TR2/3/3A rear mount does not line up

[TomMull]

My TR3 came with a TR4 all synchro gearbox with overdrive.

Since the OD gearbox was having some issues I decided to replace it, at least temporarily, with an old non synchro unit that I had on hand. It came from a fairly late TR3A.

The problem is that rear mounting flanges on the replacement gearbox fall 1/2 inch forward of the rear mount. Is there that much flex in the mounts? The rear mount was new a couple of years ago. Everything lined up easily with the OD gearbox. The front mounts appear good but age unknown. I cannot lever it back with a punch as seems quite happy where it is. I've measured the tail sections on both boxes and they are as close as I can get with a tape; certainly not 1/2 inch off. What am I missing?

 

[CJD]

That's the rub with the newer tranny in the old cars. The easiest fix is to mark and notch the metal part of the engine mount...elongate the holes for the tranny bolts. Mine has been running that way for a decade.

 

[TomMull]

I'm going from the newer tranny to the old one but I guess the fix is the same. Thanks John.

Tom

 

[CJD]

That's a new one! I read it fast and assumed the opposite.

 

[charleyf]

That's a new one! I read it fast and assumed the opposite.

I believe if you were to obtain an original rear plate for a TR3 your bolt holes will line up. As in my TR3 with a TR6 tranny I had to elongate the holes.
The reason here is the thicker phlange at the front of the TR4 -6 trannies. It just moves the rear back that 1/2"+.
Charley

 

[Graham H]

As Charley said the front flange is thicker by about 1/8 of an inch on the TR6 box but the main difference is the 4 synchro box is longer to fit the extra synchro in.

Graham

 

[TomMull]

Thanks to all, this is starting to make sense. I was focused on the OD as being the difference and forgot about the case being longer to accommodate the 1st gear synchro.
According to the previous owner the car had a TR4 engine and OD transmission installed before he bought it in 1970. I now see no visible mods to the rear mount.
Perhaps the "original rear plate" was changed although I'm not sure what that is. Crossmember under the mount?
Tom

 

[TomMull]

OK, I now see that the plate under the rear mount should bolt on. Mine has been welded. I have two old TR3 chassis and that plate is missing on both but it does look like it is available. Seems like this might be an easy easy solution (at least after I grind away the welds).

Thanks again for the help,

Tom

 

[Graham H]

Just a note, the TR3B had the TR4 engine and box fitted when they were assembled so some adjustments would have been made to them but I don't know what or how.

Graham

 

[TomMull]

Thanks Graham,
The modification is simple, requiring only the replacement of the bolt on cross-member on the chassis under the under the rear mount. Of course you need to know this and I did not at first. My car had must have had the TR3B/TR4 crossmember installed when the TR4 drive train was installed and I simply need to go back to the original one. It appears to be still available, part no 031-665 or 128055.
Moss has it listed for the TR2 thru TR3B with I think is incorrect. I'm quite sure he 3B should have the TR4 one, 031-655. I'll keep you posted if I'm wrong.

Tom

 

[TomMull]

Even easier and no parts needed. I guess this is what John meant in his reply. It takes me a while.

This is apparently the x-member for the earlier TR3s drilled out to TR4 specs. Simple to do. (I don't know it the holes placement is the only difference.)
Tom