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MG TD TF 1500 - T-Type/Y-Type Speedo Drive Gear Sets

Y-Type/TD/TF Gearbox - So yesterday I was going a "simple" gearbox swap in a Y-Type, but found the speedo drive would not "just slip in" like it had always done.
Closer examination and I discovered the shaft gear and speedo pinion were stamped "LL" while all the Y-Type spares I have are stamped "L", and you cannot mix gear types.
Does anyone know what model the "LL" gears are from?
Thanks in Anticipation...........Safety Fast!
Tony Slattery
The Classic Workshop
Black Moutain
Australia
A L SLATTERY

I’m not familiar with the intricacies of the Y-type and TD/TF differences. I believe the transmissions are basically the same with the exception of the shifter remotes. I presume that since the Y- type uses 16” disc wheels vs the 25” T-type wheel/tire combinations and tire combination that may have a taller overall profile that the gearset may not be the same for speedo accuracy. Are you trying to mix and match the wrong worm gear and pinions? What are their tooth counts ?

Bill Chasser
TD/c-8151
W A Chasser

I will do some measurements & count some teeth.

Tony

A L SLATTERY

the YA had 16" wheels but the YB had 15" wheels so the TPM would be different between YA/YB.
The YA had a similar axle to the TC, YB same as the TD/TF
I also think the YA gearbox is more related to the TC
Ray TF2884
Ray Lee

I can only find the drive gear to pinion ratio for the YB, which is 6/15 (and it will be the same for the TD).
The pinion on the left has 18 teeth, so it is not for the YB, which means it is probably for the YA or YT.
R WILSON

MG Speedo drive codes research progress update.......

AA - ?/9 - TB/TC
L - ?/4 - ?
LL - 18/4 - ?
N - 16/4 - ?
S - 13/4 - TD
AK - 17/? - TD MkII
T - 15/6 - TA & TF1250
U - 14/6 - TF1500

While the number of teeth on the mainshaft are sometimes the same (4), the OD changes to match the speedo pinion, so you need to check the letter on BOTH gears to match a set.

If anyone can assist with the unknowns, I would appreciate it.

Thanks in anticipation....

Tony Slattery
The Classic Workshop
A L SLATTERY

I have found the attached page in the Scientific Publications workshop manual for the TC, TD, TF & MGA.
It might clarify matters, but probably not.

R WILSON

No, I don’t think the above attachment clarifies matters at all.
The YB speedo drive gear to pinion ratio of 6/15 is given in the YB parts list. The YB and TD had the same rear axle with the same crown wheel & pinion, so the speedo drive gear to pinion ratio should be the same – but the attachment shows 5/13.

I wonder if my local MG dealer will be able to help.
R WILSON

Only if you speak Chinese
Bruce Cunha

I have taken photos of two speedo drive pinions.
The pinion with the T on the left is from my own TD gearbox.
The pinion with the T on the right is from a YB gearbox.
Despite a powerful lamp, I was not able to see inside the gearboxes to count the teeth on the drive gears.
My Scientific Publications page is in doubt.





R WILSON

R Wilson - that is very interesting.

If the pinions fit easily, you can be assured that the mainshaft gear is correct, so likely 6 teeth.

The big problem with all this is that we cannot know what gearbox/diff/speedo swaps have happened over the years, and with Special Tuning parts thrown in the mix - who knows.

I'm working on an Excel spreadsheet with all the variables to determine what speedo gear set should match which diff ratio and speedometer. I will also add rolling radius of the tyres.

Cheers

Tony
The Classic Workshop
A L SLATTERY

I have listed the various part numbers for the gearbox drive gear & pinion for the speedo in the attachment, and it shows that the TD, YB, TF1250 & TF1500 all had the same 6/15 speedo drive gear & pinion. However, the TD & YB had the 5.125:1 diff, and the TF1250 & TF1500 had the 4.875:1 diff, but the appropriate correct speeds would be shown as the cars had different speedometers.
So for a particular RPM in, say, top gear, the TD speedo might show 30mph, so the YB speedo should show 30mph - but the TF1250/1500 speedo will show 33mph. This is 5% more, from the calculation; 5.125/4.875.
The oddity is the TD Mk II, as this had the same speedo as the standard TD, but had the 4.875:1 diff. Thus the Mk II must have had a different speedo drive gear & pinion in the gearbox, as indicated by the different part numbers.
Perhaps someone who has a TD Mk II gearbox on his garage floor can withdraw the pinion assembly to determine the letter and number of teeth on the pinion gear. The number of teeth on the drive (or worm) gear in the gearbox would also be useful, but that is more difficult.
NB. I can no longer save a Word document in jpeg format – what has changed?
R WILSON

A more comprehensive attachment.
R WILSON

Tony, with regards to the TD-C with the 4.875 gear ratio I think a speedo with 1525 turns per mile (same as TF) was supplied. At least I assume this is correct since I have a couple of these 1525 TD speedos.
Hugh Pite

H.D. Pite

Sorry, in my latest table, I forgot to move a part number across so that it was in the correct column. I have therefore attached issue C.
R WILSON

Well, as some people may have noticed (but too polite to point out), my latest table issue C still has errors – so please ignore it.
I have established the following;-
1. The YB, although not of interest to T type owners, was available with three options of rear axle diff ratio; 8/41 (5.125:1), 8/39 (4.875:1) & 9/41 (4.555:1). I must assume that the YB gearbox I have as a spare came from a YB with a 4.875:1 diff.
2. According to the parts list, the TD was available, even before the Mk II was produced, with an optional 8/39 diff (4.875:1) and an optional 9/49 diff (5.444:1) – although the parts list may have an error here, should it be 9/41 = 4.555:1.
My car was a RHD export model (where to not known), so I must assume that it was fitted with the 8/39 diff (4.875:1) – and I never knew!

Thus, as the pinions in my photos have 15 teeth and are stamped with a T, they match Tony’s table.
MG Speedo drive codes.
AA - ?/9 - TB/TC
L - ?/4 - ?
LL - 18/4 - ?
N - 16/4 - ?
S - 13/4 - TD
AK - 17/? - TD Mk II
T - 15/6 - TA & TF1250
U - 14/6 - TF1500

NB. The following is extracted from the Brown & Gammons TD/TF catalogue, normally a very reliable reference:
5.125 diff (standard TD): gear AEG3110, 5t; pinion X22797, 13t.
4.875 diff (opt TD, standard TF): gear 168210, 6t; pinion X17260, 15t.
4.555 diff (opt TF): gear AEG3109, 6t; pinion X19101, 14t.
Unfortunately, it does not show the stamped identifications.

Out of interest, I will try to prepare an issue D table, but this may take a little longer as the parts lists are not very clear!!
R WILSON

Perhaps to simplify what I was trying to say.
The only way that my two pinions could each have 15 teeth, be stamped with a T, match Tony Slattery’s list and match the Brown & Gammons information, is for them to have been fitted in gearboxes (with the appropriate drive gear) that drive through a rear axle with a 4.875:1 diff.
I now know that both the standard TD and the YB could have been ordered with the 4.875:1 diff, so that is what must have been in the rear axle of my TD and the YB.
However, it does mean that anyone who changes their gearbox should check the number of teeth on the pinion gear before installing the gearbox, so that the drive gear & pinion combination is the correct one for their rear diff ratio.
R WILSON

This thread was discussed between 08/09/2023 and 04/10/2023

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