New Mac's Drums Locking Up Tight

[philipswanson]

I put a pair of Mac's replacement drums on the rear of my 40 wagon. The original ones are way over .060 worn. They fit great and brakes don't grab. They spin freely until I tighten the lug nuts. Problem is when I tighten those lug nuts, the drums both lock up tight. I have noticed that when I measure the new drums and compare them to the old ones, they are not as wide across the lining contact area. My guess is they are making contact with the outboard edge of the shoe. The shoes are original 1 3/4" so they are not too wide which might cause this. Anybody else have problems with Mac's drums? They are made in the US by Vintage.

[19Fordy]

Which drum did you buy?
https://www.macsautoparts.com/ford_m...+drum&x=23&y=9

[flathead48]

I had the same problem with one of two I bought from Joblot, the problem I had was it was pressing against the backing plate. The drum was too deep, I had a machine shop take 1/8 inch off the outside edge and problem solved. Something to check.

[john in illinois]

To locate rubbing spray a thin coat of paint on the rim where backing plate goes and the area of Brake shoe edge. Tighten it up till it rubs. I use marking dye for this but paint or primer rattle can should work.

John

[19Fordy]

Or, if it's just a little too deep and you don't have a lathe you can make a shim out of sheet metal. Do not be tempted to grind the drum down unless you have a very accurate way to do it. In a pinch, a pedestal style belt/disc sander may work, like the kind used in a wood shop. Not a hand held belt sander. In the case below the backing plate lip was rubbing against the bottom of its corresponding groove in the drum, so a shim was used. Don't try to grind the backing plate lip either.

[philipswanson]

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Thanks for all the suggestions. I thought of the shim idea but didn't want to interfere with the hub centering in the middle. If you run the drum out .028" you might risk missing the edge of the hub that does the centering to the drum.

I am beginning to think it is caused by oversized linings. I ran masking tape across the new drum and installed. Didn't rotate it, only tightened. When I took it off, the tape was torn off in the lining area but hadn't been touched anywhere else. Maybe the linings are worn less on their inboard edge, causing the drum to lock up when the lugs are tightened. I am going to try the paint route next.
Thanks, Phil Swanson

[philipswanson]

Originally Posted by 19Fordy

Which drum did you buy?
https://www.macsautoparts.com/ford_m...+drum&x=23&y=9

Bought the Vintage Made in USA ones. Their P/N is 64-12905. See post above.

[rotorwrench]

Another thing to think about is condition of the hub tapers on the rear axles. If the tapered surfaces are worn on either the shaft or the hub, the hub is increasingly closer to the backing plate as they wear.

[Bored&Stroked]

Keep in mind that the correct way to do this is to have shoes 'arced' to the radius/diameter of the drum. Not only is it important that the drums actually fit, but maybe even more important that the linings have the proper arcs and clearances on top/bottom - such that they don't grab. What I normally do is take a new set of linings and the drums to a good ole' fashioned brake shop --> I have them do a slight 'clean up' pass on the drums (to insure they're perfectly round), then arc the shoes to match. Each set of shoes is arced for a specific drum (usually). Hope this helps.

[philipswanson]

Originally Posted by rotorwrench

Another thing to think about is condition of the hub tapers on the rear axles. If the tapered surfaces are worn on either the shaft or the hub, the hub is increasingly closer to the backing plate as they wear.

That's a good point but I haven't even tightened them down all the way and still have the problem. They are going to go down further when I do the 200 lbs. torque when I'm done.

[Mart]

Philip. Take a methodical approach. Remove the shoes. Fit the drum and a wheel. Torque the lug nuts. Tighten the axle nut while rotating the wheel and see if it fouls or clears. If it clears then turn your attention to the shoes.

It might be that the drum does not rotate true and will need a skim to just clean up. You might need to remove the backplate and use a dial indicator on the braking surface of the drum to check for concentricity. If the mating surface of the hub is not perfect it may throw the drum off true.

You might need to get the drums skimmed to the hubs.

What studs are you using?

Lots of variables here, hard to tell what the problem is without being able to lay hands on the parts.

Mart.

[Andy]

I like Mart's approach a lot!
I bought some Speedway drums. They would not let the hub seat because the surface in the drum was not wide enough. I had to bevel the hubs. Just a thought. I also used spray paint to find the interference. The drums would cock when tightening the lugs as one side would seat and the other side would stick up.

[philipswanson]

Originally Posted by Mart

Philip. Take a methodical approach. Remove the shoes. Fit the drum and a wheel. Torque the lug nuts. Tighten the axle nut while rotating the wheel and see if it fouls or clears. If it clears then turn your attention to the shoes.

It might be that the drum does not rotate true and will need a skim to just clean up. You might need to remove the backplate and use a dial indicator on the braking surface of the drum to check for concentricity. If the mating surface of the hub is not perfect it may throw the drum off true.

You might need to get the drums skimmed to the hubs.

What studs are you using?

Lots of variables here, hard to tell what the problem is without being able to lay hands on the parts.

Mart.

I am going to pull the linings off again as you suggested and check to see if I still have the interference. After the paint process, I see a faint line around the bottom of the recess. That would indicate the lip of the backing plate is hitting the drum. I don't like the idea of running a shim between the hub and drum. So if this is the case I will look into cutting down that lip. Maybe just scribe it and sand to the line. I don't think this is a super precision part.

[Seth Swoboda]

Another thing to consider is when doing a brake job on the rear, you will want to disconnect the parking brake at the clevis where the cables attach under the car. If you don't you will have trouble installing the drums and experience "grabbing" if you do manage to get them installed. If you do not disconnect the parking brake cable you may get one side installed but the other will be nearly impossible.

There is a technique to adjusting the parking brake when you have the drums reinstalled. Pull the parking brake handle so that it engages the first or second notch. Then get under the car and re-attach the cable clevis so that the slack is taken out. When you release the brake handle you should then have the appropriate adjustment for the parking brake to engage.

Also, if you are running the stock 1940 brake drums the drums should tighten to the axle shaft via the castle nut on the axle shaft. Installing the wheel and running up the lug nuts should have no bearing on drawing the brake drum up tight. Perhaps you already mentioned this and I missed it?

[philipswanson]

Originally Posted by Mart

What studs are you using?


Mart.

I am using the non-swedged studs that Mac's called for. Their P/N 68-1107-F. They fit very snuggly to the drum as they should.

Phil

[philipswanson]

Originally Posted by Seth Swoboda

Another thing to consider is when doing a brake job on the rear, you will want to disconnect the parking brake at the clevis where the cables attach under the car. If you don't you will have trouble installing the drums and experience "grabbing" if you do manage to get them installed. If you do not disconnect the parking brake cable you may get one side installed but the other will be nearly impossible.

There is a technique to adjusting the parking brake when you have the drums reinstalled. Pull the parking brake handle so that it engages the first or second notch. Then get under the car and re-attach the cable clevis so that the slack is taken out. When you release the brake handle you should then have the appropriate adjustment for the parking brake to engage.

Also, if you are running the stock 1940 brake drums the drums should tighten to the axle shaft via the castle nut on the axle shaft. Installing the wheel and running up the lug nuts should have no bearing on drawing the brake drum up tight. Perhaps you already mentioned this and I missed it?

I already pulled that clevis pin so E brake is disconnected. Thanks for the instructions on the adjustment. I will use them when I get this drum interference fixed. I think it is the edge or flange of the backing plate rubbing on the recess area on the drum. Still trying to figure it out.

Thanks, Phil

[philipswanson]

[QUOTE=19Fordy;1554716][B] Don't try to grind the backing plate lip either.

Why not??? It seems to me this is the best alternative. A shim might cause interference with the centering of the hub.

Phil

[19Fordy]

I used a shim with no problems. I would not grind the backing plate because it would be very difficult to make it accurate all around the perimeter. Plus, being on the car makes it even more difficult. And , it's easier to replace a drum than a backing plate if something goes wrong.By the way, I can't find that 65-12905 on Mac's site. Grinding dust will go everywhere. Not good. Just for the heck of it try shims made out of thin paper board like you see on back of note pads. Cheap and easy way to start solving the problem without damaging anything.

[Mart]

Don't rule out the possibility of a bent backing plate. That can happen too.

Mart.

[PeteVS]

I'm putting hydraulics on a Model A rear and this is a guaranteed problem situation. I had a machine shop take 1/16" off the backing plate flange on the axle housings. I then made up 1/16" shims for between the axle flange and the drum. I haven't gotten it all together yet so I'm not fully sure that it's going to be enough. (I'm using MT drums.)