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MG TD TF 1500 - Speedometer Gear Differences?

Gentlemen:
I'm rebuilding a spare gearbox and have got to the point where I'm attaching the speedo gear (no broached keyway slot) to the output shaft (which has a keyway cut). so a mismatch; however, the speedo gear and the pinon both have "AK" stamped on them. I have another speedo gear without a pinon that has "T" stamped on it and what appears to be a slightly different worm thread. Does anyone know what the differences are between the "AK" and the "T"?
Many thanks and happy new year to you all:)
Regards
Rod Murray
Rod Murray 54TF 3006

Rod
You are actually seeing an "AA" on the gear. That is for the TB/TC gearbox. The pinion gear for the TD/TF has a "T" stamped on. There are a different number of gearteeth. Be sure to install the correct one!

Tom
'54 TF
Tom Norby

Does anyone have the correct number of teeth on the "T"?
I did check on both the gear and the pinon even with a magnifyer and the letters stamped are definitely "AK" "Alpha Kilo" on both!I ask because I could put the "T" gear on then find a "T" pinon but that still leaves me not knowing what the AK is or for!
The pinon stamped "AK" has 17 teeth on it!
Thanks
Rod Murray
Rod Murray 54TF 3006

Rod,

The standard pinion for the TD with the 5.125 cwp has 15 teeth. The pinion I have with the 4.3 cwp I believe has 13 teeth. This has balanced out the speedo with very little error.

George
George Raham [TD4224]

Hi Rod;
The correct number of teeth for the pinion gear stamped "T" is 13. If indeed your pinion gear is marked "AK" it may be from another MG gearbox of that era. Y maybe?
Refer to the article on gearbox pinion gearing from Doug Pelton in Issue 1 of the on line Totally T Type.

I had my TF pinion out of the gearbox last month. I bought one of Doug's upgrades to stop that pesky speedo cable leak. I saw the "T" and counted the 13 gear teeth.

Quote from article here:

"However, the pinion gears are different. The TC/TB pinion gear is distinguishable by having 9 teeth and is stamped “AA” on the end. The TD/TF pinion gear can be identified by having 13 teeth and are normally stamped “T” on the end of the gears. So check to make sure the gearing is correct for your car."

Tom
'54 TF
Tom Norby

George,

So do I understand it correctly that you can change the pinion in the trans and get the correct speedo read out? I'm in the process of doing the 4.3 and also have the engine and trans out. Would it be possible to change out the pinion on my trans and get pretty durn close with my speedo? If so, where would I source the 4.3 pinion and how hard are they to swap?

Alex
ABW Waugh

George R.
I am confused. Are you talking about different teeth numbers in Moss #117 in the TD/TF gearbox illustraion? A part with no number, as it is not available? If not this, what exactly are you referring to?

Tom
'54 TF
Tom Norby

Rod,

I counted the gears on the pinion that was used on my TD with the 5.125 cwp. It has 15 teeth and the "T" stamped on the end of the gear. When I changed to the 4.3 cwp, I changed the pinion to the one from the old Morris which I think has 13 teeth. It has "TT" stamped on the pinion gear, and if I get a chance I'll pull it and be sure of the number of teeth. I am surprised that the TF has the same marking as the TD when it has a different cwp gearing.

George
George Raham [TD4224]

Alex & Tom,

I think I can answer both of your questions together. The pinion I am referring to is listed in the Moss parts on page 12 [TD-TF Gearbox], part no. 119. As noted, this part is NA. This is probably why they have the same part no. as they are both NA.

George

George Raham [TD4224]

George,

So, where did you get your old Morris pinion with 13 teeth? Does this give you accurate speed information after switching the CWP set?

Alex
ABW Waugh

Alex,

From reading your question, it seems as if you want to change the speedometer ratio by changing only the pinion. You cannot do this, you need to have a matched pair (same stamping) for both the worm and pinion.

John

J Scragg

I became curious about the MKII gearbox components, since the rear end became 4.875 vs. 5.125. Sure enough, the SPL shows a different pinion/gear set for the MKII.
The set was changed, not just one component. Bud
Bud Krueger (TD10855)

Rod,

My '53 TD gearbox which has never been opened by anyone but me during a current rebuild has a "T" stamped on it and has 15 teeth. The rear ratio is a 5.125. Ties in with the comment by George. The drive gear on the output shaft has a keyway so I can assume that it is original as well. I have a spare '52 used box that has the same number of teeth and a "T" on it as well. It does not, however, have the keyed drive gear or keyway cut into the shaft. Go figure.

Mark
Mark Strang

John,

Okay... so you have it right. I was assuming you could change just the pinion. I was going off George's post though that was further above:

----------
The pinion I have with the 4.3 cwp I believe has 13 teeth. This has balanced out the speedo with very little error.

----------

I'm trying to find out what George has done to to his gear box pinion to balance out his 4.3 gears in relationship to his speedo.

George...

Can you further clarify? I still don't understand what it is you did.

Alex
ABW Waugh

Gentlemen: I pulled the pinon out of the other gear box and it does have "T" stamped on both and it also has 15 teeth as you say George! I will use that pinon and package up and put away the "AK" until after I have finished the car and might experiment then! When I had the instruments redone by John Marks in England, he had me identify what rear end I would use (with some arcane mathmatics) so therefore I assume he would have based that calculation on the "T" gearing at the gearbox end and whatever gearing ratio he would have put in at the instrument end. George he probably did the same instrument gearing on your instruments when he did yours.
I'll keep my eye open for another 15 tooth pinon for a spare.
Gentlemen; many thanks for your help!
Regards
Rod Murray
Rod Murray 54TF 3006

Gentlemen;
Remember, we are talking about the pinion gear in the gearbox, which mates to another gear on the gearbox mainshaft. This gives a reading of rotations per mile of the mainshaft to the speedometer. This is all in front of the differential, so any gear ratio changes back there allow the car to travel further with each rotation of the rear end, causing the speedo reading to be too low. The distance traveled per mile is changed by those rear end ratios.

I have been researching this week, all the methods for recalibrating my speedo after the 4.3 differential upgrade I did this past summer. I have read no reference to changing that speedo pinion gear. And, to do that, it seems to me that you would also have to replace it's mate, the gear on the mainshaft #117 in the Moss illustration. All unavailable. That would require opening up the transmission. If we want accuracy of MPH, the choices are internal work in the speedo, or buying a gear reduction attachment that fits onto the speedo cable line at some point, likely at the gearbox end.

I looked through The mg workshop Manual by Blower this morning. He mentions that the gear reduction for the YT, the TD and the TF are all 6/15. Not sure exactly what 6/15 implies, but those three cars all used the same gearbox, so I would think that the speedo pinion gear would be the same as well.

Tom
'54 TF

Tom Norby

One more thing, to correct myself. The pinion gear marked "T" does indeed have 15 teeth, not the 13 I said yesterday. I went to the website for "from the fame up". Doug has a photo, face on, of both these gears, the T and the AA. His text says 13 teeth, but you can clearly see that there are 15.
Rod's "AK" gear is a mystery.

Tom
Tom Norby

Rod, Ken Miles was talking about building a speedo gearbox for my TD - it seems that he's done some for some of his Minis. Why don't you talk to him - it may be that he can do both yours and mine at the same time.

He told me last week that things are a little slow and he's got time to do some small projects - maybe this is the time.

Dave
Dave Jorgensen

Alex,

In answer to your question of "where did I get the pinion with 13 teeth". About 45 years ago, a friend offered me the engine and tranny from a Morris 10 he was scrapping. The block was an XPJM LX [1200cc]. The tranny was some what similiar to the TD. Not being one to throw anything away that might be useful in the future, it was put in cold storage. In the mid 90's when the 4.3 cwp was installed during the restoration, I considered what would happen to the speedo and wondered what I could do to correct it. I remembered the old Morris tranny and dug it out. Sure enough, it had the old speedo pinion and the 13 teeth on the pinion gear. This proved to be my lucky day with this discovery. The 5.125 cwp gave me 60 MPH at 4200 RPM. The 4.3 cwp gives me 60 MPH at 3500 RPM. The 13 tooth pinion gear has the speedo reading 60 MPH at 3600 RPM as near as I can tell. This is close enough for me. I still give thanks for the decision I made 45 years ago. The old block proved to be very valuable to me later, but thats another story.

George
George Raham [TD4224]

Dave & Rod;

From the thread "recalibration methods for the speedo" of a few days ago:

R Taras, California, USA, mgyowner@yahoo.com
Hello Tom. The problem with the suggestions above is that those sources do not make the boxes with the correct threads to attach to the gear box output nor the correct thread to attach the cable to. Contact me off list, I make them.
Butch
VMG

He e-mailed me some photos. He has experience making conversion boxes for MG's that attach down at the gearbox end of the speedo cable. I expect to hear from him later this week. I'm sure he'd rather make 3 than 1.

Tom
'54 TF
Tom Norby

Tom-
6/15 means there are 6 teeth or threads on the mainshaft gear, which is classed as a worm gear, mated to a 15 tooth pinion. Within limits, it is possible to use different numbers of pinion teeth on a given worm, provided that the tooth form and pitch diameter can be adjusted suitably. This would normally be 1 or 2 teeth difference for a given worm. I have no idea if this (George's) 13 tooth pinion is actually correct for the 6 thread worm, or if the fit is a fortuitous mix of tolerances, wear, and luck. Lightly loaded loose fitting worm/pinion combos can work even though failure would be quick on a heavily loaded similar drive. Some systems use an eccentric pinion bush to allow center distance adjustments to suit different pitch diameter pinions over a wider range, like maybe +/- 4 teeth.

FRM
FR Millmore

Tom and Butch --- Just remember that the speedo cable comes out of the tranny and curves forward to go up to the proper hole in the firewall. When you add the correction box the curve becomes tighter. So the object is to make the correction box is short as possible. This leaves the cable with the largest radius possible.

If I remember correctly the box I got, the output was lower than the input (correctly installed). Which helped a wee bit.

I can't find the address of the outfit that supplied the correction box. But it was something like API Engineering up in Michigan or Minisoeta. I thought I had it but that has proved not to be the case. If one of you guys up that way could give me a couple of clews maybe I could reconstruct the address.
Cheers,
Bob
Bob Jeffers

This thread was discussed between 01/01/2011 and 02/01/2011

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