SOHC.co.uk Forums > CB500/550

Master cylinder cap seal

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Bryanj:
Can i say told you so, as long as you give partnumbers and dont tell them how old the bike is most dealers will order parts

fogrider:
The new seal was in a Honda packet , from a Honda main dealer. It does'nt appear to be the right one and is a poor fit. One way up it leaks, the other it does'nt fit properly. The original was a two sided soft lip seal, the shallow lip sealing in the centre of the recess in the cap. The new seal is a hard seal, lip one side only. It is too small in diameter too . As it has the correct part number, I'm wondering if the master cylinder is a wrong look-alike ?

The reservoir is 43mm inside diameter, can someone check the int dia on their their 550f please ?

Regards, Terry.

Seabeowner:
Just measured a couple 43.3 and 43.5.
You do have the diaphragm do you, I think that does most of the sealing?
https://www.cmsnl.com/honda-cb550k-1978-usa_model7222/diaphragm-non-oe-alternative_45520300000p/

fogrider:
Many thanks for the measurement, looking good there, and yes, the diaphragm is missing !.  It's how it was when I bought it, I assumed it was original and realised it was missing when I had a better study of the parts diagram this afternoon.

That explains  everything. Back to the main agent.....

Regards all, Terry.

kent400:
By some coincidence I had to replace the master cylinder diaphragm on my 400 four early this week as the original had rotted and almost falling apart. I'm assuming that the master cylinder is the same as fitted to the 500 four.

I ordered the genuine diaphragm part number 45521MJ6006 and plate 45520300000, the plate is nothing more than a large plastic washer. The purpose of the washer is to secure the diaphragm in the master cylinder cap as it fits above the threaded portion of the cap.

I'm absolutely sure that the master cylinder and cap are original but the retaining washer although genuine is too small so it won't retain the diaphragm in cap above the threaded part. The original washer can't be used because the lip on the new diaphragm is smaller.

This means that when the cap is removed it leaves diaphragm in the cylinder. I don't think it's important but if it is the only remedy is to make a plastic washer using the old one as a pattern with a smaller hole for the new diaphragm

   

   

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