Hotrodding drum brakes

15 nov 2021

Early Rambler drum brakes, at 9 x 2 inches, are adequate at best, but drum brakes have advantages for modest braking needs, mainly simplicity and the built-in servo action keeps pedal effort low. However the skinny brakes on these cars is not confidence-inducing.

A rational upgrade is a Scarebird disc brake conversion and for most people most of the time that is the only sensible option. Disc brakes do require more pedal pressure, or a master cylinder swap for a smaller bore. This is easy to do especially if you are already messing with new and upgraded brakes. Power brakes are not a practical option for 1961-1963 Americans, and virtually impossible for 1960 and older.

However another option if you have access to the donor car parts is to install later (1970's) 9 by 2.5" drum brake systems. These bring modern levels of braking to these small cars, retaining the simplicity and low pedal pressure of modern servo brakes. 9 by 2.5"e; parts are plentiful; the smaller front drums are no longer made (and are a different, incompatible part than the also-common 9 by 2" rear drum brakes).

And with minor modifications, these larger drums can acheive performance nearly equal to small discs. Those modifications are documented below.

Donor parts

My brake donors have been 1968 Rambler American, and 197x AMC Gremlin. Both use 9 by 2.5"e; front drums. Get everything, including the drums even if you know the drums need replacing. Front brake drums are an assembly of drum and hub, and you need to reuse the hub.

For projects like this I take all donated parts to bare metal, cleaned, wear grooves welded and ground flat, then painted. You only need to do this once (or

Both used wheel Cylinder Raybestos WC37014 (left), WC34015 (right), 1-1/8" bore. Front/rear braking balance is excellent since the donor and target cars are so similar.

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.

Note that the original American wheels, from the 1950s and earliest 1960s, will not fit the wider drum. If you want to retain the AMC hubcaps you will need to obtain later AMC wheels, probably a good idea anyway, as the original wheels are a very narrow 4" wide, newer are 4.5 to 5" and easir to fit tires to. If you absolutely must retain the original steel wheels then got the Scarebird route.

Hot-roddind drum brakes

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).

The modifications turn the brake drum itself into a centrifugal pump, a pattern of holes spread evenly around the drum, such that the entire shoe surface is swept by the holes in every rotation. As the drum rotates air and dust is spun outward, exiting via drilled holes. The holes are small, totalling under a half-square-inch of area Of the drum's 197 square inches (for a 9 x 2.5" drum). When the brake pedal is released the shoe-to-drum clearance is typically .010 to .020" and dust and heated air on the shoe surface is swept as it is created.

Backing plate modifications

It is likely that no modification to the backing plate is needed at all, and that simply leaving out the adjuster-hole plugs is enough air inlet capacity. My most recent set I bored a 1" hole and screened it;

my first set, I drilled 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. I now consider this excessive.

The first set I did, in my 1963 American, I used to commute 95 miles/day, Los Angeles to Irvine, and through one rainy winter in 2009. 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.

I eventually drove that 1963 American Twin Stick in some casual sports car events, up and down mountain roads braking very hard, adn not once did I experience brake fade.

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, as-is drums pulled, 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 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.)

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.



Additional drilled-drum photos


Master cylinder, brake lines and hoses

For the 1963 American,a t least, the correct replacement master cylinder was $220, rebuilt, and the 1964 AMerican master cylinder was $27, new (in 2009). 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)

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!



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