Bore finish for master cylinders/calipers

[Matt Gignac]

For the teams that have done custom master cylinders or calipers, what surface roughness of the bore did you aim for? The mfg. of our seal does not give any concrete values, but it seems that par for the course is 10-20 micrometer RMS.

Alternatively, has anyone measured the rougness of a purchased MC/Caliper?

Is turning and reaming sufficient, or do you also hone the bore?

Matt Gignac
McGill Racing Team

[Matt Gignac]

For the teams that have done custom master cylinders or calipers, what surface roughness of the bore did you aim for? The mfg. of our seal does not give any concrete values, but it seems that par for the course is 10-20 micrometer RMS.

Alternatively, has anyone measured the rougness of a purchased MC/Caliper?

Is turning and reaming sufficient, or do you also hone the bore?

Matt Gignac
McGill Racing Team

[rjwoods77]

For calipers it doesnt really because the piston doesnt ride against the piston bore. Industry typically uses 0.0025" on the size (.005" overbore. The only time the pistons would touch is if something causes it to moment about the seal like debris, suck pad, etc but even then it is not even that much. Also the psitons are ususlally harder than the caliper material so galling/scoring isnt a problem if this does happen.

You are on the ball with the master cylinder. The surface finish has to be good but ridges or oddball machine marks(chip gets jammed during cutting) are the dangereous parts. Particulate in the fluid is what messes up master cylinder seals. Specially aluminum from a not properly deburred and cleaned casting.

[rjwoods77]

Sorry for the spelling errors. I shot something quick at work. I meant to say .0025" on the side.

[Matt Gignac]

Makes sense for the caliper. I guess since the seal is in the bore, the critical surface is the OD of the piston, and this can be ground really smooth.

My main concern is wear of the seal for the master cylinder, with the seal on the piston, and also, wear resulting from when the seal goes over the inlet port. Currently, I have the piston OD .002" less than the bore (as per seal mfg directions), and the way I have my piston/pushrod supported, I don't think that bore scoring/scratching will be a big issue.

What I'll end up doing likely, I'll build it, test it on a bench, and then see if i need to make it smoother or not.

Matt Gignac
McGill Racing Team

[rjwoods77]

You do have to grind the pistons. We were making they at SSBC with a 25 surface value that we got from the tooling that was cutting them to begin with if. They are pretty forgiving of surface finish. I do have one question. How did you derive your seal groove geometry? I hope you didnt just cut a square groove into it and I really hope you arent using an o-ring. If you have any furthur questions call my cell at any time. I know a whole bunch about this. I just sent it to your PM.