2021 at 03:27 PM.
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Old 06-21-2021, 04:36 PM   #4
Cydways
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Default Re: 21 stud domed vs flat top pistons.

thanks for that info Terry & jack

looking on Vanpelts site they give two different compression ratios for '37 aluminum vs CI Is this just a CC Head difference or are different pistons used? Would a Domed '37 piston work with a '36 head with clearancing?


1935 221 3.0625 x 3.750 85 6.30:1 (alum heads) 21
1936 221 3.0625 x 3.750 85 6.30:1 (alum heads) 21
1937 221 3.0625 x 3.750 85 6.12:1 (alum.heads)
7.50:1 (cast iron)
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Old 06-21-2021, 04:37 PM   #5
bobH
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Default Re: 21 stud domed vs flat top pistons.

Quote:
Originally Posted by Cydways View Post
I have recently acquired a '37 short block with flat top pistons and have been looking for a set of heads. That search has brought me to flat top or domed heads but I can not seem to figure out if that was based on year or did they have both as a option on all the years? Did the compression stay the same with both setups? Any info to clear this up for me would be greatly appreciated

-Douglas
Here's some info out of a 35 - 50 Motor's Manual... I suspect this will only muddy-up an answer to your compression question.
35 (generally believed to be flat pistons), CR 6.30
36 (generally believed to be mostly domed), CR 6.30
37 (generally believed to be domed), CR 6.20
38 (could be either 21 or 24 stud, domed), CR 6.12
39, CR 6.15
40, CR 6.15

So, leaves me wondering. I don't see a definite answer to your question.

If I was looking for heads for flat pistons, I'd expect they would be 35-type heads, with water pump provision. If I found 37-type heads (no water pump), I would expect they would be for domed pistons. If I wanted 37-type heads (no water pump) for flat pistons, I expect I would not find any, at least not original Ford heads. Maybe someone can correct me here?

But, how about all the 37 engines that Ford produced, that were replacement engines for earlier cars (32-35)? They would have come with flat-top pistons.

Last edited by bobH; 06-21-2021 at 04:44 PM.
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Old 06-21-2021, 06:31 PM   #6
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Default Re: 21 stud domed vs flat top pistons.

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Quote:
Originally Posted by Cydways View Post
thanks for that info Terry & jack

looking on Vanpelts site they give two different compression ratios for '37 aluminum vs CI Is this just a CC Head difference or are different pistons used? Would a Domed '37 piston work with a '36 head with clearancing?


1935 221 3.0625 x 3.750 85 6.30:1 (alum heads) 21
1936 221 3.0625 x 3.750 85 6.30:1 (alum heads) 21
1937 221 3.0625 x 3.750 85 6.12:1 (alum.heads)
7.50:1 (cast iron)
Same pistons, different head volume.
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Old 06-21-2021, 07:14 PM   #7
Terry,OH
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Default Re: 21 stud domed vs flat top pistons.

You may be ahead to change the pistons to domed and run the original style 37 heads. I believe your going to save $. What vehicle are you going to install this engine into?
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Old 06-22-2021, 09:30 AM   #8
Cydways
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Default Re: 21 stud domed vs flat top pistons.

Thanks Terry. Jack, it will be going into a fenderless model A coupe. It appears to be a standard bore with some tin sleeves currently. Maybe i can push it out to an 8BA standard piston size and shave a Stock '37 head down. I will have to see what pistons are available.
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Old 06-22-2021, 02:24 PM   #9
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Default Re: 21 stud domed vs flat top pistons.

If you peel the sleeves out carefully, you might find some .083 over pistons which would just require a hone job. This was standard practice in the day.
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Old 06-22-2021, 02:56 PM   #10
Ggmac
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Default Re: 21 stud domed vs flat top pistons.

If you find .083 over pistons , even nos style let me know if you DO NOT want them .
Thanks , Gary MacDonald
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Old 06-22-2021, 04:47 PM   #11
Cydways
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Default Re: 21 stud domed vs flat top pistons.

Thanks for that info, Jack. It does appear to be a difficult size to find. Not boring this out to its limit also seems like a good idea even with the advance of the 4th ring with the 8ba. I will call some piston manufacturers and see what a custom set runs these days.
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Old 06-22-2021, 05:31 PM   #12
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Default Re: 21 stud domed vs flat top pistons.

Were all the '37 85 hp engine blocks sleeved? The one I have is at 3.125 so it must have been bored from 3.062 out to the 3 1/8" or else the sleeves would be paper thin. Not much wear or ridge so I might be able to hone and reuse pistons.
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