Hi, I'd like to contribute about these copper rocker box gaskets. I've been making and selling these under the Ebor Bikes badge for a while, both from my website and eBay UK. From what I've read on this thread, many people but not everyone have found joy with them. When problems occur, I believe they come from the annealing which to date has been done by the customer, and it is possible that if significant amounts of oxide have to be removed as a consequence of prolonged heating, that the gasket thickness may no longer be satisfactorily uniform.
So as from now, the gaskets I sell are both made from C106 copper (as before) but are now also vacuum annealed after the laser cutting. These are about as soft as you can get copper, a Vickers hardness test of them is typically 42.5 on the Vickers HV1 scale. Remember that, as supplied, C106 copper has an allowed hardness range of 40 to 160, so the annealing is necessary to get them really soft. That it is done in vacuum means that no clean-up job is needed.
Copper is a good gasket material for this application. It's used with success on Triumph twins and triples too. It is able to withstand the lateral forces imposed on the BSA twin rocker-box well. It does pay to have the two joint faces in good condition. Flatting the faces down with abrasive paste on such as a sheet of plate glass can work, though it may end up with a joint face that is smooth but not very planar. My favoured way is to run the end of a really sharp end-mill over the joint faces on a milling machine, removing just a thou or three till it's cut all over. If the engine has a light alloy head, and this is not done, attention should be paid to ensuring that the cast-in bronze inserts in the head for the rocker box bolts have not been pulled slightly out above the joint face surface. If this has happened, they can be flatted back using a smooth file very judiciously.
I think that generally there's no harm in using small amounts of a sealing compound with the copper gaskets. This will seal any minute nicks in the joint faces, but if the faces are in really good condition I don't think it's vital.
I have from time to time wondered about another approach: machining a small groove round the joint face of a rocker box, to take 1mm or 1.5mm O ring cord (probably Viton). I do know that japanese bikes using this type of arrangement do NOT exude oil! I have a CNC mill that could do the job and it could be easily programmed off the drawing for the gaskets, but it's only about number 423 on the list of projects
I hope that helps,
Jon