Wrong Assumption???

Special K · 04-18-2002, 10:44 AM

I just got done milling down a set of 320i brakes for the '73. The spec in the tech article said to bring them down to .587" +/-.005. Since I did the milling myself with a 1" back spotface cutter and an electric drill they didn't come out absolutely perfect. The mounting lugs on each caliper are the same, but one set of lugs (l/h caliper) is about .002" beyond the lower limit set by the tech article. Here's what I'm thinking. Since the calipers are the "dual piston" type, won't any difference be made up by the travel of the caliper pistons (i.e. the outer piston will have to travel out .002" farther than the inner piston)? .002" isn't much at all, but that makes a total of .007" from ideal (as stated in the article). Anyone see a potential problem that I'm overlooking? I guess if it's that critical, I could always make some shims out of some .005" stock to make up the difference, but I'd rather not have shims in that area.

Ron Meier · 04-19-2002, 03:24 PM

Since no one has put up an answer:

If you leave it like it is, it will "cock" the caliper relative to the centerline of the rotor. The only thing for sure is that it will wear the pads funny.

No proof guesses:

Could "side load" the caliper pistons and cause a piston to stick or the seal to leak.

My opinion:

Cut shims and put them in to "square" the caliper to the rotor ~ They ain't gonna fall out.

Special K · 04-19-2002, 10:31 PM

Thanks Ron, the calipers would be square (parallel) to the rotor, just off center a few thousands of an inch. I made sure that each mounting lug was the same, but I got "mill happy" on the left caliper and took one lug down about .005" more than I wanted. To even things up I milled down the other lug (on the same caliper) to match. I could put some .005" shim stock between the caliper and the spindle, but I'm "Ass-uming" that the caliper pistons will even things out by one (outer) coming out of its bore .005" more, and one (inner) staying in its bore the same amount. At least that's what I'm trying to convince myself will happen (Knock on wood, or rust, or whatever it takes to appease the 914 rebuild God's).

Ron Meier · 04-20-2002, 03:46 AM

You solved your own problem. While wanting to maintain the mounting ear as thick as possible, I do not believe that 0.005" less is going to seriously threaten the integrity of the caliper.

When mounted on a '70-'72 (doesn't require machining), the caliper is skewed about 1 mm? relative to the centerline of the rotor. According to the tech article, placing a "wavy washer" between the ear and mount restores the centerline match.

Good Luck!