Brakes - Turning brake shoes that are too thick
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
NOTE: The following information is about removing excess material from the brake shoes, not the brake drums. I do not ever recommend turning the brake drums on these machines. I suppose, very occasionally, it may need to be done. However, I would certainly never perform this as part of any
It is an unfortunate fact that many new, replacement brake shoes are too thick. Mount them to your brake plate, and they simply will not fit back into the brake drum.
One option would be to turn the brake drum. But that is a horrible idea. The drums are already thin and no one is making replacements drums or hubs. I would never turn a brake drum just to fit thicker brake shoes. Never. Take good care of your drums!
The much better option is to remove the excess material from the brake shoes.
In the past, I've done this laboriously using a belt sander. It is very dusty and the results are anything but professional. Others have taped 60 grit sandpaper inside the brake drum, then sanded the shoes to fit. This is less dusty, but even more labor intensive.
Fed up with these methods, I sent my new brake shoes mounted to my brake plate to Charlie Mullendore of Antietam Classic Cycle. He has a large lathe pretty much set up just to perform only this task. He clamps the brake plate to the axle, then mounts the axle in his lathe. He then removes the amount of material needed.
How much material needs to be removed? Well, that depends on the size of the ID of your corresponding brake drum. Since I do not have a 12 inch dial caliper (6 inch is the largest that I have), I took my wheel (complete with drum) to my local machinist. It only took him a couple of minutes to provide me with a good, accurate, absolute measurement of the ID of the brake drum. In the case of the 850 T cush drive rear wheel I have installed on my Ambassador, the ID was 8.68 inch. The ID of your drum is likely to be different.
Thanks to Locky who posted this information on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In (mostly) Locky's own words:
...in regards to your question about the specs for the brakes, what I think the brake shop is after is the internal diameter of the drum. I'm not sure about a 2 leader brake but the 4 leader is 219.9 mm - 220 mm or 8.657 inch - 8.661 inch. The brake shoes need to be turned while fitted to the backing plate to 219.88 mm - 219.95 mm or 8.653 inch - 8.659 inch. But they need to be turned while the cam is in the open position and the cam operating lever at 119.8 mm - 120 mm or 4.716 inch - 4.724 inch from the anchoring lug. That will make sure your break shoes are the same radius as the internal hub when the breaks are applied. The measurement to lock the leaver to is taken from the back of the lever where the cable attaches to the back of the cable anchoring lug. You will have to make something to lock it in this position then have it machined, and that how its done. Hope this helps, Locky.