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Good day, I have two brake drums. I believe the GM part number is 3830166, well thats' what it says on the rather old boxes from GM. The numbers stamped on the drums are 3828671, which I believe to be correct. The other is 3813851, this one has no adjustment knock out. Any thoughts as to both stamped numbers.
The drum part number and casting numbers are NOT the same. The same castings were used to for at least two part numbers - one for base brakes and one
for J-65 metallic brakes. The J-65 drum has a finer finish on the friction surface. Does this drum have a max diameter? Is it die stamped or embossed from the casting tooling? Also, the casting numbers should be "embossed" in the casting from the casting tooling, not "stamped". Embossed or stamped can be an important distinction when it come to maximum diameter marking.
Is the 851 casting number a 2" or 2.5" drum?
There were several 2" and probably more than one 2.5" casting numbers. Both the casting numbers and part numbers also appear to have changed as the result of the FMVSS requirement to include the maximum diameter, so it's important to know whether the max diameter appears or does not appear, and whether it is die stamped or embossed in the casting.
I do not show the 166 finished drum number as a service replacement in the late seventies AFTER the FMVSS max. diameter requirement when into effect, and I do not know the part numbers for the pre-FMVSS drums.
From what I've been able to determine, old stock was die stamped with the max diameter and assigned a new part number. At some point the casting tooling was modified to include the embossed max diameter and a new casting number assigned, and the finished part numbers may have again changed, so it can get very confusing to figure out what you have.
The drum with the 3813851 casting number is most likely a 62 passenger car drum. Starting in 1963, the brake adjusting hole location was moved from the backing plate to a "knock out" on the drum. It can't be a later unit because the part number changed again in 1965 and beginning in January of 1971, the "Max Diameter" was cast in the drum.
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