Triumph TR4-5-250 Rear Axle Components - IRS
The shims used to correctly position and pre-load the crown wheel and pinion and bearings did not change from TR2 to TR6 (including Stag, Sprint and Innsbrock saloons). The IRS axles require a single spacer, in addition, between the pinion and the pinion head bearing. It is argued that this spacer could be replaced with an equivalent number of shims between the pinion and head bearing as both achieve the same net result.
Rebuilding of many hundreds of differentials has only ever-revealed spacers 0.040 thick. No parts book listed this spacer though a Triumph service release seems to indicate a range of thickness’ (0.030 to 0.051) was available, part numbers 140643 to 140657.
In the absence of full engineering drawings it can only be guessed that the internal machining of the differential casings between solid axle TRs and IRS versions is different. With the introduction of the 3.45 to 1 ratio in 1967 the pinion teeth would foul the casing in some circumstances due to the larger pinion diameter, but that doesn’t explain the spacers presence from 1965 in the TR4A differential which used 3.7 and 4.1 to 1 ratios.
If the spacer is left in place and the contents of an IRS differential are fitted to a solid axle, the pinion face will usually foul the hub (or crown wheel carrier) as it rotates.