Hi all,

as above in the title.
having issue with one big piston in my front brake caliper. It just doesn’t want to slide into the hole and seems to get stuck about 1/5th of the way when pressed in.
the smaller piston slid in no problem at all.
am wondering now if supplier made an error in sizing ?
im replacing OEM with stainless.
seals & dust seal and piston all cleaned, lubed with brake fluid before attempted insertion.
would appreciate if someone could help , if I’m missing something.
thanks

 

You said the piston was all cleaned up, what about the cylinder in the caliper? Sometimes corrosion can get in the way if not removed. Otherwise, I would say you got a incorrectly sized piston or cylinder sufficiently damaged.. They usually drop right in as long as you stay true to the cylinder.

 

Thanks for the reply.
3/4 new pistons fit fine just one large bore piston giving trouble.
all 4 oem pistons drop right in.
Im thinking the bore must have a squared side though it slots in fine and evenly flush upto the dust seal point about 3-4mm down the bore 🤷
measures all and they seem to be the correct size . Really quite stumped.
excuse the mess of them , residual grease from the gun used to push them out.
*edit - pistons & calipers were completely cleaned after I took them apart, red rubber grease held the seals to their recess’ , and brake fluid lubed both caliper bore and piston.

from my searching (to no avail) there doesn’t seem to be much discussion/problems posted about caliper cylinders deforming.
though this is a 20 year old bike that lay neglected for years anything is possible I suppose.
possibly the aftermarket pistons are to blame, I have new oem ordered so these will tell all when they arrive next week.

 

Will be interested to hear how this turns out. I could see corrosion causing the bore to be smaller I suppose. Maybe a light run of a brake cylinder hone?