caliper piston sizes & binding

[fattyjr]

Well just overhauled my brakes on the tiger. New pads , seals and stainless pistons.  Put it all together, went for a run and brakes binding.

Pulled them apart and couldn't move the new pistons.  I had to use the brake levers to push them out.  Cleaned up an old piston and it slip in and out like a glove.

Got the micrometer on them and found the old piston is 26.94mm dia and the new ones are 27.01mm.  0.07mm bigger.

Anyone else had similar issues

[Sin_Tiger]

Yet to fit mine, who did you get your SUS pistons from?

Any evidence that they are not parallel?

Got access to a lathe?

[Bixxer Bob]

Are they not a ground finish Sin?  Or were you going to suggest high revs and fine wet and dry?? 

[fattyjr]

I got a machine shop at home and at work.  I checked to make sure they are parallel.

They guy i got them from charged me £9 a piston vs. £21 I have seen elsewhere.

I can only assume that they didn't allow a tolerance for heat expansion etc.

[Sin_Tiger]

Are they not a ground finish Sin?  Or were you going to suggest high revs and fine wet and dry?? 

They should be. I was going to suggest checking them with a DTI to see if they were actually round.

You've got more resources than most but I think you've probably answered your own question there and I suspect BB's first comment regarding the surface finish is probably the most likely culprit

[fattyjr]

Well I have been talking to the guy who manufactered this pistons and he just wants to argue the toss.  Just doesn't want to accept that using his pistons are LOCKING up my brakes where the OEM pistons are just fine.

I have ordered some pistons from squaredeals to verify the sizes. but the guy just doesn't want to listen. 

Does anyone know of any place online that shows actual piston sizes?

[JoeDirt]

Normally, when making sliding fit your piston is slightly smaller than the nominal diameter. If I had a machine shop make pistons for me... I would stay under the 27.00mm on the piston. I think your guy thought he was doing you a favor by leaving you a little on the "fat side". If you put your manufactured piston in a lathe and hit it with scotch bright or emery cloth while the work is spinning at a high rate... it would knock off that extra 0.01mm.

Below picture is a "perfect world" sliding fit:

[fattyjr]

Well, I finally got through to the seller and he sent me another full set of pistons that are machined to match the OEM pistons.  I only mentioned working clearances and thermal expansion and he sorted me out.