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TR5/TR250 TR250 Differential

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RagTopMan

Senior Member
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Well, maybe not a TR250 differential...

Yesterday, I pulled the differential from my TR250 in preparation for new seals and bearings. Looking at it more carefully, I have discovered that the differential has a TR4A rear cover plate and mount. Here's one on Ebay that looks exactly like what I have on my diff: https://www.ebay.com/itm/TR4A-differentia...7a7&vxp=mtr

Furthermore, the number found on top of the diff is CTC 60762. Researching this number seems to indeed point to being a TR4a diff. Could this have been possible? Or just something replaced on the car over the years. The car's commission number, engine number, obvious original color, and heritage certificate all jive.

How can I tell what gear ratio is in the diff? There are other numbers on the casing but I'm not sure which, if any, denote gear ratio.

Should I just swap out the rear cover and mounting bracket? Replace the whole diff rather than bother with rebulding and switching the mount on this one? Or leave it the way it is?

The adventure continues...
 
According to the SPC, the main housing assembly is the same, just the rear cover is different. I wouldn't be surprised if the TR4A arrangement will bolt up to a TR250 frame, but I don't know that.

My guess would be that the diff has been replaced before, with one from a 4A. I don't know why they didn't keep the TR250 cover; maybe it was traded in before the difference was discovered.

My inclination would be towards rebuilding the one you have; but that's mostly because the last "rebuilt" diff I bought turned out to just have a new coat of paint (which fell off easily to boot). The rebuild process isn't too hard, but it can get very tedious getting the preload and contact pattern just right. During the rebuild, you can also add a drain plug if you want.

You can get by without the magical Churchill tools mentioned in the manual, but will need to make a few of your own plus have a dial gauge, micrometer, etc. A hydraulic press is really useful, too.

Be sure to check the measurements, don't assume that the new bearings are the same thickness as the old ones. My new input bearing turned out to be over .030" thinner than the original.

Here's a few photos I took of the process. This is a 2.5 Estate diff (sold to me as a Stag diff), but it's nearly the same as the TR version.
https://s258.photobucket.com/albums/hh260/TR3driver/Stag%20diff/
 
Oops, forgot to mention: To check the ratio, you can either pull the cover and count the teeth, or turn the shafts and count turns. Odds are good it is still the original 3.7 ratio, but it won't hurt to check.

Hold one output shaft stopped, and turn the other output shaft 20 turns while counting revolutions of the input shaft. If it turns 37 times, you've got a 3.7. 41 times would be a 4.1; 34.5 would be 3.45. It's unlikely you'll find anything else unless you're working on an old race car.
 
Thanks for the tip on counting the shaft turns to determine the ratio. Fun exercise to keep oneself sharp! Turns out to be 3.7 as you guessed.

The outer housing number on my diff (305815) is the same as TR6 differentials so I too am of the belief that all that is different is the rear cover plate and the mount.

I think I am going to keep this diff, replace the rear cover and mount with the "correct" set up and rebuild it as planned. It just confused me when I saw the TR4A mount set-up on my car. The rubber mounts I have already purchased (correct for TR6 diff mounting brackets) did not look like they would even fit in the rear bracket. And they don't!

As far as the rebuild... I'm farming that out. The diff was quiet, no clunk or whine, but leaked like heck out of the pinion seal. I'll let my local shop give it a once over and likely/hopefully just replace seals and bearings.
 
Definitely looks like the original differential was replaced with a TR4A differential given the serial number and mount. I've never heard whether the one piece TR250/TR6 rear rubber mounts were "stronger" or perhaps stiffer than the 2 piece TR4A rubber mounts but I wonder if they redesigned them for the TR5 and PI version of the TR6.

You could certainly keep the differential and just swap out the rear cover to get the correct rear mounts.

Scott
 
I have a TR250 differential, and the cover is as pictured on ebay. That is it has the separate aluminum mounting brackets.

So you are a go to use it with the knowledge it is original equipment.
 
FYI, here are my diff numbers
• CTC 58339 (stamped on bottom of case)
o 212047 on rear carrier
o 305721 on rear cover

The car was build in Nov 1965 and sold as a 1966 model year.

Bob
 
TR6oldtimer said:
I have a TR250 differential, and the cover is as pictured on ebay. That is it has the separate aluminum mounting brackets.

So you are a go to use it with the knowledge it is original equipment.
Click to expand...

Ray,

What's the serial number on that differential?

Scott
 
HerronScott said:
TR6oldtimer said:
I have a TR250 differential, and the cover is as pictured on ebay. That is it has the separate aluminum mounting brackets.

So you are a go to use it with the knowledge it is original equipment.
Click to expand...

Ray,

What's the serial number on that differential?

Scott
Click to expand...
I would not worry about the authenticity of your differential. It is original as installed at the factory and is listed as such in the Moss parts catalog.

When I bought my TR6, a friend had a TR250 that he launched over a center divider. He parted out the car and I bought the engine, transmission, rear end, and running gear.

At this point in time, the 250 differential, with the TR6 cover is in the car, so getting the serial number would be a chore.

The other thing to keep in mind is that the TR5/250 was a transition car with limited production. It was in essence a TR4A with a six cylinder engine, nothing more, nothing less.


Also, the differentials used in the TR4A,TR5,TR250, and the TR6's were basically the same as used in the Triumph 2000 and 2500 and perhaps other cars.

TR's were very dependent upon the design of components from other car production considerations, and occasionally the other way as well.
 
Ah, I think I see some of the confusion. The TR250 and perhaps even some of the early TR6's did have a rear mount bracket that bolted on similar to a TR4A differential but the bracket is different and the rubber mounts are different for the TR4A. The TR250 and TR6 used the same rear rubber mounts (single piece versus 2 pieces)

TR4Adifferential.jpg

TR250differential.jpg

TR6differential.jpg


RagTopMan's post definitely seemed to indicate he has a differential with the TR4A rear bracket which can't use the TR250/TR6 rear mount. He certainly should be able to just unbolt the TR4A bracket and bolt on the TR250 bracket though without removing the rear cover if he wanted.

Scott
 
RJS said:
FYI, here are my diff numbers
• CTC 58339 (stamped on bottom of case)
o 212047 on rear carrier
o 305721 on rear cover

The car was build in Nov 1965 and sold as a 1966 model year.

Bob
Click to expand...
The rear carrier and rear cover part numbers on your diff match those found on mine. So, I've got definitely got a TR4A diff on my TR250.

Internals are the same and its just the rear cover and carrier/bracket that I need to swap out for "correctness". Luckily I've already found what I need.

Always fun trying to piece together the history of a 44 year old car. You should have seen all the miscellaneous wiring found in this car when I bought it. Engine compartment light, interior lighting, car alarm (NO KIDDING), cruise control (SERIOUSLY), a couple of good ole' toggle switches operating the headlights and such and a second fuse box for all the extra circuits! I spent the first month just removing and correcting all the crapola. What is funny is that in the box of papers and books that came with the car was found an old newspaper classified ad for the car dated 1972. The ad touts the fact that the car has cruise control!

Did I mention the TR4 bench seat cushion that had been thrown onto the deck behind the seats and the third seat belt that had been bolted in? Or the half-assed vinyl glued on top of the crash pads and center support? Or the plastic simulated wood that had been glued over the original dash panel? Needless to say, I had to overlook a lot of little/cosmetic stuff in deciding on this car. An almost completely rust free body and frame, non-smoking engine with good compression, and a well negotiated purchase price won the day!

Discovering that the diff was an obvious exchange at some point with no attention to the way it mounted should come as NO surprise to me!
 
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