Author Topic: CB750F2 .... silk purse from a sows ear.  (Read 6992 times)

Offline UK Pete

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Re: CB750F2 .... silk purse from a sows ear.
« Reply #210 on: June 01, 2019, 08:02:07 PM »
Wow you certainly have had your work cut out with this bike, keep up the good work fella

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Offline SumpMagnet

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Re: CB750F2 .... silk purse from a sows ear.
« Reply #211 on: June 01, 2019, 10:17:13 PM »
She's not too bad .... It wasn't expensive but was actually pretty complete, and the engine internals were sound. I could just chuck away things like this and replace...but where's the fun in that :)

I want to try and get every bit working as it should, and try and make each piece look as nice as it ever did. Just looking at the smooth and shiny frame paint makes me happy ( is that wierd??? )

I want to try and complete the rebuild without spending more money than the bike is ever going to be worth....so...perhaps the slower I work....the more it will appereciate in value...so the more I can spend...Hmmmm......
CB750F2 - in pieces
CB900F Hornet - the daily transport

Offline SumpMagnet

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Re: CB750F2 .... silk purse from a sows ear.
« Reply #212 on: June 16, 2019, 03:53:16 PM »
Gods Holy Trousers..... I give up!

Not on the project...but on a weeks worth of effort to break down the rear master cylinder. It's not happeneing.

The adjuster nuts on the rod are welded together by rust and won;t come off...and the centre section of threads look to made of iron oxide...which isn;t helping.

Worst of all..the circlip simply will not shift. Both ends where the circlip pliers should go snapped off in the struggle, and where I can see the groove....it looks....wrong. The profile should be  |_| .... but its more ... \_/ and the circlip will not move at all. Heat didn't shift it, soaking in penetrating oils/WD40 failed, and I can't even punch the ends and make it move.

I am at the point of drilling into the groove from outside to punch an end free ..... but I don't think that's a good idea. I have run out of other ideas....so I think this part is heading for the scrap pile and the quest is on for a servicable replacement.

So...in order to make myself feel better....I dug out the solvol and started cleaning the comstars. Not that they are bad...the alloy is really unpitted ...but really more because I felt I needed to achieve something!

EDIT:
actually...drilling 2 small holes through the alloy behind the circlip, and whacking it with a drift ( well..actually a snapped off old 1.5mm drill bit ) got the clip. It came out in 2 pieces, but it came out. Now...I have to make the piston move, and it won;t right now. Unlike the front MC, you can't get at the back of the piston to drift it out. There is no 'back' you can get at. So....more WD40 has been poured in through the available holes in the back of the unit, and it will sit for a few days to marinade. ....

And I have had zero like finding a replacement unit ...so this has damn well better work!
« Last Edit: June 22, 2019, 02:17:59 PM by SumpMagnet »
CB750F2 - in pieces
CB900F Hornet - the daily transport

Offline SumpMagnet

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Re: CB750F2 .... silk purse from a sows ear.
« Reply #213 on: July 13, 2019, 05:13:43 PM »
Well...I didn't give up..... I don't think I actually know how :)

I had decided my rear master cylinder was scrap....and that kind of gave me the leeway to get a little bit....er...brutal.

So...I drilled the head off the piston. Cored out the middle and took the head off. This was the bit that had glued itself to the bore with crud and solidified brake fluid.

Got the piston top off...and soaked in some more WD40. Then...using a pair of forceps and a needle...I teased a split into the rubber seal, clamped it and tore the sucket out of there. With that out...the rest of the piston could be pulled out.



The wreckage of the tear down looks pretty sorry for itself....

Now I will see if I can polish the bore to the point where I reckon it can actually seal on a fresh piston/seal. A little irregularity can be taken up by the seal....as long as it's not pitted or scored to ruin a seal of let fluid past. The Solvol will be broken out and I have made up some round polishing mops gluing sponge onto some old sanding tools for the Dremel...so tomorrow...I shall polish and see if I can get a smooth durface. Worth a try! If I can get it smooth and shiny...I will buy a rebuild kit and see if I get pressure.

Victory.... OK...a small one....and not by means I would recommend to anyone.... but a victory none the less!
CB750F2 - in pieces
CB900F Hornet - the daily transport

Offline Spitfire

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Re: CB750F2 .... silk purse from a sows ear.
« Reply #214 on: July 13, 2019, 06:37:36 PM »
Nice one, hopefully it will seal, fingers crossed for you.

Cheers

Dennis

1976 CB750F

1977 CB750F2 In bits

1964 BSA A65R In bits

2000 NT659V DEAUVILLE