My parts book shows a washer 40-0574 fitted beween the sprocket and the rotor. It isn't mentioned in the BSA Service sheets. Also, there wasn't one fitted on my bike (but that proves very little...)
I am reliably informed that the 'washer'is 0.093" thick. Does anyone have any reasons why I should or shouldn't fit one?
Quote from: Tigerfeet on 25 Nov, 2025, 18:16 My parts book shows a washer 40-0574 fitted beween the sprocket and the rotor. It isn't mentioned in the BSA Service sheets. Also, there wasn't one fitted on my bike (but that proves very little...)
I am reliably informed that the 'washer'is 0.093" thick. Does anyone have any reasons why I should or shouldn't fit one?
Probably to centralise the rotor or align the primary chain better.
I have just bought the same item shown on the parts listings , but do consider how much actual length you have on the drive side crank.
If I measure my crank from the outside face of the engine sprocket(large boss outermost) , then my crank is 41mm long.
if i then put the rotor on the crank with the 4mm thick spacer , then my nut does up pretty much flush with the end of the crank.
if I then offer the stator onto the studs, then the stator is virtually centralised over the rotor.
i haven't looked across all the C15 parts books yet , to see which models or all the spacer was fitted to.
the primary chain was aligned via the bevel inner spacer which is available in different thickness.
I have 3 parts books for the C15 - 2 books show the distance piece and 1 book does not - says for C15 star and SS80, but I can't see that it does much - also take care on the nut that you have - stepped or plain.
Isn't the Rotor made of Aluminium..? So the washer would be for load spreading purposes.
No,
The aluminium magnetic body has a steel core and it is this core that the nut does up on and the spacer bears on.
The reason for it's existence is not obvious certainly.
As always with this C15F, it has been an interesting interlude. I assembled everything as a trial run - and found that the primary chain was clearly out of alignment. (This was despite me having checked this, months ago.) I then realised that there wasn't really any way of knowing that the sprocket was all the way home on the crankshaft. After a happy hour playing about with torches and a dentist's mirror and feeler gauges... I gave up, took it all apart and went the way Rhyatt outlined. My crankshaft measured 52.54mm from the distance piece up against the bearing to the shoulder. The sprocket measured 23.70mm and the rotor 28.74mm - so with the two items on the crankshaft they should be 0.10mm shy of being flush with the shoulder. Armed with this knowledge, and a dab of marking ink on the back of the sprocket, and a large washer behind the crankshaft nut to ensure I didn't bottom the nut out on the shoulder, I wound it all back on - and this time the sprocket definitely went all the way to the distance piece. I hadn't factored in that as more of the spline is engaged, more effort is required to wind it on. Anyway - chain alignment was restored, and I was able to check the alignment of the rotor within the stator. This was offset about 2.5mm - and also there is about the same amount of thread showing through the crankshaft nut. So I am fairly confident that the 40-0574 spacer (when it arrives) will give me perfect rotor alignment and the crankshaft nut outer face will be flush with the end of the shaft.
Trial assembly without the 40-0574 spacer.
you and I are at the same point it seems - with my spacer on and using a plain nut (thread issue now resolved !!) the nut is about 1/2 a thread out from the end of the crankshaft.
i treated the bike to a chain tensioner as well.
to check alignment , I used a steel rule straight edge along the clutch wheel teeth and against the engine sprocket .
when it comes to fitting the grommet for the cables, I used a rubber top-hat as the outside area of the chaincase, does not allow the grommet lip to sit square.
works well https://www.ebay.co.uk/itm/254515406378?_skw=rubber+top+hat+grommet&itmmeta=01KB2XKTKG218NT7WGN59TKAF9&hash=item3b424ce62a:g:QU0AAeSwkjdosFdl&itmprp=enc%3AAQAKAAAA0FkggFvd1GGDu0w3yXCmi1cmDu1HtqSwCcPLJjfFFJXy%2B3g6f2eB%2BUrDi%2FsUAIEZSNs7YvKj54QjWyq3Chs5VobBrlEjVqyW0MNDpSNNhMo9ooZWmzZA8zSsRDvSvBqV3lsZ3Z%2FfKgajYxbkcrgc3lNt2utqeHtvZem5utC5KoUXa5Bdag6%2F3nr7eaWDwQCAwvTC5YNbYI2D%2FAm5CIKKYcg9FRUdHcXtM7fgQSw2owZ9ekjey5Vv4ZRE7qnq%2FmyRuRgHYrQHfDPcWV%2BqZUjNZh0%3D%7Ctkp%3ABk9SR4aqz93YZg
Just an observation born of many years fixing BSA and Series Land Rovers:
99% of dimensions are fractional inches. Most everything is inches or half inches and few things are 1/16ths or 1/32nds and annoyingly there are few 1/64ths. It pays to covert all those metric units to imperial fractions. Yes this is a pain as calculators don't help and it seems that engineers in the 40-60's preferred fractions over decimals but what it does is give you a feel for what is going on. If the end result is a whole fraction in imperial there's a good chance its right.