View Full Version : transmission lengths ??

02-25-2013, 09:03 PM
So I am working away trying to get this TR3 sans body ready to go to down to Hickory to have the nice "new" body set back on the frame and ran into yet another minor set back today. I have overcome it though in what I would consider an elegant fashion but a question remains.

The engine was sitting where it was suppose to be in the frame this morning and today I bolted the transmission to it. In doing so it became obvious that the mount bosses at the back end were going to be a tad farther back than the holes in the cross-member. In other words the transmission was about a half inch too long. I fixed that by reshaping the cross member a little and having new holes punched in it.

Everything lines up perfectly now, but of course the drive-shaft is going to have to have a little cut out toward the rear. Ordinarily I might just have the splined section give me what I want, but the shaft barely fits between the transmission flange and the axle pinion flange with the whole thing collapsed. No big deal really, but it does beg the question as to WHY all of this happened.

The transmission is one that came to me in a sorry state separate from the rest of the car and I rebuilt it as a TR4 full synchro unit. The engine mounts are new of course and the plates that they are attached to are straight as a stick. The only thing that I can guess is that TR4 transmissions are in fact a little longer than what came in TR3s.

Anyone have any comments on same?


02-25-2013, 09:33 PM
The TR4-6 transmission is indeed a bit longer than the TR2-3 unit, by about 3/8" or so.

But there should be plenty of travel in the driveshaft spline to accommodate that. I ran a TR6 gearbox in my TR3A for many years and it's now in my TR3; both times with the stock driveshaft. Might be worth pulling it apart, make sure there isn't dried up grease inside the sliding yoke or something. Also the flanges mate together by a small amount, so you'll get a bit more clearance when they are mated.

02-25-2013, 09:35 PM
Jack, I took out the transmission in my TR3 and replaced it with one from a TR4 to get the full synchronized gears. It was a perfect fit. Mine came out of an early TR4. I didn't think there was any difference between the early and later models of the TR4. There might be with the TR4A's.

02-25-2013, 10:36 PM
Thanks so far guys ! The extra length is indeed about 3/8", 16MM by my measuring stick. This must be a TR4A box then, I just didn't remember what I ordered all those innard parts for when I did the overhaul.

I was surprised too Randall when I was able only to slip the companion flanges between the output and pinion flanges. I would have thought that there would be more clearance even with the small increase in transmission case length. In any event it is what it is, so its off to the drive-shaft fix-it shop tomorrow when this winter blast leaves the scene.


02-28-2013, 12:33 AM
FWIW as a reference for the common good should anyone plan on swapping the OEM three synchro transmission out of their TR3 for a four synchro box from a TR4A....

The transmission, a TR4A unit is indeed longer than the original TR3 by about 3/4" or 5/8". Randall said 3/8" and I said 16 MM earlier, but the final measurement, on at least this installation, was 3/4". This places the rubber mount toward the rear of the car by that amount and requires the modification of the cross-member plate.

The way that I did that was to flatten the front flange down-turn in an effort to produce as much "meat" as possible just forward of the original front holes, and then, using a big deal hydraulic punch, create a set of new holes forward of the original ones. This placed the new holes directly above the frame bolt bosses and all was well in the world until I tried to install the drive shaft.

As noted in another thread, the shaft would just barely fit between the axle pinion and transmission flanges, so I had to cut the shaft down by 3/4" in order to end up with both good contact within the splined that section and to still allow enough movement in length as the shaft lengthend and shortened up as the rear axle moved up and down.


02-28-2013, 10:11 AM
Late TR4A gearbox housings were slightly longer again due to an increase in the thickness of the front flange. Beforehand in the event of clutch slave cylinder stay 128043 being mislaid, over time the bellhousing flange could fatigue and a big chunk snap off.

Once had to makeshift repair a broken flange to help a stranded owner. What a hassle.


02-28-2013, 10:39 AM
Which brings up yet another thought Viv.:D

Does anyone have measurements of the thickness' of the bell housing? I'm thinking both the bolt bosses and the other "in between" sections?

I had to weld several cracks in this one and even refit a small section that had broken away. Not a big deal perhaps, but having to do that brought to my mind just how crumbly this aluminum alloy really is.

If there was any sort of obvious thickness differences it might be a good way to start out identifying the units without having to uncover/scrape and clean the sides of the cases, let alone taking a length measurement.