A:
There are two combinations of second gear bush and gear. What can happen over time is that the gearbox can be rebuilt with "early" and "late" specification mixed. It is often thought that the later specification must be stronger and Triumph must have made the changes for that reason, technically this is so, but we have found that the dog teeth on the late specification fail much more quickly than the early type did, although the bush part doesn't.
What often occurs is a late specification bush UKC956 and gear TKC454 are mated with the early thrust washers in the range 129941-2-3-4, when what is needed is thrust washers in the range UKC958-9-60-61
The late specification bush is shorter and the bore of the gear is correspondingly short, but the thrust washers are that much thicker, by around .080", which does make the 2nd gear thrust washers less likely to break.
We recommend fitting the early specification gear 105629 plus uprated steel bush 129939X (which is 1 x 153237+1 x 153239 sold as a kit). These must be used with thrust washers 129941-2-3-4. Also we have available a specially made thicker version of 153239 (originally approximately 0.123" thick), which we stock as 153239A (0.126" thick) which may be required to achieve the correct end float, measured using a new circlip 055707.
When correctly fitted to the mainshaft, if (say) 153237 is clamped against the 2nd gear thrust washer, using 153239, it should have 0.003" end float, irrespective of the total end float of the whole cluster on the mainshaft, which we suggest should be 0.004"endfloat. The same end float applies to TKC454 on UKC956, against thrust washers UKC958-9-60-61.
An alternative would be to fit TKC454 gear, UKC956 bush, with thrust washer UKC958-61. More often than not UKC959 is needed. If the gearbox has TKC454 fitted, carefully inspect the dog teeth, and if they are chipped and rounded, throw it away.
If you measure the length of the BORE of the gear, 105629 is approximately 1.375" and UKC956 is approximately 1.187", so it will be hard to mix them up. This measurement can be done with a steel rule for the purposes of establishing which gear you have.
All the thrust washer dimensions are shown on our website against individual part numbers.
