Hotrodding drum brakes on a 1963 Rambler American

The original brakes on this car were 9" drums, with shoes 2" wide, terrifyingly small on today's highways, and I plan on driving to work (100 miles a day) on Los Angeles freeways. That's 139.5 sq.in. total (front plus rear!) braking area, or about 35 sq.in. per wheel -- about what today's motorcycles get at 1/5th the weight.

Clearly an upgrade is called for. AMCers generally bolt on late-model (sic, this means late 70's/80's) disc brakes. But purely for aesthetic reasons I want to go with hot rod drum brakes.

[NOTE: Scarebird disc brakes work just fine on this car; i ran them for a couple of years. though the information here on hot-rodded drum brakes predates this, i've since moved on to the Rambler Roadster project on which i've moved back to the drum brakes -- yes they were that good.]

I really want to stuff 10 inch drums in here, but they collide with the odd little steering arms. I may find a way to get them in there, but for now I'm sticking with 9 inchers.

First step: brake upgrade

I got backing plates from a 1976 Gremlin from an AMC-lister, which have about 30% more shoe/drum area than stock. I had a local shop hot-tank and media blast them, then I welded up the wear grooves in the shoe contact points and ground them down, and painted them with Eastwood's Chassis Black paint.

Part Part number
Front wheel cylinder Raybestos WC37014 (left), WC37015 (right) 1-1/8" bore

These just bolt on with no modifications necessary to the car. The Gremlin hub is identical to the early American hub, uses the same bearings, and bolts right on. They don't fit with the original steel wheels but that's moot, since you can't even buy tires to fit those anymore! 9x2.5 brake drums, shoes and other parts are still easily available, as they were used on millions of American (and some European) cars in the 1960's and 1970's.

I achieved perfect front/rear balance by simply using 1976 Gremlin wheel cylinders front and rear. These bolt in with no modifications (though I did replace all the ancient, corroding steel lines when I did brakes, tires and wheels).

Second step: ventilation

An old hot rodder trick for hopping up (drum) brakes, besides bolting on the largest ones that fit, is drilling them for ventilation. It was difficult to find any hard information, but I puzzled it out from a combination from brake experts C.H. Topping's website and first principles (and guesswork).

Ventilated drum brakes are designed to pump air through the otherwise-sealed interior of the brake drum, primarily to cool them, and secondarily to pump out water. Since the drums are spinning at a high rate of speed, holes drilled around the working face of the drum acts as a centrifugal pump, drawing air through the thin gap between drum and shoe.

Enough holes are drilled across the face of the drum such that every bit of the shoe surface is swept by a hole. The holes are not drilled in a row across the drum, but are staggered around the drum radially, in what ends up as a spiral pattern. This prevents weakening the drum (there's a hole only every 3.5 inches or so) and imbalance.

I did the backing plates first, while I had the front end apart for overhaul.

Backing plate modifications

The backing plate mod consists simply of dozens of #1 drill bit holes around the edge of the backing plate, such that they align with the edge of the drum, and three 1-1/4" holes down low, between the shoes, and tack-welding some screen over them. Simple.

I did the backing plates months before I did the drums (for reasons of severe laziness and impatience) and drove 5000 miles or more with ventilated backing plates and unmodified drums. I didn't get to do before and after tests, but I can tell you this: in dry weather I never, not even once, experience hot-brake fade. And I commute 95 miles daily round trip on Los Angeles freeways in or near rush-hour traffic. They stop hard and fast, truly no fade. How much ventilation vs. oversized contributes is impossible to tell.

However with just the backing plates drilled, and the drums unmodified, they are lousy in wet weather. All drum brakes are -- anyone who's driven drum brake cars on a regular basis (most American cars had drums through the late 1970's) knows this from experience. If the backing plate ventilation made this worse overall, it's hard to tell. Certainly water can enter more easily; but it can exit just as easily.

I can tell you this for certain: there is a lot of airflow in there! When I pulled the drums off for drilling I got a most pleasant surprise: almost zero brake-dust build up. Check out the photo to the right, those brakes have more than 6000 miles on them and they look one day old -- clean! I did not clean them off in any way for the photo; that's how it was after pulling the drum. So far so good.

(March 2009, after a rainy Los Angeles winter) But with the drum-drilling below, nearly all wet-drum problems disappeared. When they did get wet, there was a tendency to get grabby left and right, as normal drums do when they are drying off after a soaking. But the tendency is vastly reduced, and and momentary. Gone is the need to drag your brakes to dry them off -- they dry off driving normally, and in seconds. Braking hard while wet actually does braking; there's about the same braking loss as fully wet discs.

Last year, I would take the 1963 Classic wagon (with it's Bendix discs) in bad rain. Now I don't hesitate to take the drum-braked American.

April 2009: Here's a nice before and after -- fronts and rears with 6000 miles or so on them. Fronts are modifed as described here, rears are stock/unmodified.

Front drums, and no, I did not clean them off in any way.

Rear brakes. To be fair, the rears I did not rebuild, but cleaned off (with brake cleaner and a wire brush) when I got the car, about the same time I did the fronts; they had been very recently done with new drums and shoes, good springs etc.




Drum modifications

The drum modifications were more involved, in that it took some planning to lay out the holes, though in the end it was quite simple.

The basic idea is that you put enough holes lined up across the working surface of the drum that in one drum revolution, each brake shoe is completely swept by the holes.

The first problem is that .75" of the shoe rides on the drum under the stiffening ridge and double-flange of the drum. I did not want to drill into the stiffening ridge, which would weaken the drum. Outside of the ridge is the double-flange, which isn't wide enough for #7 holes. This part of the drum was done by hand, last, and easily enough. See below.

Nine holes 0.201" diameter (a #7 drill bit), 0.2 inches apart, covers most of the 2.5" wide shoe, the part that is under the "flat" of the drum.

The first step was to mark off the drum for drilling. Using a tape measure, find the outside diameter of the drum. (These 9" drums are 10" or so outside (31" circumference), and are actually tapered a bit; they're castings.) I made a pencil line as my zero reference (see photos for the pencil marks). From that point, I made marks every 3.375" or so that divided the circumference of the drum with nine equal-spaced lines. These marks are not critical, but I stayed within 0.1" of correct. These marks locate each hole on the circumference, but not laterally (across the face). That's done in the next step.

With a small machinist's square, I measured the depth of the brake drum working surface (eg. the shiny part) innermost edge, measuring from the outer, flanged, edge of the drum. I transferred that distance with a pencil mark near my previous zero-reference mark. Out 0.1" (half a hole diameter) from that is the center of the first, inner most hole to drill.

I then made a cardboard template that I could push up against the drum flange as a reference, and starting with that first hole mark, put nine dots on the card, every 0.2". Then I put the marked card up to every radial pencil mark (made in the first step) and center-punched for drilling; first hole on the first mark, second hole on the next mark, and so on. This creates a "stairstep" or spiral pattern of holes.

(I was originally planning on making a strip of paper 31" long by 3" wide, all marked out with the pattern; that would have been easy to photograph for illustration purposes here, but the long paper ended up too unwieldly to actually work with.)

With those holes drilled most of the shoe area is swept, except that under the flange and stiffening ridge. The ridge is simply left alone; that totals 0.5" of the 2.5" shoe left unswept. For the outer double flange area, I drilled, by eye, two 1/8" holes 180 degrees apart through both flange lips, one in, one out. This sweeps that inner .25" or .375" of drum. (See photo at right; red arrows point to one of the two.)

Last but not least, there is a few tenths of an inch of drum inside, past the edge of the shoe, that doesn't touch any shoe, and collects dust. I also assume this is a major source of water collection; any water that does get in here would simply stay pinned there by centrifugal force. Measuring once again with the square, and transferring the distance to the outside, I drilled two holes, 180 degrees apart, in the center of this area. This will pump in more air and pump out any water that collects. (See photo at right; finger points to one of the two inner holes.) March 2009: I'm convinced that these two small holes contribute greatly to extracting water from the drum.

Now I have 13 holes comprising a centrifugal air and water pump, throwing air out the drum, drawing in through the backing plate rim and center holes.




From the outside, it doesn't look like much, the few holes are almost unnoticable. The drum photos, including these below, were taken a week after the drilled drums were installed. While it is hardly enough time to show any results, note that the holes have no effect on the shoe surface. These shoes have probably 6000 or 7000 miles on them total.









Master cylinder, brake lines and hoses

Though this car wasn't maintained too badly over the years, every component in the brake system was worn out, including the steel lines and flexible hoses.

The steel lines were all rotten-looking, and I'm glad I pulled them all out. (I just hand-bent long stock lengths from NAPA, did as few double-flares as I could get away with. These cars are pretty simple and it wasn't much work.) There were many sections close to failure; many crunched/snapped in half when I bent them after removal. It wasn't leaking though, but why live with bad brakes?

The master cylinder was leaking (no surprise). The 1963 master was $220, a 1964 master cylinder was $27 new, not rebuilt. Guess which one I bought. The only difference is the outlet size, which I fixed with adapters. All other dimensions are identical inside and out. The replacement master cylinder fits lots of AMC/Jeep applications.

Part Part number
Master cylinder Raybestos MC36237 has 5/16" inverted flare connections
Tube adapter Dorman 43301 3/16" inverted flare female tubing (3/8-24 thread) to 5/16" flare (1/2-20 thread)

The flexible brake hoses are the same front and rear, NAPA had them. In these older Americans the hose attaches through the inner fender, instead of on a bracket as in later years. I don't know if the hoses here were original (they were old and stiff) but they looked like ordinary AMC front brake hoses with a plain washer with a 9/16" or whateveritwas hole.









Measurements, mockups, comparisons...

Below is some information I gathered while working out brake solutions. It seems few people hack on this chassis much (and even fewer drive them in modern heavy traffic) so if this seems really basic, it is.

In summary:

Concavity for hub position, 9x2.5 drum Concavity for hub position, 9x2.5 drum Note the difference in overall height; less visible is that the hub face portion of the drums is the same.
Stock drum height measurement off the deck. Stock drum height measurement off the deck. (Inside-to-deck measurement; these junk drums are damaged, I keep them for this sort of work only.)

Those simplistic off-the-benchtop measurements work because the dust seal lip that engages the backing plate is the same on all drums. I checked.

PS: The rear wheels don't have any of these clearance issues, though the stock American wheels would not fit anything but the stock 9x2's.

10-inch drums don't fit







I really want 10" brakes front and rear -- with drilled ventilation they would be close enough to disc performance for me, and higher Cool Factor. They're not even cheaper as replacement drums are getting expensive.

I have brakes from the front of a 1964 V8 Ambassador (10x2.5) and 10x1.75" from the rear of a 1976 Hornet. These give 168 total square inches braking surface, but the larger diameter will make for a big difference; the're probably 1.5 to 2 times braking power. Cool!

The problem is that the 10" brakes do not fit the front! The early Americans (mid-1950's Nashes with new fenders -- literally) steering geometry has the tie rod ends right where the big brakes want to live:

(The solution is to simply space the brakes, and spindles, out enough to clear the tie rod ends. This will change steering geometry and possibly cause clearance issues. I've been too lazy to tear everything apart, again, to find out, so this project will have to wait... and I'm quite pleased with how well the 9x2.5's are working out!