How far to brake pistons travel?

[Anti]

I have some old Endless 6 piston calipers I'm looking to set up on my project.

I disassembled them to work out how I'd need to adapt them to fit the rotors going onto the car and this is where I'm confused. As they arrived, all 8 calipers had the pistons protruding 8mm from the caliper. This was I thought as much as they would retract.

After popping a set of pistons out I found that their recess into the caliper is actually exactly as deep as the total height of the pistons themselves. So I'm wondering how much space I need to allow below the piston for the fluid to travel as this obviously dictates where the piston will sit and as such the brake pad and then how much I have to space apart each side of the caliper to fit around the wide rotors. It will also make mounting the calipers less painful as there are issues clearing the wheel face too.



This is a basic profile of the piston recess in the calipers. 27mm deep in total with the fluid feed at the bottom (obviously). At the base of the recess is a 4mm tall section that is cut with a slightly wider bore. Can I assume that up to this point is where I should allow the pistons to travel to and any further could cut off fluid flow?

The only reason I can see to allow 8mm as they were is a larger cushion of fluid allowing for more of a heat sink?

Let me know if I'm on the right track.

Cheers.

[Brian P]

Are you asking how much clearance the face of the piston needs to the base of the bore inside the caliper?

Even microscopic clearance between the face of the piston and the bottom of the cylinder will be sufficient for fluid to get in and apply hydrostatic pressure. The slightest bit of clearance needed in order to allow the brake calipers and brake pads to be assembled over the brake rotor, will be sufficient to achieve sufficient clearance below the piston for fluid to get in there.

The machined pocket at the base of the cylinder makes it easier for the fluid flow to get in and out and ensures that the piston, when fully compressed, does not block the fluid port AND does not block air from getting out when bleeding the calipers.

The piston needs to go out far enough to accommodate worn-out pads on rotors worn below minimum thickness without the seals letting the fluid out.

The piston needs to go in far enough to bottom out but not uncover the seals in the process.

The caliper needs to have some wiggle room laterally in order to accommodate imperfect mountings. This means the combined thickness of new rotors plus new pads shouldn't bottom out the pistons on either side of the caliper. The slightest bit of clearance is sufficient for the calipers to work. A little more clearance to allow some wiggle room makes the mechanic's job easier.

[Anti]

That was a bloody terrific explanation. Just what I wanted to know and the news is good too. Thank you so much, that was great.