Safe maximum diameter when boring/honing brake and clutch cylinders

[jbrunken]

Does anyone know how far over-size these cylinders can be bored to and maintain a proper seal? I'm working with both Ate and FAG cast iron parts.

For reference, my brake master and clutch slave currently measure 20.5mm and the clutch master measures 19mm.

I've searched around the web, but I've found nothing useful so far.

Thanks -

[Jeroen]

Stock sizes are 19.05mm for the clutch master and 20.64mm for the brake master. Looking at the design of the seals there is hardly any room for boring these. If the inside is badly rusted, replace. When I overhaul one of these, just did a clutch master the past weekend, usually very slight honing suffices. It's more like polishing up the surface and cleaning it out, getting rid of gunk.

[jbrunken]

Thanks for the reply, Jeroen.

I wrote Ate to find out if they had any information that they could share, and this is the response I recieved. I was also asking if they had a roughness spec (Ra) for the honing:

"There are no maximum machining dimensions for clutch cylinders, these are not repaired. The same generally applies to cylinders with aluminium housings or also to brake caliper housings. I do not have any information on roughness."

They included a chart that lists their tolerances for their brake cylinders, and it contained the following text (translated from German via Google Translate):

"This tolerance table only applies to main and wheel cylinders made of gray cast iron!
 Not for master and slave cylinders
 Not for disc brake cylinders (calipers)
 Not for aluminum alloy cylinders"

BTW - the tolerance listed was ~.1mm

Their response made me curious as to why they would produce repair kits for them. I'm at a loss as to when a cylinder would require replacing the internals without servicing the cylinder bore. A worn cylinder would need to be trued, and a pitted cylinder would need some amount of boring and honing to restore the cylinder surface.

I sent Ate a follow-up question regarding the repair kits. I'll send an update if they reply.

I have BMW's service manual for the E21s. It contains the diameters of the brake and clutch cylinders, but does not list any tolerances for them!

In my case, my brake and clutch cylinders appeared to be in good condition when I took them apart, but after cleaning and dipping them in Evaporust (the outsides were rusty), I found that the surfaces of the bores had very fine pitting in them. Their roughness is like a piece of worn, fine sandpaper. After viewieng them under magnification, it appears that I would need to remove at least .05mm to get them back to a proper surface.

I bought a 400 grit honing brush to try out with them, but it wasn't able to correct the surface.

I typically avoid rebuilt hydraulic parts, due to bad experiences with previous parts that were bought from an auto parts store. I was hoping that I could clean these up, refinish the outsides, and move on, but it looks like they're going to be replaced now.

[jbrunken]

I received a response from Ate regarding the repair kits:

"The repair kits could be used if only the sleeves were defective, but the cylinder bore was still in order or reworked to the extent possible."

To be clear, "sleeves" in this case refers to a sleeved cylinder, per a further clarification from them. Personally, I've never seen any of the cylinders for these cars sleeved, but I haven't seen many of them either.