Reriveted brake drum - tolerances

[dykker5502]

Hi, disassembling my broke transmission I discovered that the rivets in the brakedrum was loose. So I got a friend of mine experienced with both hot and cold rivetting as well as general machining to help me.
I'm pretty satisfied with the result...

(Yes, I know I have work to do with the lugs)

My buddy are a little concerned because when we turned the rivets and bearing surfaces we measured the axial throw as well as the varians in diameter of the drum. It turns out that the drum is very slightly "trumpetshaped" so that the outer diameter (open end) is 0,012" (0.3 mm) bigger than the inner diameter (closed end). Also, we exchanged one rivet at the time to keep the axle as close to centered as we possibly could (rather than removing all the rivets and then rivet it together again). Despite our effort the drum throw also around 0,012" (0.3 mm).

Is this a problem?

My buddy wants to put it back in the lathe and just take the slightest chip to make it concentric and square. I on the other hand do not want to make the drum any thiner.
It's a wide 26/27 drum.

[Dan McEachern]

I suggest you put the drum on 1" tapered arbor between centers and then check the low drum bearing surface for runout before drawing any conclusions regarding brake drum runout. Bottom line is .012" drum runout is too much, but you need to make sure everything else is running true before you start cutting metal. Another thing to consider is how the driven plate locating surfaces- both axial and radial are acting as these are critical in order for the driven plate to run true. The 4th main surface on the output shaft of the driven plate must run true within about .002" or so when everything is assembled. Dan

[dykker5502]

Good point, Dan.

[dykker5502]

Nobody else that want to chime in?

[Allan]

Michael, in use,the brake drum and output shaft are one unit. I index them as I assemble them before balanci g them. If you check the run out on the assembled pair of components, you may find you can improve the situation by trying up the interfaces.

With easy access to an excellent machine shop, I have them grind the drums on their c.f.
Ankshaft grinder. The finish is so much better than a machined surface.

Others do it differently.

Allan from down under.