How much brake lining is enough?

[1967 R502]

Perhaps this is an easy question, but how much brake lining is enough?

I’ve just taken possession of a R 69s project. The brake pads obviously have seen some use, but when I consult the Clymer manual, it says only to replace the brake pads when they look low.

That’s not too helpful.

New pads have about 4 mm, and I would judge these to have 2mm-ish.

Is there a written recommendation anywhere in MM as is normal with cars?

[schrader7032]

I can't find anything for the /2. But for my 1970-on Airhead, it lists 1.5mm minimum thickness. If you're at 2mm and you're there, seems like a good idea just to go ahead and replace the brake material.

[1967 R502]

I think you’ve answered what I suspected: there is no specification.

[niall4473]

Past a certain point, as the pads and drum wear down the contact area diminishes, at 2mm thickness, I would skim (true) the drum, get oversize linings fitted and skim them down to the exact new diameter of the drum with a .020" shim between the cams and the heels of the shoes, done that way, no bedding-in is required and the brakes are noticeably improved.
Very thin linings can split, with exciting results.

[cwf]

Obviously we're unlikely to be doing big mileages on these old bikes any more but, given the diminishing number of useable drum brakes around and the loss of metal involved in skimming, let alone cost, time and disruption, is it really worth doing unless absolutely necessary?
My 60/6 drum brake did 180,000 miles of hard city driving before it went oval. Of course, some drums will need skimming before that and, for racing purposes it might be desirable but, unless this bike is very high mileage, replacement shoes are probably enough. I used to get about 25,000 from a pair of shoes.

Charlie