If this were a demountable type drum, I would agree that the hole should be sized for bolt clearance and that the clamping force alone is sufficient to hold the drum in place. Demountable type drums are clamped between the hub flange and the wheel (similar to a front drum) whereas my nondemountable type is clamped between the hub flange and the oil deflector. The oil deflector looks to be made from 16 gauge sheet steel based on my measurement of its thickness at .050 in.
My old drums have 1968 casting codes on them so I'm pretty sure they are the original drums. Based on their construction it looks to me that the original design of the nondemountable drum’s bolt hole is for an interference fit with the bolt and the bolt provides shear resistance upon braking in addition to whatever clamping force is applied to the drum once the wheel is installed.
On 2/2/11 I talked with a tech rep at Centric who said the new drum’s holes should provide an interference fit with the bolt. Centric’s specifications for this drum call for a ".53 bolt size" which is quite a bit less than the .627 in. (approx) diameter of the bolt holes on the drums I received. He was at a loss to explain the countersinking. He recommended that I return the drums that I purchased. He also said it would be several months before Centric could sort out this issue with the fabricator. I called the vendor (racepages.com) and they authorized a return and paid for the shipping. It could take up to four weeks to credit my account for the purchase amount.
http://www.racepages.com/products/?D...let+brake+drum
I hope other C20 guys find this useful. I’m considering what to do next.
Hub and Drum Details (Nondemountable - Type Drum)
[illustration below taken from the 1969 Chassis Service Manual]
6. Wheel Bolt
7. Drum
8. Oil Deflector