Brake Drum Research

[Harry S.]

Morning all,

Just doing some research. Does anyone have a picture of a 63-passenger car (impala) front brake drum? The face where the lug nut holes are. Part number and casting number if you have them.

Looking to compare it to a 63 Corvette front drum.

Thank you.

[Duke W.]

Re: Brake Drum Research

The drums were the same and I recall the casting number ends with 671. Drums finished for the J-65 metallic brake option have a "X" stamped on them and a slightly thicker web. They have different finished part numbers, but those were only on the box, not the drum

Some years ago I did research on OE and replacement drums. I was going to write an article for The Corvette Restorer, but never got around to it. I'll look for my research notes over the weekend, and if I find them I can provide more inf0.

This has never been a "hot topic" because the drum data cannot be seen during judging, so it's only of interest to we "history freaks" of the drum brake era.

Duke

[Harry S.]

Re: Brake Drum Research

Duke, I'll post info as it becomes available.

I have two NOS drums, PN on box is 3830166, cast number on drum is 3828671. Both stamped with an X. I had one stamped with a F that I sent to a member that needed one. I did not write down the cast number, should have.

From eBay today ---> 1963 Regular drum, PN 3758538, Cast Number 3813851 or 3S13851 ( hard to read ), F stamp

59-64 CHEVROLET IMPALA 63-64 CORVETTE NOS GM FT BRAKE DRUMS PT# 3758538 | eBay

[Duke W.]

Re: Brake Drum Research

Found my notes; 3828671 is an OE 2.75" front drum casting for both Corvette and full size pass. car (Biscayne, Impala...) This casting was used to produce both base and metallic (J-65) drums. Even though casting numbers are the same, castings for base brakes have a 0.109-0.119 thick web, and those cast for metallic brakes have a 0.125-0.135" thick web. It's always been my observation that these thick webs have "X" stamped on them and this was probably done when the webs were formed in order to differentiate them from base drum webs. My notes indicate that these web thicknesses are listed in the AMA specs

Finished drum numbers are as follows

Base: 3830166

Metallic (J-65): 3830167

I don't have any info on the 3758538 Drum including whether it is finished for base or metallic brakes or even thickness, but given the part number it looks like a late fifties release, so it could have been an earlier drum casting that was released for full size passenger cars that was replaced by the 3868671 casting for '63

I do have a photocopy of an old part book page, Group 5.809, July 1 1963, that has drum listings back to the 40s including the following drum part number listings. I did not see 3758538 listed, so I must have been superseded at this point

59-60 PASS. w/metallic linings.............. front...................3764682

59-62 PASS, (exc. metallic linings).........front...................3771853 (The last three digits are somewhat obscured so may be wrong)

61-62 PASS w/metallic linings...............front....................378 2993

From the above there was clearly a change (or multiple changes), but was there a new casting number, or not? Also, what about '61 without metallic linings? Parts books often read like a mystery novel.

Then later on the page:

63 PASS. CORVETTE w/ metallic linings, (exc.Sp. Perf.).......front..............3830167

63 PASS. CORVETTE (exc, metallic linings, Sp. Per).............front..............3830166

So it appears the ...671 casting may have replaced an earlier casting for '63. Assuming the Ebay listing is really a 2.75" front drum it may be functionally correct for a '63 Corvette, but it is not OE.

Also it doesn't make any sense that the part number starts with 37 and the casting number starts with 38.

Maybe Joe Lucia and give us some more history.

Duke

[Tim S.]

Re: Brake Drum Research

Pardon my ignorance. I have a question, as I am working on a 63 metallic brake car. When you speak of the "web", what portion of the drum are you speaking of?

Thanks,
Tim

[Duke W.]

Re: Brake Drum Research

It's the center part of the drum, which is a piece of steel of the thicknesses I mentioned. It's inserted into the mold and then the molten iron surrounds the outside edge and bonds to it as it solidifies. That's why they are sometimes referred to as "composite" drums because they are made up of two somewhat different materials - steel and cast iron.

If cast in one piece the web would likely have to be thicker in order for the molten iron to completely fill such a large narrow cavity, so composite drums are lighter.

Duke

[Joe L.]

Re: Brake Drum Research

Found my notes; 3828671 is an OE 2.75" front drum casting for both Corvette and full size pass. car (Biscayne, Impala...) This casting was used to produce both base and metallic (J-65) drums. Even though casting numbers are the same, castings for base brakes have a 0.109-0.119 thick web, and those cast for metallic brakes have a 0.125-0.135" thick web. It's always been my observation that these thick webs have "X" stamped on them and this was probably done when the webs were formed in order to differentiate them from base drum webs. My notes indicate that these web thicknesses are listed in the AMA specs

Finished drum numbers are as follows

Base: 3830166

Metallic (J-65): 3830167

I don't have any info on the 3758538 Drum including whether it is finished for base or metallic brakes or even thickness, but given the part number it looks like a late fifties release, so it could have been an earlier drum casting that was released for full size passenger cars that was replaced by the 3868671 casting for '63

I do have a photocopy of an old part book page, Group 5.809, July 1 1963, that has drum listings back to the 40s including the following drum part number listings. I did not see 3758538 listed, so I must have been superseded at this point

59-60 PASS. w/metallic linings.............. front...................3764682

59-62 PASS, (exc. metallic linings).........front...................3771853 (The last three digits are somewhat obscured so may be wrong)

61-62 PASS w/metallic linings...............front....................378 2993

From the above there was clearly a change (or multiple changes), but was there a new casting number, or not? Also, what about '61 without metallic linings? Parts books often read like a mystery novel.

Then later on the page:

63 PASS. CORVETTE w/ metallic linings, (exc.Sp. Perf.).......front..............3830167

63 PASS. CORVETTE (exc, metallic linings, Sp. Per).............front..............3830166

So it appears the ...671 casting may have replaced an earlier casting for '63. Assuming the Ebay listing is really a 2.75" front drum it may be functionally correct for a '63 Corvette, but it is not OE.

Also it doesn't make any sense that the part number starts with 37 and the casting number starts with 38.

Maybe Joe Lucia and give us some more history.

Duke

Duke-----


I can't be a lot of help on this one. As far as brake drum casting numbers, I gave up on trying to figure those out a long time ago. There were quite a few of those and I've never been able to co-relate them with particular finished part number(s).

I'm not going to get into the C1 series brake drums because there were lots of different part numbers and it's very confusing. Of course, some of these would be forward compatible with 63-64 Corvettes. However, I will address the one linked in Harry's original post. The GM #3758538 was initially catalogued for 1959 passenger cars front brakes and that's the ONLY application it was ever catalogued for. It was discontinued in October, 1961 and replaced by GM #3771861. Of course, it might be forward compatible for other applications but I cannot determine that. As far as the casting number on one of the drums being 3813851 while the finished part number is reportedly 3758538, that sort of casting number-to-part number relationship, while unusual, is possible. However, it's not possible in this case. That's because the 3758538 was discontinued way before the 38xxxx series part (or casting) numbers were ever released (except for some 383xxxx mostly engine-related part numbers released in the early 50's.). So, the particular brake drum pictured could not have originally been supplied in a box labeled 3758538. The seller does make note that one of the drums was "slightly used" and, thus, its origin is questionable.

As far as the 63-64 front brake drums, it's like this:

1963-64 with standard brakes used brake drum 3830166. This part was discontinued in February, 1972 and replaced by GM #3872326. The latter was discontinued without supercession in November, 1987

1963-64 with J-65 brakes used brake drum GM #3830167. This part was discontinued in April, 1970 and replaced by GM #3872384. The latter was discontinued without supercession in March, 1979.

Curiously, 1965 with drum brakes may have used the GM #3872326 from the beginning.

So, what's the difference between the above part numbers? I really have no idea.

Addendum:

I should also have mentioned that the part numbers I provided above for 1963-64 Corvettes were the same as those used for 1963-64 passenger cars

[Dorsey F.]

Re: Brake Drum Research

To add more. Confusion, EBay #3830166.

[Duke W.]

Re: Brake Drum Research

I have two NOS drums, PN on box is 3830166, cast number on drum is 3828671. Both stamped with an X. I had one stamped with a F that I sent to a member that needed one. I did not write down the cast number, should have.

As I previously stated, my understanding is that if the web is stamped with a "X" the drum should be for metallic brakes and the web thickness should be 0.125-0.135", which should have been finished for metallic brakes, part number 3830167. The base brake drum would be 0.109-0.119".

Also for what it's worth the metallic drum surface finish should be 20 microinches as stated in the 1963 Corvette Shop Manual. The non-metallic drum surface finish is 60-120 microinches. I'm not sure if these surface finishes can be visually distinguished as I have never seen both new examples side by side.

So can you measure and report the web thickness of the drums you have with a micrometer or caliper? It would be best to measure at three or four points and check for consistency.

P. S. Thanks, Joe for chiming in. Thinking back I recall the primary reason why I never wrote the intended article for The Corvette Restorer was that there were too many dead ends/unanswered questions, and now I know that if you couldn't sort it out, there's no way I would have been able to.

[Duke W.]

Re: Brake Drum Research

1963-64 with J-65 brakes used brake drum GM #3830167. This part was discontinued in April, 1970 and replaced by GM #3872384. The latter was discontinued without supercession in March, 1979.

Curiously, 1965 with drum brakes may have used the GM #3872326 from the beginning.

So, what's the difference between the above part numbers? I really have no idea.

I think the difference is that the last drum part numbers have the maximum diameter embedded in the casting. In the mid seventies I did a complete overhaul of my SWC's J-65 system including drums, shoes, and springs that were still available from GM, So I got the 3872384 front metallic drums. I don't recall the casingt number (and I'm too lazy to take off a wheel to look), but I'm pretty sure that both front and rear drums have the casting number embedded.

I recall that the shoes and drums came in the old brown/yellow GM logo boxes and figured they must have been sitting in a warehouse for at least ten years.

Do you remember when those old box designs were replaced with the white,blue design?

Duke

[Joe L.]

Re: Brake Drum Research

I recall that the shoes and drums came in the old brown/yellow GM logo boxes and figured they must have been sitting in a warehouse for at least ten years.

Do you remember when those old box designs were replaced with the white,blue design?

Duke

Duke-----

A LONG time ago. The transition to the green/blue/aqua packaging began about 1965 or earlier.

[Joe R.]

Re: Brake Drum Research

I have a pair of brake drums with casting number of 720004.

What are those for?

Thanks, JR

[Joe L.]

Re: Brake Drum Research

I have a pair of brake drums with casting number of 720004.

What are those for?

Thanks, JR

JR----


It's not a Chevrolet part/casting number if that's the entire casting number. However, if there's a leading "3" (perhaps difficult to discern), then I would say it would be a Chevrolet casting number probably from the early-to-mid C1 period.

It could also be that they're not GM parts, at all, and the number is some sort of aftermarket part/casting number.

[Dorsey F.]

Re: Brake Drum Research

Great, even if confusing, information. How about rear drums? Nothing at all shows in my 67 P&A for rear with metallic linings.

[Duke W.]

Re: Brake Drum Research

Even if you had part numbers they're decades gone from GMPD, but FWIW...

C2 Rear metallic/base drums (2"wide) part numbers (also applicable to C1 front and rear)

59-62 3755504/3752622

These may have also been used on early '63, but the ...504/622 was replaced by 3828687/3828686, likely before the end of '64 production.

The ...687/686 was replaced by 3869537/3868801 about the time that the new FMVSS went into effect (1971?) that required permanently marked maximum ID (11.090"), so these probably had new castings with the max ID embedded. Earlier unsold stock may have been removed and hand stamped with the max ID.

The ...537 was the last part number for the 2" metallic drum, but the base drum number changed to 3885944 circa 1977 and this was the last part number for base 2" drums.

Note that the above are part numbers, but not even Joe was able to determine casting numbers for the various part number over the years.

I'm not sure how long these 2" drums were used on full size passenger cars, but it looks like all the metallic drums were gone from GMPD stock by the early eighties

Duke