Hi Bill
The camshaft nut should be fully tightened!!
It sounds like the key has not been seated correctly and is being pushed back up against the cam bush???
The cam position is governed by the shoulder on the cam inside the trough (crankcase) and the gear being fully pulled onto the cam, up to the outside bush.
As the gear is being fitted keep a close watch on the key to make sure it stays fully seated in its keyway!
If the cylinders have already been fitted, DO NOT TAP THE GEAR ON!!
This will drive the cam inwards and the key will damage the bush, If the cylinders are off you can put a piece of alloy or soft copper between the leftmost cam and the case while you fit (hit
) the gear
I know there are special tools to fit Truimph cam gears so as to avoid the same damage issues to the cam bushes
but I have not seen a BSA version????
I have always fitted the gear when the cylinders were not yet fitted!!
HTH
John o R