Chaterlea John was correct as usual.
A couple of weeks ago I got around to turning up a mandrel to locate the mag end cap on its cam ring bore. I kept it a thou or so oversize and warmed the casing so it’d grip the mandrel. Also cut a slot for the camring locating screw, to act as drive peg should the cap try to slip whilst being machined. The idea was to rebore the armature bearing so the camring would be concentric with the armature. I was amazed how much out of true it all was. The bearing bore clocked .005” (.0025” off centre) but the real culprit was lack of squareness. I had to take .020” off the flange that locates on the mag casing just to get it to clean up. I also deepened the bearing bore a similar amount, as I didn’t fancy making any more of those outer shims.
Today I re-assembled the mag and remounted it on the engine. Success, the points gaps are now equal and the timing between cylinders spot on (instead of 20* difference). What amazed me was the amount everything was out of kilter. This was not wear or abuse, but sheer bad machining from new. Casting the cap would be dead easy and machining it should only require two or three jigs, so why Lucas made such a pig’s ear defies understanding. Maybe there was crud in the jig that didn’t allow the casting to sit properly when the ‘technician’ did his particular part of the manufacture. Wonder how many came out like that before quality control or the shop’s fitter spotted the problem
