Nissin brake piston size

Hi,

Can anyone tell the the size, particularly the diameter, of the brake pistons in the early, Nissin, front brake caliper? My bike was first registered in 1994 and has a drum rear brake, I've always understood it to be a CB 500 R.

This afternoon I tried to strip and rebuild the front brake caliper and discvered that the pistons I'd been supplied didn't fit. But the seals from the same source did... The seals were a bit awkward to fit but I've always found brake seals tricky and once these were in place they looked fine. But when I tried to fit a piston it wouldn't get anywhere near, it wouldn't go through the cut-outs in the side of the caliper opposite to the pistons.

Measuring the pistons I found that the ones I removed seemed to be about Ø25.354mm using a micrometer, but it was a 0-25mm mic and so out of range and uncalibrated. And looking back on it now I'm not certain I read the mic right.

But the ones I'd bought were definitely bigger; too big for the mic. My vernier calipers (also uncalibrated) measured them as Ø27mm.

I looked in the Haynes manual and that just made me even more confused.
It says "CB500-R and T models ... 26.935 to 26.968mm", so that would agree with what I've been supplied.
It also says "CBF models - with ABS ... 25.400 to 25.450mm" which is close to what I've measured, but mine isn't a CBF and definitely doesn't have ABS.

I've always bought pads described as for the Honda CB 500 R 93-95, and they've always fitted. And I'm confused by the fact the seals fitted. I can imagine you might be able to squeeze seals in a bit but not that much.

I've email'd the shop who supplied me the pistons and seals and maybe they'll be able to clear it up but if anyone has a proper measurement I'd appreciate it, if only to clear up my confusion; the shop is shut until into 2018.

Oh, and if anyone is wondering, I cleaned up the old pistons as best I could and re-fitted them, with the new seals. I'm midway through trying to get the air out of the line but it seems to be working OK.

Thanks for your help,

Doug.

When I stripped my Nissen calipers I measured the pistons according to the Haynes Misinformation manual and can confirm that the measurements are correct. I have a 1994 CB as well and it is indeed the "R" model. I assume (as you so) that the pistons that have been supplied are the incorrect ones. As you mention the old pistons fit so the new ones are oversize. I also cleaned up the old pistons and refitted them, with new seals of course and it's been fine since. As regards bleeding the line I used a vacuum bleeder (you need a source of compressed air) and once the fluid starts to come through I go back to manual bleeding. The brake was still spongy the next day so I bled it again and some minute air bubbles came out and it was sorted then. Hope you get is sorted soon mate.

Hi Skyerae,

Thanks for responding, but can I check something?

You said that you measured your pistons and that the measurements are correct - do you mean the Haynes manual measurements or my measurements? The first time I read it I thought you meant mine but then read it again and thought maybe you meant the Haynes was correct (it does happen occassionally!).

I bled the brakes as best I could using the old fashioned manual method and as expected the brake is spongy but overall much better than it was before. I expect I'll get a bit more air out this weekend.

Thanks again for your help.

The shop I bought the bits from said they'll take the pistons back, but they're confused as to how I managed to fit the seals. They seem to think that the seals were the right size for the pistons they supplied so should have been too big to fit in the calipers or work with my pistons.
I forgot to mention that all the bits I'd bought were pattern, maybe next time I need to go to Honda.

I ended up buying some Ø25.4 mm pistons from ebay and they fitted fine. I did use the seals that had been supplied for the bigger pistons, the dust seals needed modifying but the main pressure seals went in with a bit of effort. That was over a year ago and the brakes have been OK since.

I recently purchased a used front brake calliper, mainly because the pins and other bits were getting worn out on my original one. I've not properly stripped and cleaned it yet but I did put a vernier calliper across the pistons and it indicated Ø27 mm.

This replacement calliper has "K 27" stamped into it, there's nothing in the same location on my original calliper. I wonder if there was a change in size? Once I change the calliper it won't matter but it seems odd.