C3 rear brake caliper inlet.

[Tom B.]

What size are the threads on a C3 rear brake caliper inlet? I have one on my '81 that I cannot get the hard line to thread into without it cocking. I won't force it because it will just cross thread or ruin the line fitting. I can't see that the threads are damaged. They look just like the caliper on the other side of the car. I have tried both ends of the line with the same results. The line will thread into the other side with ease so it's not the line. There must be an edge or something there that is sending the fitting off in the wrong direction. I'm thinking that if I can run a tap in there it will straighten out the problem. It has to be on the first thread or so.

Tom

[Joe L.]

Re: C3 rear brake caliper inlet.

What size are the threads on a C3 rear brake caliper inlet? I have one on my '81 that I cannot get the hard line to thread into without it cocking. I won't force it because it will just cross thread or ruin the line fitting. I can't see that the threads are damaged. They look just like the caliper on the other side of the car. I have tried both ends of the line with the same results. The line will thread into the other side with ease so it's not the line. There must be an edge or something there that is sending the fitting off in the wrong direction. I'm thinking that if I can run a tap in there it will straighten out the problem. It has to be on the first thread or so.

Tom

Tom------

Most, if not all, late 1980 thru 1981 Corvettes used calipers with METRIC brake line/ bleeder valve fittings. This was a VERY dumb thing for GM to do and, for 1982 they went back to SAE. Sometimes, these calipers can go through rebuilding operations and end up on non-80-81 Corvettes and really throw someone a curve.

The standard bleeder valve threads are 5/16-24. The metric threads are 8mm X 1.25. If you were to look at these threads they would appear virtually the same. But, they're not.

[Tom B.]

Re: C3 rear brake caliper inlet.

Thanks Joe. I have run into that metric or SAE before. Last night I finally found on NAPA that they listed the specs for the brake hoses to the trailing arms. Most places just list the application without details. Since the fitting on both ends of the caliper hard line is the same I discovered that it must be 3/8 24NF. I grabbed a 3/8 24 NF nut and threaded it over a same size tap by hand just to be sure it was clean. Then I found that the hard line would easily thread into the nut. I secured the caliper on the bench and carefully run in the tap. It went in straight but I could feel some roughness. It only took a couple of turns to clear out whatever deformity was causing the line fitting to not thread in straight. Got it back on and bled and I hope I'm done with brakes on that car for another 37 years.

Tom

[Mark E.]

Re: C3 rear brake caliper inlet.

Good deal. Don't forget to flush every two years to minimize corrosion.