Brake Piston size query

[downsy]

Hi guys , I’ve recently stripped and removed my seized piston from my brake caliper ( thanks to the advice found on forum using a grease gun) . My issue is , following the comparison pic ( from previous post I ve stumbled across) my caliper looks like an early type . However the piston length is 36.5 mm , now according to Yambits this is a piston for a later model. From the pics can anyone shed any light on what piston size I should be fitting . Obviously my bike is quite mongrelish but would someone really change the sizing of the pistons ?.

[stusco]

Norbo has pistons for £20 in his shop

[downsy]

Mar 5, 2024 22:16:41 GMT 1 stusco said:

Norbo has pistons for £20 in his shop

[downsy]

I ve seen them on there but could nt see any size reference but that could ve been as I was looking at it on my IPhone on not on laptop 🤷‍♂️!. As soon as I can sis out the correct size I ll most prob be ordering from his shop 👍

[JonW]

piston length or width?

Ive seen larger pistons jammed into smaller bores, but only 0.5mm or so different.

[downsy]

Piston length , apparently early ones are 38mm dia and 34.5 mm length .
Then later ones 38mmdia and 36.5mm length ( these are the ones currently in mine ) both these sizes are relevant to 4LO models .

[JonW]

Look at where the seals are, ie their depth. measure where the piston would be on full stroke with well used pads. if the smaller ones will span that gap with some overlap then you'll be ok i would think.

The extra length is only 2mm... but hey 'it is brakes'... so dont listen to the internet.

I would also say that if the caliper's depth can take the longer piston, just use the same as what came out if it worked fine before. 2mm extra length if it doesnt protrude would be fine etc...

Both these are the same width, so...

[bezzer]

Mar 6, 2024 0:50:38 GMT 1 JonW said:

Look at where the seals are, ie their depth. measure where the piston would be on full stroke with well used pads. if the smaller ones will span that gap with some overlap then you'll be ok i would think.

The extra length is only 2mm... but hey 'it is brakes'... so dont listen to the internet.

I would also say that if the caliper's depth can take the longer piston, just use the same as what came out if it worked fine before. 2mm extra length if it doesnt protrude would be fine etc...

Both these are the same width, so...

I’m guessing the O/P wouldn’t know if it worked fine, as his piston was seized in?

[JonW]

Fair point...

[stusco]

Mar 5, 2024 22:42:25 GMT 1 downsy said:

I ve seen them on there but could nt see any size reference but that could ve been as I was looking at it on my IPhone on not on laptop 🤷‍♂️!. As soon as I can sis out the correct size I ll most prob be ordering from his shop 👍

Send him a message maybe he can help

[tony2stroke]

Measure the depth of the bore, the piston when fully inserted should be flush with the top of the bore, so measure the bore and match the size.

[paulrs]

I can say with 100% confidence that the longer pistons do work in the early calipers, however, if the disc isn’t worn and the pads new it won't assemble because the pistons stick out a couple of mm to far to allow the pads to fit. The later caliper body is a few mm wider, hence needing the longer piston.

My bike has been running with the long pistons in the early calipers for about 25 years and the brakes have been excellent. Recently got hold of some later calipers and currently rebuilding the brakes using the long pistons from my old calipers as they were in perfect condition.

Regards
Paul

[mangocrazy]

My 350LC is one of the early models with the slightly shorter pistons, and I bought a pair of stainless pistons to replace the corroded to f**k originals, but they were for the later calipers and a few mm longer. When I tried to assemble the refurbished caliper with new pads the extra height of the piston was enough to cause the brakes to bind when re-fitted to the bike.

As tony2stroke says, the piston should be flush with the top of the bore. This is easily checked by reassembling without the seals in the caliper. I had a couple of mm machined off the pistons until they were flush with the top of the bore and everything played nice.

[paulrs]

Going back 25 years to a time when I paid £300 for a complete running (but a little scruffy) 350LC and £1,500 for a very tidy 500LC, the pistons for the early calipers were already NLA from Yamaha. Back then stainless pistons were not available, or if they were i didn't know about them. Discussing this with the Yamaha spares man at the time (Granby Motors I recall) I was assured it was OK to fit the later/longer pistons and worst case rub a bit off the brake pads with sand paper if needed. Didn't need to do this as the discs were a little worn.

To be honest I was never happy with this and felt it was a bit of a bodge, so 25 years later I have finally got round to rectifying the situation 😁

Obviously todays solution would be to buy stainless pistons of the correct length, although in my case the pistons I got from Granby all those years ago are still perfect and I am changing the calipers to suit.

[downsy]

Thanks for info guys , much appreciated. I ve checked the depth of my caliper and it’s 34.5 mm so assume this is an old version . Measured the discs and one is 4.8mm thick and other 4.6 mm . So although getting close to the recommended service point reckon with new pads and 34.5 mm pistons should be fine !! .
Will checkout Norbos shop on laptop and get some bits sorted .
Paul