Replacing pistons and seals on Zee calipers

Afternoon,

Ive picked up some Heppe (AliExpress) replacement pistons and seals for my 640 zee calipers. I’ve cleaned up the calipers, a little mineral oil on the seals and around the piston but the piston won’t go in. Is there a knack? Or possibly the curse of cheap pistons and poor tolerance? Thanks in advance

Did try replacing an old piston with new ring and wouldn’t easily go in either. How much force do they need to go in?

Some blogs suggest silicon grease but some say mineral oil. Any suggestions here?

shimano – mineral oil.  You have to get them exactly square to go in.  Try without the seal to see if they go in then

They need to go in perfectly straight and they should slide in with just finger force.

Do you need / have you got the system open or callipers off the bike so you can displace oil / air as you push them in?

Do you need / have you got the system open or callipers off the bike so you can displace oil / air as you push them in?

Yes

Thanks @twonks, yep, calipers off, hoses off, calipers split open. Nothing blocking the ports.

Digital caliper available to measure old diameter Vs new?

Diameter are the same, got a few in with lots of wiggling. Will persevere!

My experience is that when pistons need replacing on callipers it’s usually just late to replace them, they need replacing before or what happens is that SLX callipers get bought to replace Saint callipers because the pistons are sh*gged…