A65 idler pinions- differences?

[1660bob]

Hi All, just pondering about idler pinions.......

My 1962 A65 Star parts list has the idler pinion and shaft as an assembly, under part number 68-0114, and it fits all engines from 1962-1968. Its runs in the flanged bronze bushes 68-0214 and 68-0028.
This arrangement continues up to 1968, but the part number for the idler in 1968 parts book changes to 68-0978??, the flanged bushes are still in use as before, just the idler seems to change.

From 1969 onwards the idler was changed, along with the bushes and the whole plot re-hashed....
My question is: why did 68-0978 appear as a 1968 idler only?- what was the difference?

[Spaceman]

The changes to the idler from 1967 to 1969 were probably relatively minor related to changes in the auto-advance mechanism that was bolted to it. For example, the 1968 bolt used a BSC thread while the 1969 bolt used a UNF thread. The change in idler part number may simply have reflected the change in thread at least from 1968 to 1969.

[1660bob]

It could be something to do with the auto advance units as they do differ in part number from `67-`68, but curiously, although the auto advance bolt changes part number also, from 76-0290 (`62 to`67) to 70-7831 for `68 only, both these bolts are 1/4 BSC by 2 3/16" long and described as interchangeable..... a BSA policy mystery no doubt .

So the `68 year only idler is the same thread as the earlier version. The thread changed to UNF in `69 with the later  stepped shaft/non-flanged bushes arrangement.
One thing I have noticed, is that the gear blank for the later pinions is considerably "chunkier" and heavier (100G or so) due to less metal being removed from the sides during the machining process. The critical dimensions are obviously the same, but much less metal machined off so heavier. I have an idler that is the heavier type, but has the 1/4 Cycle thread,non-stepped shaft, so its pre `69, but different (heavier) to the earlier type. Does this make it a `68 only pinion? Well confusingly its hand etched 68-0114, not 68-0798 so I`m none the wiser... I wonder if BSA altered the weight of the later pinions for some reason and the `68 version was simply the first of the heavier type, but interchangeable with the earlier (68-0114)Perhaps to damp out noise in the gear teeth backlash??