When you think you've got something sorted, a piece of evidence turns up to make you think again. So it is with the idler saga. I thought it was fairly straitforward: the first Cents had a "long spoke" idler which cracked. It was replaced with a "short spoke" idler and eventually with a "dished idler", something like this:
So to confirm, why not have a look at the first production batch of Mk 1 Cents?
Well this one is P1, the first prototype and it has a dished idler but with smaller holes than the later one and a splined hub - so it's a fourth type. Perhaps it was the idler from the Comet? It turns up again later in the Mk 3 handbook. Then there are these:
And:
So from this evidence, the "short spoke" idler looks to pre-date the "long spoke" variety. I then looked in the illustrated parts list, which seems to confirm it:
On the left is the "short spoke" which is referred to in 1947 as the Split Idler (ie two piece). On the right is its later replacement, the "one piece" or "long spoke" idler, which is definately the sort which cracked in Vietnam. So what caused the split idler to be replaced? I found some reports in the National Archives into the tendancy of Cent to shed its tracks. Whilst it doesn't mention idlers, it is noticeable that the one piece idler is assymetric and much wider. Maybe it was introduced to help with the track throwing but had in turn to be replaced with the dish type when it was found to be cracking. So the order of batting is probably this:
ie split idler, one piece idler, dished idler. This seems to be born out in part in the User handbooks:
Top right is a drawing from the Handbook for the Mk 3,5,6. The drawing depicts an early Mk 3 and it has the dished idler with splined hub seen earlier on the first production batch. Contrast that with bottom left which is representative of the later marks, sporting the one piece idler.
I'm still not convinced this is definitive but the research has been interesting, if you like that sort of thing..... Roll on the boxes arriving.
Regards
Stephen