Sandcast Master Cylinder Piston Bore Size - smaller than later versions?

[Joe K]

Hello All,

I have a master cylinder, which I believe is from the year of the Sandcast bike, 1969.  I purchased a re-build kit for it on Ebay.  I have rebuilt at least 2 other CB750 master cylinders before, but I am having trouble with what may be an ealier version.  The spring, piston and seal slide into the bore hole OK, but the washer and rubber fitting with a metal ring in it are a little to large to fit in the bore hole.  Has anyone else ever had the same problem and could offer some advise on what I should do?

Thank you,
Joe K.

[Steve Swan]

I have not heard of a smaller bore on any m.cyl; early or late.  i fitted a K&L rebuild kit to the early m.cyl which came with 232.

[Joe K]

Thank you for letting me know, Steve.  I am still scratching my head on this one.  I purchased the kit so long ago, I cannot verify the manufacturer.  I don't have the packaging either.  All the part look OK.  I will keep at it and try to make it work.

Joe K.

[Steve Swan]

Joe, my disclaimer:  there's a helluva lot more i don't know than i do know.  and what i think i know, we sometimes discover new info.

[Joe K]

Well said, Steve.  I took my Dremel tool with the round sanding attachment and bored the aluminum enough to get the washer and rubber thimble into place.  I ripped the rubber thimble though and now have to purchase a new kit.   Enjoying myself, though.  These are fun bikes to work on, but sometimes they are a real wrestling match.

Joe