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I am about to install a 4 synchro gearbox with A-type OD to my TR3A, instead of the 3synchro-A-type OD she had.

The box is fitted now, but the mounting holes of the OD are 2 cm further to the back of the car than original. I expected the 4 synchro box would be "somewhat" longer say 5 mm, and therefore easy to overcome, but did not expect it to be 20 mm. Now I am forced to alter the complete gearbox mounting plate 2 cm(!) backwards or fabricate a new one.

Does anybody has any good ideas how to do or comment?

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Rudi, I'v told you wrong things when I said that  a later box is only 5mm longer. Later boxes have a thicker flange on the bellhousing, about 5 mm, but also the housing for the gears is about 1/2" longer to accomodate the synchro hub and ring for the 1st gear. Sorry for that. I wasn't aware of than because I've a J type on my TR3. Perhaps it's simplest to make an extension on the mounting plate, with holes for the bolts to the gearbox. Perhaps the new holes need to be drilled through the vertical rear part of the original mounting plate : it's easer then to grind away some material first.

Alternatilvely, isn't it possible to simply weld the 2 big bolts to the rear side of the original mounting (with the heads of the bolts downwards)?

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No problem Marcel. Even if you wouldn't I would have believed it to be an "easy" fit. (should have known better as even original parts are never an easy fit). As in none of the reknown books one can read a single word about this. They all let us know that all TR boxes are interchangeable just like that. (Or did I misunderstood?)

Now I have drilled 2 new holes to fit the mounting rubber and slightly grinded away some material at the back and front of the mounting plate. At the back to make it possible for the rubber to pass and at the front for the OD casing. The mounting rubber sits 20 mm more to the back now and slightly passes the backside of the plate. I believe, or better I hope, this alteration does not weaken the plate itself.

Hope this will all fit now. For today working is finished as my wife has blown the horn to stop the working day.

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