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CB750/500 petrol tap renovation

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Oddjob:
Just taken apart my petrol cap that came with the new tank today. All the O-rings have hardened and the 4 way rubber doughnut behind the tap itself is also hard as nails.

Got me thinking, has anyone ever worked out the Viton equivalent sizes, the parts book show them as available separately but fails to give the sizes, the most you can get is the one between the cap and the tank described as 23mm but no section width. What with DS wanting £5.54 for one and £6.01 for the other (you can bet these are old parts so most likely nitrile and not Viton and due to the exposure to modern fuels we should be looking at Viton ones as a replacement) these aren't exactly cheap but most likely the Viton equivalent will be miles cheaper. The 4 way doughnut thing I'll have to source as genuine. Float bowl is rounded off so a new one of those is needed as well.

I can guess the sizes but the ones I've taken off are really flat so getting a section size could prove tricky.

So anyone already done this?

Seabeowner:
I've used 23 x 2mm c/s for the one between petcock and tank and 28 x 2.5 (which may be a bit big but squeezed it in) for the bowl. If you haven't moved since 2017 (have an address) I'll send you them to try and maybe adjust if you feel necessary.
Presume this is about tap and not cap.

Oddjob:
I did measure the one between tap and tank and I also thought it was around 2mm, the one under the bowl I made out to be 30mm though, that was by measuring the old one across it's widest point. Saying that I'm wondering if the sizes relate to the internal diameter rather than the external, as 28mm plus 2.5 each side would make it 33mm? I have a stack of orings lying around, I may try and see if I have any 26mm x 2mm ones, the bowl oring was actually not in bad condition and looked to be 2mm rather than 2.5mm.

I'm also wondering if the doughnut shaped one can be home made rather than paying over £6 for what is bound to be Nitrile and thus not resistant to the Ethanol in fuels these days. I measured the thickness last night and it looks to be around 3mm, I have some really good hole cutters so I just see if I can find a sheet of 3mm Viton and punch out my own, may be the way to go.

Yes, about the tap not the cap, I bought a few spare rubber gaskets for the cap some years ago, never had to use one but the cap gasket is very perished on this one.

Oddjob:
Seems you do measure the ID of then ring, then add the cross section of the rubber. So 26mm + 2mm + 2mm is the 30mm I measured.

Found some viton 3mm sheet on ebay so I'll give that a go as well.

Cheap enough to buy 10 of each rings but appreciate the offer.

Oddjob:
Been doing a little work on the petrol tap from the second hand tank. Just mucking about really, tidying the tap up, what started as just a little tidy up ended up being a bit OTT to be honest, let me be clear this is NOT polished, I don't intend to polish it, I may polish the lever if I feel like it. This has just been dismantled and sanded using the little abrasive discs from Amazon, anyone can do this. What started as a pretty rough casting has slowly been sanded, started with 80 grade disc, just to get rid of most of the casting marks, then progressively down the grades, 120, 240, 400, 800 and finally finished with 1000 grit disc, light green to be exact. At each stage it got shinier and shinier, however the last 2 grade discs were contaminated with buffing soap and this transferred itself onto the tap and in the end I needed to clean the surface with some Brasso felt wool, which to be honest dulled the surface down a little.

Anyone can do the same, all they'll need is a set of discs and a Dremel. I have replaced the 2 small screws holding the lever plate in place with some stainless ones, you could of course replate the originals instead or even just use the existing ones but they will rust.





It's not really that visible but I feel it looks better than that dirty grey look is normally has.

The only worrying thing is that I reinstalled the doughnut washer behind the lever, mine for some reason seems to be made of cork not rubber, it doesn't seem to be sealing as I can still blow down the brass tube with the lever in all positions, saying that the cork doughnut isn't in the best of conditions so that may be the reason.

When I get round to assembling it back together I intend to replate the plate with the markings on so it doesn't rust.

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