by Speeding Big Twin

On H-D BT trans cases the earliest letter-number date code I am aware of is from April 1938. The letter-number format seems to have then continued through the 1940s. Numbers-only codes appeared as early as January 1950. Listed below are date codes I know of so far on 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

A 0, C 0, E 0, F 0, G 0, H 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, L 3
C 4, D 4, E 4, H 4, L 4
C 5, F 5, H 5, I 5, J 5, K 5
D 6, E 6, G 6, H 6, I 6, J 6, J6, L 6 (See below re two codes with J)
A 7, C 7, D 7, E 7, G 7, H 7, J 7, K 7, L 7
A 8, B 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
(Another photo I have appears to show F 9 for June 1949 but the image isn’t clear.)

1 0, 3 0, 6 0, 8 0, 9 0, 100, 110
10 2
2 3, 3 3
1 58, 2 58, 6 58, 8 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, 8 60, 10 60
1-61, 6 61, 9 61, 11 61
2 62, 4 62, 8 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. Notice the period August 1948–March 1949 where there is no space. Notice J 6 (spaced) and J6 (unspaced) were apparently both used for October 1946. Notice on occasion a hyphen was used. Regarding the letters I and M, see below for further mention. Notice 1959 codes sometimes had 59 indicating the year but sometimes only 9 and I have seen the same thing on some other 1959 castings such as crankcases and heads.

Date code 3 3 at first may have indicated casting in March 1953 but it remained for some time and has also been found on trans cases of bikes as late as 1956–57 models that are unaltered from original. I cannot explain why 3 3 was used for so long. As you see above, I have no examples between 3 3 and 1 58.

Codes in my list from the beginning through 2 63 are on cases with casting number 121 35. On occasion it has worn down but the two examples below are still fairly good.


P1. Casting number 121 35.jpg
P2. Casting number.jpg


That casting number may have first appeared with Knuckleheads that had 1935 serial numbers (at least four were built as prototypes for 1936 models). The same casting number also appears for Knuckleheads 36–47, BT Flatheads 37–48 and Panheads 48–63. For 1964 Panheads, some had a 121 35 case while others had a case with casting number 34703 64 and I’ll come back to that later.


 Vent in the case or not? For the first few years the 121 35 case had no boss for a vent—for example, this one cast in September 1938.


P3A. September 1938 casting.jpg
P3B. September 1938 date code.jpg

 In those instances, venting was usually handled by a nipple in the starter cover. However, some cases with no vent boss have a machined hole as per the next example cast December 1938. A vent nipple (old #2265-36, new #34720-36) is installed.


P4A. December 1938.jpg
P4B. December 1938 date code.jpg


 I don’t know who machined that hole. Was it done at the factory? By a dealer? By someone else?

Also with no vent boss but a machined hole is this April 1939 case.

P5A. April 1939.jpg
P5B. April 1939 date code.jpg



But as early as May 39 (E 9) there was a cast-in boss for a vent nipple.

P6A. May 1939 vent nipple boss.jpg
P6B. May 1939.jpg


And because the boss appeared as early as May 39 casting, it may be possible this type of case was first used for some late-39 model machines.

The vent nipple remained in the case boss through 1955 models but as of 1956 models the boss was originally left unmachined and H-D changed to vent screw #34720-56 placed in the lid.

P7. Unmachined boss. Vent screw in lid.jpg

However, if such a case was a replacement to be used for a 55 or earlier machine then the case boss may have been machined as original. Also it’s possible for anyone else to drill a boss at any stage so always check for other ID.


 Bronze insert or steel insert? As seen above, the Sept 38 case and May 39 case have a bronze insert underneath for the mounting bolt on the R-H side. Also fitted with a bronze insert is this July 41 (G 1) case.


P8A. July 1941 has bronze insert.jpg
P8B. July 1941.jpg


 But the May 44 (E 4) case below has a steel insert. The change from bronze to steel apparently occurred around late-43–early-44.

P9. May 1944 has steel insert.jpg

 NB: recently on another site I ran across a D 9 trans with a steel insert but it wasn’t a 1949 case because three other features definitely indicated April 39 as opposed to April 49. In that instance, the case probably had a bronze insert originally and then it had been replaced later on with a steel insert.

Also, there are reports that the mounting bolt underneath on the R-H side debuted on some early-37 models as opposed to mid/later-37s. There are other things too that can assist with ID of a 121 35 case cast prior to date code introduction so if your 121 35 case has no date code then please let me know and I’ll try to help identify it.


 Two bosses at upper left front: in the next picture the two rounded bosses normally help indicate a 1946 or earlier model case. This example was cast in June 1945 (F 5).

P10A. June 1945 round bosses.jpg
P10B. June 1945.jpg

But as of 1947 models those two bosses were flat to provide clearance for the (hand) gear shifter lever which was changed from pointing up to pointing down. Clearance at first may have been achieved by the factory machining bosses that were cast round but at some point the bosses were cast flat. The next case was cast in January 1947 (A 7) and has flat bosses but I do not know if they were cast flat or machined flat at the factory.

P11A. January 1947 flat bosses.jpg
P11B. January 1947.jpg

Also be aware that flat bosses are not always a good indicator because some people flattened bosses on some pre-47 model cases. Always check for other ID.


Bosses for two mounting studs at lower left: this case was cast in August 1948 (H8 is upside down) and has the normal lower left bosses for that time.

P12. Slim bosses.jpg

I used the August 48 case because the mounting studs are absent and the view of the bosses is unimpeded but other examples I have suggest those two bosses were still like that in March 49. I don’t yet know what they were like in April and May 49. A June 49 case appears to show beefier bosses but a photo I have of what appears to be F 9 isn’t clear. Anyway, it seems by July 49 at the latest that more material was added to those areas, as illustrated by the G 9 case below.

P13. July 1949 bosses.jpg


For comparison of the front boss from the side and top, here is a case cast in December 1948.

P14. December 1948.jpg


And an August 1950 case showing the beefed up boss.

P15. August 1950.jpg

These are the clearest examples I have to demonstrate the difference from the side/top but as I mentioned earlier, the change seems to have occurred sometime during 1949.

 Getting back to date codes, was the letter I skipped on occasion? In discussions on a couple of forums several years ago it was said the letter I was omitted from all codes on aluminium parts (trans and engine) cast in 1948–49 due to confusion with the number 1. Some engine pieces showed up with code M8 and M9, as did 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 so now we have some 1948 trans lids with I8 and some with M8. Why? I don’t know.

For trans cases cast in 1948, was code I8 used? Haven’t seen one yet so if you have an example please let me know. There is, however, M8 on a BT trans case from 1948.


P16A. 1948 casting has M8.jpg
P16B. 1948 date code.jpg


P16C. 1948 casting.jpg

 Next code looks like I 9 and it appears to indicate September 49 as opposed to September 39.


I also have other examples that appear to be I 9 on 1949 cases and it is something I am still looking into. On a 121 35 case, M9 has yet to be seen by me so if you have an example then please let me know. L 9 is the latest I have for 1949 so far but regardless of which letter(s) indicated Dec 49, I do not know if letter-number was the only format used for BT trans cases that month. For BT aluminium engine parts cast in Dec 49 I have seen two code formats: letter-number; and numbers-only. These suggest Dec 49 was a changeover period for engine code formats so a similar thing may have happened with date codes on BT aluminium transmission parts.

Other markings: as seen above, some trans cases have an indented 1 or 2 near the casting number and I thought these may be inspector markings done at a foundry but a few months ago someone said they were a mould form number. I cannot say for sure what 1 and 2 mean but if anyone does know for certain then please comment. Other cases have an indented 1 E or 2 E but thus far I have only seen the E on cases cast July 49 and later.

Sometimes characters are found on the left side and on the back near the hole for the adjusting screw.

P18. H on left.jpg
P19. S and 40.jpg

Apart from H, S and 40, other characters seen either on the left side or at the back are A, B, D, F, G, L, T, W, Y, 0, 1, 5 and 9 but I have no idea what they mean and my best guess is that they may have been stamped by an inspector. Anyone know for certain?

Machined for reverse gear or not? Some transmissions had reverse gear installed as original while others did not but even some of the latter may still have been factory-machined to accommodate an idler gear shaft. On these occasions, with reverse not required, the inner hole for the shaft was left open but the outer hole was plugged as per the next case.

P20. Idler gear shaft outer hole is plugged.jpg


But the case below was not machined to accommodate an idler gear shaft. Date code is 11 61 for November 1961.

P21A. Idler gear shaft hole not drilled.jpg
P21B. November 1961.jpg

Two cases for 1964 Panheads: as I mentioned earlier, some 1964 Pans had a 121 35 case while others had a 34703 64 case. Here is the front of a 121 35 case cast October 1962 (10 62) and it has a recess on the starter side.

P22A. Front 121 35 case (Oct 62).jpg
P22B. October 1962.jpg


But the front of a 34703 64 case has extra material as per this one which was cast October 1963 (10 63).

P23A. Front 34703 64 case (Oct 63).jpg
P23B. Underneath 34703 64 case (Oct 63).jpg


 Next is the left side of the 121 35 case from Oct 62.

P24. Left side 121 35 case (Oct 62).jpg


But a 34703 64 case has longer versions of those three bosses.

P25. Panhead trans case 34703 64.jpg


P25. Panhead trans case 34703 64.jpg


Another difference is the top opening. Again the first photo below shows the 121 35 case cast in Oct 62.

P26. Top 121 35 case (Oct 62).jpg


But the opening is different with a 34703 64 case.

P27. Top 34703 64 case.jpg


For 34703 64 cases, apparently a change to the inner wall on the starter side meant spacer washer #33237-64 had to be used if an idler gear shaft for reverse was installed. Anyone know of other changes introduced with 34703 64 cases?

And although some 34703 64 cases have codes such as 10 63 and 12-63, the two examples below bear no sign of a date code.

P28. No date code.jpg



P29. No date code.jpg

 After 34703 64 came casting number 34703 65. This case retained some changes introduced with the 34703 64 case but incorporated other changes to accept an aluminium inner primary.

P30A. Casting number 34703 65.jpg

It debuted for 1965 Panheads but the same casting number appeared on Shovelhead cases for 1966 and several more years. I have never seen a date code on a 34703 65 case.

Other casting numbers seen on Shovel trans cases are 34703 77, 34709 78, 34709 79, 34709 80, 34709 82 and 34707 82A.


Additional original component identifying numbers: these are sometimes referred to as security codes or anti-theft numbers. Additional ID began with 1962 models and for Panhead trans cases I find it stamped on the front just below where the lid attaches. The ID consists of one capital letter followed by either three or four numbers. The first letter used was A so it is on some 62 models but there is also evidence that A was used for some 63s and the proof came via a confirmation letter from the factory to the owner of a certain 63FL.

The earliest cases I know of to have additional ID were cast in Jan, Aug and Oct 1960 but I used the Jan 61 (1-61) case below as an example because the letter A is clearer.

P31A. Additional original component identifying number.jpg
P31B. January 1961.jpg

I don’t know exactly how Harley assigned the additional ID but B is another letter apparently used for some 1962–63 models. B has also been confirmed by the factory as original on the 121 35 trans of a certain 64FLH and it can be found on some 1964 Pans with 34703 64 cases as well. However, some 64s may have C or D.

Pans for 65 also had ID on the front, as did many Shovels. Apparently the letters I, O and Q were skipped, with Z being the last single letter used. By that stage (Z), Shovel transmissions had casting number 34709 78 but that case also appears to be the first to receive double letters: AA. The ID seems to have remained on the front at least into the double letters BC. At some later point it moved to the top of the upper rear ear where the inner primary attached, two examples being BF and BG. But shortly thereafter the ID shifted to the back of the case and BH is the first I’ve seen there, while the latest so far is BV.

Questions and/or comments welcome regarding any of my above posts. Or if anyone has a trans case I can help ID then please feel free to contact me anytime.

And thank you to panhead (Site Admin) for your assistance with forum settings. Your help is much appreciated.