Machining the Ovality out of TD/TF Brake Drums

4 Jan

I had a very frustrating couple of years trying to find the cause of a judder transmitted through the brake pedal of my TF.

During the investigation I talked to many “experts” and followed up on several suggestions, one of which was to check the brake drums for ovality – the fronts were fine but the rears had wear ridges and a few thou of ovality. The drums were in good condition and the splines excellent – amazing really on a car some 55 years old. So I decided to machine them back to true as replacements are not available.

The solid rear drums on the TD & TF are driven by the half shafts but locate on the oil seal collar – a split cone that tightens up in a similar way to a collet. To replicate this and to machine the drums concentrically I made a mandrel as shown in the drawing and picture accompanying this article.

The shaft of the mandrel is a sliding fit in the bore of the splines. The split cone is tightened into place through a washer and by a locking nut. This arrangement fixes the drum braking surface parallel to the mandrel. The mandrel and drum are rotated between lathe centres for machining.

To remove the wear ridge and machine out the ovality, I increased the i/d of the drums by 40 thou over the original diameter of 9 inches. The drums are 3/8” thick at the outer rim and reducing this by 20 thou (approximately 0.5%) will not, in my opinion, compromise their mechanical strength.

After machining and using the original brake shoes, the adjusters ran up 18 clicks (out of 20 max.) to lock-up. I solved this issue by having thicker linings bonded to the shoes. This was done by Brake Re-Lining Services, Unit 2, West Point Industrial Estate, Penarth Road, Cardiff, CF11 8JQ. Telephone 029 2070 2900 – contact Richard who is very helpful.

Since asbestos was banned and steel drums and pads are used more-or-less exclusively on modern cars, brake friction materials have become much “harder” and consequently more abrasive when used with the cast iron drums on our cars. Richard recommended using a “softer” woven compound and I have covered several hundred miles with this material fitted to both front and rear shoes. The brakes are very positive and efficient, they bedded in nicely and the rate of wear is not excessive.

The mandrel now sits in my tool box and should anybody wish to borrow it please contact me via email at: keithdouglas1938[‘at’]btinternet.com

Did machining the drums solve the judder? No, but perhaps the rest of the investigation and solution of the problem will be the subject of another article.

Keith Douglas


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