[TR] Replacing gearbox cover type-A overdrive

[David Griffiths]

Dear list.
I would like to ask for advice about converting a single-gear to a  
multiple gear type-A OD. I have a January 1957 TR-3 LO-15005,  
originally fittted with OD. It was restored by a company, and they  
found the original OD to be shot, and replaced it with an OD from a  
TR-2, with OD only on gear 4. It works well, but I am thinking  
originality a bit and fun also a bit, and am considering the thought  
of swapping out the cover with a TR-3 cover, which should give me OD  
on gears 2 - 4.

Bill Piggott talks a bit about this in his book Original Triumph,  
pages 116 and 74. At car 5980 the OD piston diameter was increased to  
1-3/8", up from 1-1/8" in preparation for the improvement, however OD  
was still confined to gear 4. From 6266 OD on gears 2 - 4 was  
introduced. He has to following to say: The factory supplied a top  
cover conversion pack, to allow three overdrive sppeds to be  
incorporated on pre-TS6266 gearboxes, although if the smaller pistons  
were fitted the resulting unit could be marginal under full torque.

The Bentley manual says the following: To enable the unit to transmit  
the maximum available torgue in the lower gears, it is necessary to  
use larger clutch operating pistons than those fitted previously. It  
then goes on to talk about the new top cover conversion assembly, and  
also how to modify the old top cover assembly to permit use of OD in  
2, 3 and 4.

I can buy a TR3A top cover assembly cheaply, and fit it onto my  
gearbox, and that should, in theory, give me OD on gears 2 -4. From  
reading it seems that Standard Triumph sold the conversion to fit both  
the new and old gearbox, but this bit about "marginal under full  
torque" has me wondering. Has anyone done what I am contemplating,  
installed the conversion or a later lid on an earlier gearbox, and if  
so, what was your experience with it? And, what might "marginal" mean,  
knowing the English love of understatement? Will the gearbox explode,  
or just slip out of OD? Of course, I will pull the tunnel off and look  
at the serial no on the gearbox, because the early and later gearbox  
had different nos. If its the later with the larger pistons it's plain  
sailing, but I'm really asking this question in case it turns out to  
be the early type.

thanks in advance,
Dave