TR4/4A TR4 Overdrive Conversion

[TRDejaVu]

So I read a great deal about overdrive vs 5-speed conversions and I am now the new owner of gearbox CD9551 and A Type overdrive model 22/61985. I read somewhere that the 6 in the OD number referred to it being a rebuilt unit. The seller said that it came out of a 71 TR6, but I think that the numbers say that the g/box is earlier and the OD is later. Anyone know of any cross reference charts?

I have done a few engine overhauls over the years, but never a transmission, so I am going to give it a try. I have the Buckeye files on DIY transmission overhauls on both, but I am of course open to, and appreciative of, the collective wisdom of others who have already gone down this path.

 

[TR3driver]

My first advice (in addition to Nelson's excellent information) is to not skip any steps. NEVER assume that it was right before, or that new parts fit correctly. Get an accurate scale and check all those strange forces, especially the shift hubs. Several of the new bushings I got did not fit my gearbox properly, requiring some amount of modification. In one case, after checking 5 different bushings (including the one that came out), I had to machine a gear thinner to get enough endfloat on the bush.

There are also some good articles on the VTR web site, written by Larry Young (no relation) and Sam Clark.
https://www.vtr.org/maintain-index.shtml

My experience has been that the seals sold for the shift rods do not fit snugly in the cavities, resulting in leaks. Herman van den Akker once told me the same thing. In the past, I've cobbled up spacers from thin copper washers (dunno what they are for, but I had several in my "plumbing junk" bin). But this time I bought Teflon "backup rings" from MMC that fit perfectly (P/N 9560K53).
https://www.mcmaster.com/

Not sure if it's really any easier than Nelson's method, but the way I remove the plugs for the shift rod detents is to run a suitable tap (3/8" I think) into the plug, then drop a 1/2" od spacer over the tap & screw a nut down on the spacer. The plugs pop right out, and if you desire, can be reused. I didn't though, instead tapping the holes for threaded plugs (as Nelson suggests).

 

[PeterK]

Wasn't there also a differential change with o/d? 3.7:1 for non-OD and 3.9:1 for o/d or something like that?

 

[TR3driver]

Well, not quite, though that is a common misconception (even repeated by some book authors).

Instead, I believe what happened was that the optional 4.1 final drive was only available in combination with the overdrive. But the vast majority of cars with factory overdrive still had the standard 3.7 ratio.

One easy way to tell if a TR2-4A had the optional 4.1 is to check the calibration number on the face of the speedo. The 3.7 cars have an 11xx number, while the 4.1 cars have 13xx. Tony Rhodes once told me that he had searched for a long time for such a speedo, without success. (I have the only one he was able to find, and it's still the only one I've ever seen.)

Certainly you could convert to a lower final drive ratio, even lower than 4.1 if you want. But it's not required for an OD conversion, nor even desirable IMO unless you drive mostly on a racetrack. My plan is to go the other way, with 3.45 gears from a Triumph saloon.

 

[CinneaghTR]

Congrats on the OD conversion! I hope to get to that soon after the other little projects that need to be done.

Two things:

Randall,

You're doing better than Scott at Team Triumph (I hope he is better). I asked about a 4.1 when I needed a "new" rear and he said had never seen one (!).

Also, the Spitfires and GT6s undergo a diff change with the numbers quoted above.

Jeremy

 

[PeterK]

One in a while, one (4.1 r&p) will come up on ebay NOS. I even saw a "rare" 4.55 r&p on ebay too, once.

My HVDA 5-speed conversion has 3.7 and hums along nicely @2250rpms @60mph and also has plenty of torque left in 5th.

 

[Andrew Mace]

...Also, the Spitfires and GT6s undergo a diff change with the numbers quoted above.

Jeremy, I'm not sure what you mean by that. Yes, most GT6s with factory overdrive had 3.89:1 diffs rather than the normal 3.27:1. But I don't believe OD-equipped Spitfires ever had different rear end ratios than did their non-OD counterparts. (On the other hand, there were a number of "catalogue option" ratios for Spitfires, and there were different rear end ratios for various models and, in some case, different markets.)

 

[TR3driver]

FWIW, there was a fellow in Germany who claimed to sell gearsets that will fit TR3-6, in your choice of 4.1, 3.9, 3.7 and 3.45 (and possibly some others that I've forgotten) at a very reasonable price. Not sure if he still does, but his name was Carsten Conrads, email carco@t-online.de

And I should perhaps explain that, while I have the speedo for a factory 4.1, I don't actually have the 4.1. The entire rear axle assembly on TS39781LO was swapped out before I got the car, to a later one with 3.7 gears & 9" brakes.

Somewhere in my Dad's garage should be a 4.1 axle, as I removed it from his (non-OD) TR3A back in 1975 or so. But I've searched for it several times without success; don't know what happened to it. Perhaps it was left behind as junk when they moved.

 

[CinneaghTR]

Hmmm, I'll recheck where I thought I read that, but to be truthful, I should have said GT6 only since I was assuming the Spits would be the same. Thanks for the clarification.

Jeremy