CB750/500 petrol tap renovation

[Oddjob]

Just ordered a new bowl and doughnut seal from DS. Also after a re-measure with a digital vernier I concluded that the bowl O-Ring is more 31mm than 30mm, the thickness was around 2.67mm and concluding that it must have been squashed in use I think the initial thickness was more like 3mm, so I ordered some Viton O-Rings at 25 x 3mm and just in case some 26 x 2.5mm, I'll see which suit better when they arrive. The tap to tank O-Ring looks to be 2mm but again it's almost flat, so to cover all the bases I ordered some 23mm x 2mm and some 23mm x 2.5mm and again I'll see which suit better.

I'll wait for the new doughnut to arrive to see what thickness of Viton sheet I should order to make some new ones.

[Orcade-Ian]

Ah yes Ken,
The long brass tube goes right down to the body of the tap with no side hole and there's a separate hole for reserve.  Otherwise you would have no reserve!  Must go back to skool!

Ian

[Seabeowner]

As I say I've been using 23 x 2mm c/s and 28 x 2.5 for about 4-5 years on a couple of bikes and I think they may be nitrile. Know that as the bowl one expanded a bit and had to stuff back in but actually seals just fine.

[Oddjob]

It's odd, don't you think, that people and recommending we use Viton O-Rings because of the Ethanol in todays modern fuel. We are advised to replace all the O-Rings in the carbs due to the exposure to the new fuel YET no one says anything about the petrol tap O-Rings. Yet if anything the tap is exposed to more fuel than the carbs, we drain the float bowls in Winter be we don't drain the tank so the tap is constantly exposed to the Ethanol. We also say nothing about the inlet manifold O-Rings which are exposed as well, not as much as the carb and taps admittedly but they act as a seal against the outside environment so they must be exposed to some of the petrol vapour but we don't say anything about replacing them.

At 28 x 2.5mm I'd say the ring was 2mm oversize, which may explain why it needs to be stuffed back in. I'll see when the new rings arrived. I think the 23 x 2mm was pretty much spot on. I'll see though if 2.5mm would be better or worse.

[Trigger]

Inlet manifold o'ring is a air seal and not a fuel seal and it does not come in contact with fuel 

[Oddjob]

Got to disagree Graham, it’s both, stops air getting in and thus stops petrol vapour getting out. There must be some exposure to the petrol or even if the oring fails it wouldn’t be able to access the air inlet tract. The main difference is that the vacuum in the inlet tract mainly encourages ingress of air rather than the other way round but something like a backfire through the carb would allow exposure the other way as well.

It can’t hurt to eliminate the chance of ethanol exposure to the oring by replacement with viton rather than nitrile. Same to some extent could be said about internal engine orings, we’ve all smelt petrol in engine oil for example, is that harming the nitrile rings in the engine?

Not arguing this is the case just seeing what the general thoughts on this theory are.

[Nurse Julie]

We use Viton O rings on all engine oil ways and of course the oil pump. The only Nitrile we use are on the tappet caps as its not an oilway and Viton is too hard for there anyway. Having had 2 jars of Ethanol petrol on the bench for 3 years now, one with Viton and the other with Nitrile submerged, yes, the Nitrile has disintegrated totally, ie, gone all stretched and sort of melted, the Viton hasn't been affected. But, the O rings suspended in the jars on lengths of wire, suspended above the fuel, therefore only in the fumes, neither have disintegrated........so far 😊😊😊😊

[Oddjob]

Interesting Julie. So if that is correct and I have no reason to doubt it, then only by being submerged or constantly exposed to fuel causes the Nitrile rings to deteriorate, exposure to vapours and such either doesn't or takes far far longer to become apparent.

I wonder as the levels of Ethanol increase whether the deterioration increases as well. A couple of years ago it was 5% now IIRC it's already at 10%. Saying that they have E85 fuel in the US and certain European countries, that's 85% Ethanol and only 15% petrol. You can almost see the orings melting can't you? 

[taysidedragon]

I'm sure that when Ethanol was introduced into the petrol the recommendation was to replace o-rings and rubber seals in the fuel system as a whole. That would include the fuel tap.

[Oddjob]

Got to say that nowhere to my knowledge has anyone said anything about replacing any orings except for this forum. When I mention it to other bikers for instance they are totally unaware of it, as I’m sure are 99% of car drivers.

[Laverdaroo]

Surely if and when the nitrile rubber starts to swell, disintegrate the surface area will increase slightly due to pits and holes and larger overall size?
This would suggest that when they start to go the process would only accelerate as more of it would be exposed ?

RE. If a ring has gone somewhere in /on the bike and it’s an OE piece, it could be assumed that the majority of the other seals in the machine that are subjected to similar conditions will be going or gone also meaning that as soon as you see one, it’s time to get all the others changed also?


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[Oddjob]

As it's been said Roo, it's really only O-Rings and possibly seals (unsure what they are made of TBH and may already be immune) that are essentially submerged in the Ethanol inside modern fuels, carbs and taps for instance. I'd advise doing the internal engine O-Rings as well if possible to eliminate the potential of a failure in the future. That is of course if Viton rings are available in the sizes required. TBH I wouldn't advise pulling an engine apart to replace the O-Rings as the amount of Ethanol they could be exposed to doesn't make it worthwhile and the amount of time it would take for them to become a problem means it's just not worth it. However if you are already replacing O-Rings in the engine then I'd say might as well fit Viton ones instead, if possible of course.

[taysidedragon]

Got to say that nowhere to my knowledge has anyone said anything about replacing any orings except for this forum. When I mention it to other bikers for instance they are totally unaware of it, as I’m sure are 99% of car drivers.

First post in this topic.
http://www.sohc.co.uk/index.php/topic,26449.0.html

[Oddjob]

That post was a comment and post from a friend of mine after I’d brought the subject up on another forum,  he was trying to clarify all the mistakes and myths regarding Ethanol. The only reason he did that post was in answer to my comments in relation to the problems ethanol could cause, not in our bikes though as they are all post 2000 models but in any other bikes the members may also own. Members were surprised that the government hadn’t bothered to warn anyone about what introducing this into our petrol could cause.
 

[Bryanj]

To do that would mean they were not all lying