HOME and how to join Forum Open Area General Scott topics Think twice, act once! a Confession— Re: For Richard re case cups

Roger Moss

Hi Richard
Main bore in case as produced was 2.625″ +/- 0.0005″
Standard cup OD was 2.630″ +/- 0.0005″
Shrink ring is 0.009″ less than groove inner surface (I can not remember original figure.
Due to cases being made from aluminium less capable that we are now used to, the metal collapses over time.
Example. I warmed up a case with cup and shrink ring in situ to a fairly normal operating temperature.
When the cups are ground, the 3 security screws usually have their heads ground flush with the edge face of the cup.
They grind this face, so that when the case is turned round, it is located in the cup bore and clamped back to this face, so as to grind the opposite cup.
I use a bit of brass bar and gently tapped the back of the cup by going through the bore of the opposite cup.
The cup moved slightly in relation to the heads of the screws.
I drill a hole in the shrink ring to break it and remove it.
I warmed the case slightly and the cup dropped out
I measured the cup. It was standard at 2.630″
I let the case cool and measured the cup bore that had originally been 2.625″ It was 2.6298″. The metal had collapsed 0.0048″
The shrink ring had impressed itself into the groove face.
There was no longer the interference to control the cup at working temperature.
Scotts use to remove cups when they reconditioned engines and fit oversize cups and undersize shrink rings.
They obviously had many early development problems with this steel / aluminium interface.
I confess that I did not discover the extent of this problem till recent times.
I had an engine from Italy to rebuild.
Ressurection more like
It would have been cheaper to build a new one.
The cups fell out and with them strips of brass shim someone had used to try and give some security as the hole had stretched so much.
So. If you are going to rebuild yourself, do warm the case up and tap the rear of the cups to be sure that at something approching operating temperature, the cups are still secure.
Otherwise, some remedial action is called for if you want a good engine
Very deceptive these “Simple” Scott engines!
Kindest Regards