distributor shaft play

does anyone know the amount of shaft play i should have in my 3874598 distributor? apparently there is what looks to be about an 8th of an inch of up and down play. that seems excessive to me. i guess i should also describe why we are looking at the distributor. we have been experiencing a detonation at a certain rpm...no amount of retarding the distributor seems to correct it. we are running the highest station octane we can get, and it's without the vacuum advance on. i know i need to get a tach and be able to give an rpm amount, but without it i can say it's at the same rpm range, and it isn't a stomp on the pedal test, it's a gradual increase in speed when test driving. this is on a 66 225, the distributor is from a 76 225.