Twin Dating Questions x 2

[peterg123]

I's like to ask two items...

(a) What year did they stop using 4 screw Rocker Box caps?

(b) That years are the frame with the large "donut hole" shape on the rear downtube, as illustrated here, please

I might also ask, why the donut shaped fixture, what attaches to it??

https://i.ebayimg.com/images/g/G5UAAOSw ... -l1600.jpg

https://i.ebayimg.com/images/g/G5UAAOSw ... -l1600.jpg

[dave16mct]

I think the 4 bolt rockers were used up to 1952. The donut hole in the frame was for an airfilter hose to pass through on a twin and was used up to 1954. There were a few other differences along the way ie in 53 a lug on the front downtube for a cylinder head stay. For '55 the lug for the airfilter hose was a completely different shape.
Dave.

[Groily]

https://www.jampot.com/article_read.asp?id=20
answers quite a few questions Peter.
Probably the best short essay on the changes made over the years and it covers your engine issues. A quick re-read didn't seem to mention the air filter frame hole thing though - but I might have missed it.

On your parallel thread ref availability of usable cranks, maybe if the usual suspects in the trade can't help, an advert here or in the Jampot? There must be some around and about. The last 500 one I got came from Russell Motors - but it was a loooong time back.
The 650 'problem' (which I think was somewhat exaggerated) was part-solved back in the day with the change of material from meehanite to nodular iron, and more recently by the Club's having a new batch made. There may still be a small handful of them available (but no good without the rest of a suitable engine, which is another whole series of hassles / decisions / compromises).

[peterg123]

Thanks Groily and Dave 16....... very helpful so far.