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09-05-2021, 12:25 PM | #1 |
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21 study cylinder head differences
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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1936 pickup, stock, black 1965 Mustang coupe 289/4bbl, black/red 1971 Alfa Romeo GTV 1750 coupe, dark red/tan 1970 911E 2.2 litre dark blue/black 1968 BMW R50/2 US, black (m'cycle) 1967 Triumph TR6R , sea foam/cream (m'cycle) 2022 Royal Enfield Interceptor 650 red (m'cycle) 1974 Honda CB750 red (m'cycle) 2000 Kawasaki W650 blue/silver (m'cycle) |
09-05-2021, 12:45 PM | #2 |
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Re: 21 study cylinder head differences
The short explanation? The one with the larger chamber is for dome top pistons, the other is for flat top pistons.
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09-05-2021, 01:03 PM | #3 |
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Re: 21 study cylinder head differences
From my side of the monitor, I can't really see a dome shape chamber on either of them. Is it my eyes?
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09-05-2021, 01:14 PM | #4 | |
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Re: 21 study cylinder head differences
Quote:
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09-05-2021, 01:16 PM | #5 |
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Re: 21 study cylinder head differences
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
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09-05-2021, 01:34 PM | #6 |
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Re: 21 study cylinder head differences
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Mart, it is not your eyes. Rather, it is mine. Yikes.
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"I can explain it for you. However, I can't understand it for you". Last edited by Kube; 09-05-2021 at 03:17 PM. |
09-05-2021, 07:15 PM | #7 |
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Re: 21 study cylinder head differences
By any chance has anyone actually cc'd these types of chambers, or come across a value for that measurement?
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1936 pickup, stock, black 1965 Mustang coupe 289/4bbl, black/red 1971 Alfa Romeo GTV 1750 coupe, dark red/tan 1970 911E 2.2 litre dark blue/black 1968 BMW R50/2 US, black (m'cycle) 1967 Triumph TR6R , sea foam/cream (m'cycle) 2022 Royal Enfield Interceptor 650 red (m'cycle) 1974 Honda CB750 red (m'cycle) 2000 Kawasaki W650 blue/silver (m'cycle) |
09-06-2021, 01:18 AM | #8 |
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Re: 21 study cylinder head differences
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.
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09-06-2021, 01:19 AM | #9 |
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Re: 21 study cylinder head differences
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
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09-06-2021, 07:59 AM | #10 |
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Re: 21 study cylinder head differences
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.
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09-06-2021, 01:50 PM | #11 | |
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Re: 21 study cylinder head differences
Quote:
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. |
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09-06-2021, 07:02 PM | #12 |
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Re: 21 study cylinder head differences
It also looks like the spark plug is relocated between the 2 or does it just look like it?
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09-06-2021, 08:47 PM | #13 |
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Re: 21 study cylinder head differences
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
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1936 pickup, stock, black 1965 Mustang coupe 289/4bbl, black/red 1971 Alfa Romeo GTV 1750 coupe, dark red/tan 1970 911E 2.2 litre dark blue/black 1968 BMW R50/2 US, black (m'cycle) 1967 Triumph TR6R , sea foam/cream (m'cycle) 2022 Royal Enfield Interceptor 650 red (m'cycle) 1974 Honda CB750 red (m'cycle) 2000 Kawasaki W650 blue/silver (m'cycle) |
09-06-2021, 10:01 PM | #14 |
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Re: 21 study cylinder head differences
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.
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09-06-2021, 11:45 PM | #15 |
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Re: 21 study cylinder head differences
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
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09-07-2021, 01:15 AM | #16 |
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Re: 21 study cylinder head differences
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.
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09-07-2021, 02:57 AM | #17 | |
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Re: 21 study cylinder head differences
Quote:
Then just cut the reverse domes for the domed engines. Same casting. |
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09-07-2021, 06:28 AM | #18 |
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Re: 21 study cylinder head differences
The coolant holes in the center of the head are very different between the two heads
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09-07-2021, 07:23 AM | #19 |
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Re: 21 study cylinder head differences
Brian, have you done a compression test to see if there's much difference in the average from one side to the other?
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1936 pickup, stock, black 1965 Mustang coupe 289/4bbl, black/red 1971 Alfa Romeo GTV 1750 coupe, dark red/tan 1970 911E 2.2 litre dark blue/black 1968 BMW R50/2 US, black (m'cycle) 1967 Triumph TR6R , sea foam/cream (m'cycle) 2022 Royal Enfield Interceptor 650 red (m'cycle) 1974 Honda CB750 red (m'cycle) 2000 Kawasaki W650 blue/silver (m'cycle) |
09-07-2021, 07:32 AM | #20 |
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Re: 21 study cylinder head differences
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