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Transmission Casting Numbers

Post Wed Jun 05, 2013 1:48 pm

Posts: 349
Location: Champaign-Urbana, IL
Has Chris or anybody else been able to figure out transmission casting number date codes for the late 40s and early 50s big twin transmission cases? I just picked up one with the ca. 1940-55 drilled vent boss, but the casting numbers are either "0 - 8" or "8 - 0" ... No chance that either number is a "D" or a "B" ...

I guess that "0 - 8" could be "1950 - August" but I would just be guessing. Anybody really know the answer?

Thanks in advance,


Post Fri Jun 07, 2013 4:39 am

Posts: 32
Location: Western Australia
If you’re sure about the two characters then I’d say the code is 8 0 and I posted an example below for comparison:


The month is first and the year is second so the code indicates casting in August 1950.

Letter-number date codes on the 121-35 Big Twin trans case began as early as April 1938 casting and it seems they then remained in use through the end of 1949.

Numbers-only codes appeared as early as January 1950 but I wouldn’t be surprised to see a numbers-only code for December 1949 because both types of codes were used for some BT aluminium engine parts cast that month, possibly because that was a changeover period. There were at least two different numbers-only formats used in the 50s.

The list below is of examples I have so far for BT trans cases cast April 38–December 63:

D 8, F 8, G 8, H 8, I 8, J 8, L 8
A 9, B 9, C 9, D 9, E 9, F 9, H 9, I 9, J 9, K 9

C 0, E 0, F 0, G 0, I 0, K 0
C 1, D 1, G 1, J 1, L 1
D 2, E 2, F 2, L 2
H 3, J 3
C 4, D 4, E 4, H 4, L 4
F 5, H 5, I 5, J 5, K 5
E 6, G 6, H 6, I 6, J 6, J6, L 6
A 7, C 7, D 7, E 7, G 7, H 7, J 7, K 7, L 7
A 8, C 8, E 8, F 8, G 8, H8, J8, K8, L8, M8
A9, B9, C9, G 9, I 9, J 9, L 9

1 0, 3 0, 6 0, 8 0, 9 0, 100, 110
10 2
2 3, 3 3
2 58, 6 58, 9 58, 10 58, 11 58, 12 58
1 59, 3 59, 5 59, 6 59, 7 59, 8 59, 10 9, 11 9

1 60, 10 60
1-61, 6 61, 11 61
2 62, 4 62, 10 62
1 63, 2 63, 10 63, 12-63

Notice most codes have a space between month and year but some do not. (So those spaces would be more noticeable I made them wider but when I checked the Preview it had changed them to a single space.) Notice the period August 1948–March 1949 where there is no space. However, October 1946 sometimes had a space (J 6) but sometimes did not (J6) and I can’t yet explain the latter. Notice 1959 sometimes had 59 indicating the year but sometimes only 9. Notice on occasion a hyphen was used.

I can’t explain why code 3 3 was used for so long and I know it has shown up on trans cases of some 1956 model bikes that are unaltered from original. There are also reports it has shown up on some 1957 model machines.

In discussions on one forum or another a few years ago it seemed the letter I was omitted from all BT aluminium parts cast in 1948–49 due to confusion with the number 1. Some pieces showed up with codes such as M8 and M9, including some BT trans lids with M8. M apparently indicated December so that should have meant the letter I had indeed been skipped. However, later on some other BT trans lids that appeared to have been cast in 1948, not 1938, showed up with code I8. Also notice my above list of trans case codes includes an example of M8. I have no example yet though of M9 on a BT trans case.

I contacted Bruce Palmer regarding some casting info and other forms of ID and the letter I is something we’re trying to sort out. I emailed him photos of a BT trans case which appears to have been cast in 1949, not 1939, and it has code I 9. Exactly what happened with the I and M on certain BT aluminium parts cast in 48–49 is unknown to me at this stage but one possibility is that H-D may have used more than one foundry and maybe they didn’t all apply date codes in the same manner as each other. These things I continue to research.

On the trans case, casting number 121-35 appears with all the codes in my above list except 10 63 and 12-63, both of which are accompanied by casting number 34703 64. However, the 34703 64 case apparently wasn’t used for early-1964 Panheads, only the mid/later-64 models, although I don’t know exactly when that change occurred. And some 34703 64 cases do not have date codes.

There are a few things you can check your case for regarding further confirmation. For the mounting bolt underneath on the right-hand side the insert will probably be steel as my research so far indicates the insert changed to steel from bronze around late-1943 or early-44. (Bruce tells me the change started in 1942 and should have been finished by the end of 1943.)

Also notice the red arrow in the photo below:


That boss and the other left stud boss to the rear were beefed up in 1949, sometime after March but apparently by July at the latest and you’ll see the difference when they are compared to early bosses.

The two bosses indicated by black arrows were originally rounded through 1946 models. Both bosses were flattened as original, either by design or machining, for 1947-later models to accommodate the (hand) gear shifter lever which was changed from pointing up to pointing down. However, flattened bosses aren’t a definite means of ID because some people have flattened the bosses on some early units.

Post Fri Jun 07, 2013 8:16 am

Posts: 349
Location: Champaign-Urbana, IL

Thanks for sharing your research! As I was looking down the list, I was wondering how you sorted out the late 1930s from the late 1940s numbers, but information about the casting changes pretty much answered that ... I compared a D-1 (1941) tranny case that I have and the increased bolster at the red arrow is obvious; and as you noted, somebody had hand-cut the faces flat at the point indicated by your black arrows.

Great information, and greatly appreciated. Thanks again.


Post Sat Jun 08, 2013 2:25 am

Posts: 32
Location: Western Australia
You’re welcome, Mark. On the CAI forum in February 2011 Matthew Olsen mentioned the change to the two lower bosses. He said sometime in the early-1950s they started adding more meat around the studs on the left side so I began looking into it. Matthew was close with his estimation but my research indicates it happened in 1949.

Last October via email I mentioned it to Bruce but he was unaware of the change so to illustrate the difference I sent him photos of cases cast in Feb and July of 49. Since then I found a case cast in March 49 and it still has the slim bosses. Another case turned up even more recently and it may have a mid-49 code. The letter looks like F and the number may be 9 but it isn’t clear so I can’t be sure. The case has the two beefier bosses so the casting change may have occurred as early as June 1949 but I need to see better examples before I can be sure about it.

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