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MG TD TF 1500 - TD-TF transmission question.

Can anybody tell me what the tooth counts are for the worm gear on the mainshaft and pinion for the speedometer drive cable? Were there different pitches and tooth counts depending on the CW&P ratio or was the change made with a different corresponding speedometer?

The reason I’m inquiring is twofold. First, I’m about to do a five speed swap in my fast road TD. I want to know the tooth counts to see if I can get the same ratios for the T9 since. I have a NOS gearbox, but I need to find the output pieces for the speedo drive. Looking on eBay there are several different tooth counts available for both gears. Yes I’m aware of the adapters from both Declan and Richfield.

Second, is in regards to original speedos.
For the 5.125:1 CW&P a 1600 rev/mile is used. If there weren’t various speedo drive pinions available then what is the correct rev count for a 4.875:1, 4:55:1 option speedometer. Are the rev counts the same between chronometrics and magnetic gauges and their CW&P final drives?


Bill Chasser

W A Chasser

The tranny parts are all the same. I think the instruments are different, with a different part number on the dial. Too early to research, but I think this is in archives or maybe factory parts book. George
George Butz III

So the coffee hit. I need to correct my post. Looked at the parts number book, clear as mud. Original speedo numbers: 27H224, 500527 at chassis 10779 (Change to magnetic), and 500616 at chassis 17548 (maybe high beam indicator?). Mark II-500829 at chassis 18959, so this was different somehow. Gearbox: original speedo gear- X22900, 168210 at engine #16978. Mk II- original X24440, changed to X24442 at chassis 18959. Pinion gear: original X17260. Mk II- X24441,"finished engine 18958, but also lists X17212 with no end date. So it seems there were different tranny pinion/drive gears as well as speedos. Impossible to tell the differences, time to go to work. George
George Butz III

George is now correct - the Mark II instrument was indeed different from the TD instrument, as were the transmission's speedometer pinion drive gear and speedometer pinion - all to reflect the Mark II's 4.875 diff ratio.

Tom Lange
MGT Repair
t lange

So it’s not showing an early chrono MKII speedo PN? That’s interesting. Seems like there would be a pinion or speedo PN change.

Also , the 4.875:1 was the standard final drive on all MKIIs. But the 5.125:1 and 4.55:1 were both optional. Yet no mention of that in the parts lists(?)

So if there are no changes to the worm does anyone actually have gearboxes apart and can check or know the tooth counts
For both the worm and pinions?
W A Chasser

This was from the 6/1986 TSO article about the Mark II. The numbers above are all that I could find listed in the parts list, I think dated 1956. There is an article/write up somewhere about speedo face markings. Could have been in TSO since the old CD? George

George Butz III

Bill, I have a later TD speedo with the high beam indicator which I think is from a TD-C (4.875 diff) and the face is marked with 1525 turns per mile. I will have a look at the box of pinion gears and see if any of them are marked with anything but a "T". As far as I know this was standard for all TD's and not special for a TD-C. I always assumed the change in diff ratio was taken up in the tpm in the speedo.
Hugh Pite
H.D. Pite

From TSO August 2011, p. 33, article by Bob Grunau. A lot more info! George

George Butz III

Don't know which parts list you guys are looking at but my copy of AKD 834 from June 58 for TD's shows them.
5.125 Gear X22990, Pinion X17260.
4.875 Gear X17212, Pinion X24442.
4.55 Gear X24440, Pinion X24441.

Maybe you don't see them as they are listed under "Chassis" section R

Bernie
B W Wood

Well excuse me for trying to help. I just looked under gearbox, speedometer and the Mk II supplement sections. Bob Grunau's article actually answers Bill's question. George
George Butz III

Can anyone confirm what Bob Grunau shows in his chart above, that the TF1500 had a 4.55 diff ratio? I always thought it was a 4.875, like the TD/C and the TF1250. I thought the 4.55 was an optional ratio.

Thanks.

Tom Lange
MGT Repair
t lange

George. Thank you for digging that up for me. That’s exactly what I wanted to know. Your the man!!!m

Bill Chasser
TD-4834.
W A Chasser

My 55 built-in December 54 had a 4.8 differential. I know it was never changed from the factory. It was changed to a 4.1 by Dave Clark per my instructions. PJ
PJ Jennings

Well, the plot thickens! I just had a look at my collection of pinion gears and matching spiral gears. These were taken out of a variety of gearboxes from early TD to TF 1500. They were ALL (15 of 'em) marked with a "T" and had 15 teeth, the spiral gears were also all marked with a "T". I don't know how to count the teeth on the spiral gear.
Has anyone actually found a TD or TF set of speedometer gears that were not marked "T"?
Hugh Pite
H.D. Pite

Hugh
To count the teeth on the spiral gear, just count around the edge of the gear how many thread starts it has in a full circle---If I recall correctly their should be 6 and 15 teeth on the driven gear

willy
William Revit

Thanks Willy that is easy enough.
I did some calculations using data from a TF 1250 which are undoubtably correct:
1. speedometer is 1525 turns per mile
2. speedometer gear in gearbox is marked "T" and have a ratio of 0.400.
3. the rear end ratio is 4.875
Using these figures and a bit of simple arithmetic will show that the road wheel turns per mile is 782 and NOT the 807 as given the the TSO article.
OK, using this 782 road wheel turns per mile and a TD rear end ratio of 5.125 as well as the "T" speedo gears will give 1603 turns per mile out of the speedo cable. This is very close to the 1600 turns per mile shown on the TD speedometer dial face.
I therefore conclude that, from the factory, a TF 1525 speedometer and a TD 1600 speedometer will have a "T" set of gears to give an accurate reading for car speed.
Hugh Pite
H.D. Pite

This thread was discussed between 28/12/2022 and 02/01/2023

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