Mystery long 7/16" nut

[Invicta]

Can someone please identify this long 7/16" nut from the box of bits of a 1949 AJS model 18. It is 1.160" long and is threaded 7/16" BSC for 1/2" of its length. The rest is bored out to 0.530" diameter. I thought that it might be the inner chaincase spacer, but my parts book says that should be 7/8" long so it is not that. the fact that it has no rust on it indicates that it is an internal fitting.
Rebuilding a bike from a boxes of bits is rather like doing a jigsaw puzzle when you dont have a picture. A lot more rewarding though , as you don't have to take it apart again once it is finished.

[Steven S]

021230 nut inside chaincase. Not original fitment for 49 but a sensible retro-fit.

[Invicta]

That is what I thought at first until I checked the length and apparently the spacer for the inside of the chaincase should be 7/8" long and this nut is over a 1/4" longer than 7/8" .

[Steven S]

But remember that spacer relies on an additional std2 nut, so the combined length equals the long nut.

[Invicta]

Thank you for that bit of information Steven. I forgot about the nut. I just measured the thickness of a new nut 3/8" that came with my new chaincase stud and adding that to a 7/8" spacer makes 1 1/4"which makes this long nut a bit short , but if I add one of my new washers it will be 10 thou too long. So my question is just how critical is this chaincase distance between the inner and outer cases?

[dave16mct]

If you're using the old type rubber seal the halves have to touch but if using the new club seal (and why wouldn't you?) you need to leave a small gap. You will need to experiment with washers to get the correct fit.
Dave.

[Steven S]

The differential in length is because earlier chaincases had a different seal which fitted between the joint, requiring a bit more length on the spacing. When the spacer-nut was used, the chaincase faces were metal-to-metal with a seal going over the joint, so the overall depth was slightly shorter.

Regardless of which seal you're using (especially relevant if you're using one of the new improved seals), just add or remove a washer till you're happy there's no bowing of the outer case.