Discussion in 'XJ Technical Chat' started by jag3, Jul 30, 2021.
Thanks for all the suggestions. I'll give them a try until I get it flush.
That is even better. I have a wind in set for rear calipers which use for handbrakes on cars.
do you have a 6" or larger bench vise?
Ok. I tried all suggestions. I even remembered I had a caliper piston tool (a lot ofsearching but I found it). Nothing worked, it still wants to be high in the front. Any other ideas? The pistons and seals are brand new from Len (xj4ever) but still no go.
No bench vise but might have access to one. What would it be used to do?
Would you be able to use a G cramp?
take it out and see why it can't go flat, the bottom of the bore should be flat. could the bleeder be in the way?
if it all looks good lube it up and get a bigger hammer
Idk man, would the piston really go that far into the bore if it was cock-eyed? Think about it. I could be wrong though.
I tried a c clamp and socket to try and get even pressure and tried c clamp without socket and moved as close to the front as possible and still too high. It seems like the back bottoms out and the front won't go anymore. No bleeder in the caliper, its all apart. I used caliper grease on the piston and in the bore and the bottom of the bore looks flat. I could mic the piston and see if its even but no matter how I spin the piston its always out in the front.
take the seal out and it should just fall right in,flat, yes, no, maybe ?
To be honest, when I rebuilt my calipers (xj750) they did not go in completely "straight" meaning, like yours, when they were bottomed out they didn't sit completely even all the way around. I don't think the castings are "flat". I could be wrong.
I removed both seals and dropped the piston in and it's high in the front, exactly the same as with the seals and just the picture I posted earlier. I put the piston in the other caliper and it was the same high in the front, flush in the back. I then put the old piston in both calipers and it was the same way. Is it possible the front sits high and made this way? It would seem odd to me that both calipers would be bad in the exact same way. Thank you all for your help.
if you were to push the high side till it touches and the piston was now straight, would the whole thing fit back on ?
maybe you don't need that last 2mm
Not sure I understand. I have to wait to put the calipers back on the bike because the forks are off and the front end is apart. I'll put the caliper back together with the front being high and see if it will slide over the rotor if that's what you're referring to.
I'll put it all back together with the pads tomorrow and slide it over the rotor and post what the results are. I would think it would slide over since the piston will be as compressed as possible. I'm concerned about braking, the pads wearing even and not ruining the rotors. Thank you everyone for your help and advice.
the bench vice will allow you a straighter insertion, and will prevent the piston or caliper from inserting crooked, and wont "pinch" the seal.
First you lube the piston seals in the caliper with some good grease, I recomend the grease in the link below, its what I use. Then you place the caliper into the vice, align the piston straight, insert a long deep well 1/2" drive socket into the piston face, then slowly tighten the vise.
it will insert smooth and you dont have to use much forct to do it. once you get the piston past the two seals, it will push in the rest of the way like butter...
This grease will not contaminate the brake fluid..
NOTE: the piston will not always go fully into the caliper, some models I have rebuilt hav about 3 to 5 mm still sticking out. this keeps the mating surface of the piston out far enough to align with the pad face.
Push it in till it stops and no further.
I looked closer at the piston bores and the back is deeper than the front of the bore and the rim above the outer seal is thicker than the front rim. I tried to measure the thickness and it looks like the back is approx 3.2mm and the front is approx 1.8mm. I dont see any way for these to lay flat and don't recall how they were when I pulled them apart.
You know, there should be no reason to push the pistons all the way in or make any contact with the back of the bore. They only need to be straight when contacting the rotor. Push it in squarely, not all the way in, and call it a day. Once mounted and bled there will be a fluid filled gap at the backside anyway.
The castings are machined before being ready to use. The back of the bore is square with the bore. The outer face of the casting has a radius to it.