Magneto mounting studs

“The magneto is attached to the crankcase by 3 nuts- current models use two nuts also a bolt in the lower position-with its nut inside the timing chest. The studs also the bolt are waisted (reduced diameter) which permits limited movement of the magneto, when the fixing nuts are released.”

As given in my original (and very well stained!) factory manual on twins.

The idea of the bolt at the bottom with nut inside timing chest is that it is otherwise almost impossible to release/ tighten in situ.

Hope this helps with what you need Tony.

Treat yourself to a BSA A7/ A10 extended magneto nut. Its about 6" long and means you can easily adjust mag anytime you want to. You won't regret it.

Lots of places sell them. 

George.

Thanks George, I'd forgotten about these. Not looking for total originality, so I just bought 2 from Feked.com, £4 each!

Just found these notes in the DSNs.

Early Dominators had their magnetos held by 3 external waisted studs (T2245), positioned on the outside of the Timing crankcase shoulder. This also applied to the later Distributor models. However, after 1964 engines saw the lower of these studs replaced by a 5/16” shouldered bolt, with 1/4” thread which was inserted through from the inside of the timing cover. This item making the magneto chain a little easier to adjust but relying on friction to hold the bolt in place.

The big advantage of having 3 waisted nuts is that not only could the chain tension be set but the body of the magneto could be rotated  to help fine tune the ignition timing. 

The attachments may be of some help.

If you have a dummy timing cover the suggested distances are fine. Otherwise add just a tiny fraction of a mm more.