Whether 0.03mm difference matters can only be determined with reference to all the original data and its unlikely that you will be able to get that.

First question though is, where did you get the 30.18 dimension from, was it by measuring the old piston? If it was then it seems an odd number. If you use the same instrument to measure the originals and the ones you have made then it does not matter that it is not calibrated to a traceable standard as you are just using it as a comparator really.

If it were my vehicle I would accept that size difference and ride or drive it without the slightest concern (The diameter difference is less than a 0.1% change) Probably the most important single factor is the actual surface finish which needs to be perfect.

Ian P

Nothing odd about 30.18mm though I suspect it should be 30.16mm which is 1 3/16" What make is the bike, would it have been imperial?

What does your insurance company say about having home made parts?

J

Edited By JasonB on 10/01/2014 20:47:56

Jason

I only have casual knowledge of motorbikes but 'fitted with disk brakes made to imperial dimensions' does not seem likely.

Good question about the insurance aspect but it must be possible to insure vehicles that the manufacturers parts are no longer available. I would rather trust parts I made myself than those from some backstreet operation.

Ian P

Almost there Clive, piston seals are generally of a square cross section, and it's the base of the seating groove in the caliper that's angled, this angle tilts the seal in the groove so that the inner edge (facing the inside of the caliper and bearing on the piston) is of a slightly smaller diameter!
It's the tilt on the seal that creates both the seal and 'grip' on the piston and also helps 'return' the piston when the brakes are released so that the pads don't bind, yet keeping the piston in close contact to the pad and preventing it from being sucked back into the caliper by the hydraulic action of the master cylinder piston returning!

Such a miniscule difference in piston size in my view ain't going to cause any grief as long as there's no excessive wear in the caliper bore where it operates, the piston doesn't seal the bore, the seal does, but the clearance shouldn't be enough to cause the seal to be deformed by a sloppy piston fit and leak!
In all my years as a motor engineer, I've never seen any caliper wear to such a degree, but corrosion on cast iron or alloy calipers and pistons are generally their downfall not wear!

As for 'home made', why not? as long as it's well machined and polished and of decent material who is to say it's not an aftermarket product, there are loads of companies making these products as conversions for the classic car market!

John

0.03mm sounds a bit like 0.0011" thats one and a tenth thouths in English. Just how accurate do you think they need to be?

All machined parts have a tolerance …. a thou undersize is nothing, now if they were a bit oversize I would be more worried as they would be likely to sieze in the casting as the pistons heat up in use. As long as they are smooth then I for one would say they would be fine at the size they are. If I was a production engineer and you wanted me to put a tolerance on them then I would say they should be the nominal size +000" – 0.002" or 0.05mm in metrickery.

Mark,

It may add confidence to this discussion with some measurements from salvaged brake pistons from our old Landrover.

These were for drum brakes, but the principle concerning lip seals still applies. The larger are 31.66-31.68 mm diameter, the smaller are 25.32-25.33 mm in diameter. There were 4 of each size. They are hardened with a chromed finish.

The brake cylinders in which they operated were all aluminium alloy, it was those that corroded from the conbined effects of road salt and agricutural manure spray that found it's way into the drums. a devastating mixture. It would eat holes in a chassis in a few seasons!

I hope that helps with the tolerance concerns.

Brian

.