[billhaworth@aol.com]

My update: I did make an aluminium cover for a felt disc which surrounds the mmagneto sprocket and attached this cover using the tapped hole (the one for mounting the outrigger bearing)
I haven't got so far as getting the engine started yet, so will post an update when I am able to decide if this is ahelpful addition. Or otherwise!
Thanks to for your comments and observations

[phutton]

As a very crude and simple seal, I pushed a flat piece of flexible plastic onto the onto the magneto sprocket boss. The diameter is set by the need to adjust the chain, but with that proviso, you can make it big enough to cover the hole in the back of the case. As John says, the tapped hole is there to hold the outrigger bearing and its housing on the M90/80s.

[Thomas]

I am with John! Hence, no further comment.

And as others have pointed out there is a threaded hole in the case below this opening.

If I understand right you talk about the thread for the outer cover bolts.
Cheers, Thomas

[singleminded]

There was never a seal fitted to the mag drive opening. The hole at the back on the outside was to mount an outrigger bearing that supported the mag pinion on the model 90's.The only time i had a problem with oil spewing out was when the breather internals made a bid for freedom.
I suppose you could use the outrigger mounting to hold a felt seal in place around the mag pinion, if you used a proper oil seal you would never be able to adjust the chain without causing a leak. The other cause of a timing case leak is when the oil drain hole blocks up.
  regards John

[billhaworth@aol.com]

I know this topic has been addressed before, but I am just reassembling a Model 9A engine from boxes of parts and there is no seal to close the gap around the magdyno drive sprocket where it goes through a substantial hole in the back of the timing chain case. (why is this hole so big?)
And as others have pointed out there is a threaded hole in the case below this opening.
My thinking is there must have been a seal of some sort there because the gap is big and because controlling the crankcase pressure with the breather valve makes little sense if there is a huge leak into the space (via the drain hole into the valve timing chest at the bottom of the timing chain case, though this is only 1/16" diameter)
I am not aware of any effective seal method that could be fixed with one screw, allow the sprocket to rotate, enable the magdyno to be moved back and forth to adjust the timing chain tension and seal against the back of the chain case.
I see nothing on my 1932 parts list.
I am looking for any information to help resolve this and get something workable there in this rebuild!
All help appreciated