Bone stock 1966 300 hp small block 327 - has an auxiallery single belt pulley (GM#3751232BB) that mates into the recess of the standard two groove pulley (GM#3744043BA). The single groove pulley has the shape of a bowl with a flat bottom and corresponding mounting holes. It mounts directly to the two groove "pancake style pulley" - both of them are then bolted to the harmonic balancer. The two groove pulley mounts to the balancer and the single groove sits furtherest out from the balancer.

Enough of the description. I would expect that when these two pullys are mated to each other that there would be no movement or rocking that would occur. It appears that the mating surface of the two groove pully is not perfectly "flat" and thus allows for a rocking motion to exist between the two pulleys when the are mated together by hand only.

I'm wondering if this rocking is normal. Shouldn't the fit between these two pulleys allow for no movement? Otherwise, great care and attention would have to be given when attaching the pulleys to the balancer to be sure there is a uniform gap between the pulleys or there is likely to be a "wobble" to the whole pulley assembly when running and that spells trouble.

Is there anyone that can answer my question? Is it normal for the 2 groove pulley mating surface to not be more or less perfectly flat?