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  #1  
Old 02-04-18, 12:25
Lolat492 Lolat492 is offline
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Lip seal or O ring?

Afraid this is about my super-irritating French otorcycle, but need some advice.

I wish to add a seal in the 2 stroke engine, one each side of the crank to allow a good primary compression.
The original design is a felt seal on the clutch side and a very close fit brass bush on the magneto side, the bore of which had a helical groove machined into it to 'wind' the petrol/oil back into the crank case.

Question is:

I feel an O ring is better for this as there will be + and - pressures in the lower case, and a lip seal will seal but not resist a +ve pressure.
The pressures involved could be quite high(?).

Hydraulics use O rings, engines use lip seals but not where there is pressure differential either side of the seal.

Making the parts for an O ring will be easier than for an interference fit lip seal in it's steel outer ring, so I think the O ring is right.

Others on a classic motorcycle forum disagree, hence this question.

Hope some can help.
Graham.
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Old 02-04-18, 12:56
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AllanMcD AllanMcD is offline
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The pressures involved will not be high.
I have never seen an O ring used to seal a 2 stroke crank !
But I only go back to the 1970's
Allan
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Old 02-04-18, 13:18
Lolat492 Lolat492 is offline
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Thanks Allan, so a simple Viton 'crankshaft' seal will be ok?
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Old 02-04-18, 14:52
Lolat492 Lolat492 is offline
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Very tempted to simply replace the crankshaft main roller ball bearings with sealed bearings!
Is this a stupid thought?

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Old 02-04-18, 18:39
Rob Stevens Rob Stevens is offline
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Sealed bearing would appear a very simple solution and no mods to the crankcase?
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Old 02-04-18, 18:58
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AllanMcD AllanMcD is offline
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I am not sure the best answer but any seald bearings I have seen have very flimsey seals.
Why not just a fresh original setup.
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Old 02-04-18, 19:04
postman postman is offline
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My mate Brian , bike mechanic for 40 odd years says 'Double lip seal would be correct, as it works in both directions and an O ring won't lubricate enough. '
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Old 03-04-18, 13:27
Lolat492 Lolat492 is offline
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Thank you all for the replies.
The sealed ball bearings was my first approach as it is a very simple solution and being French, the brgs are metric, so double easy.
Others have said the seals are 'dust' seals and will blow 'out' under pressure or at best leak.
The one seal is a flimsy felt washer, saturate in oil and cross your fingers, and the other side is not even a contact seal but a close (almost transion) fitting sleeve in the case with the scroll machined in it. It is certain to leak a little or a lot.

The double lip is great but I doubt I have the space to get them in, esp on the bushed magneto side.
I'll take it part first and measure what space I have.
There are no new parts available, so everything has to be had made.
Here are the crank cases of this rocket machine 3 bhp engine....
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Old 03-04-18, 13:43
kano nordie kano nordie is offline
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O rings are for stationary use, lip seals for linear or rotary use go through the lip seal catalogue or google lip seal and the size you want?
I doubt the sealed bearing would last for too long
John
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  #10  
Old 04-04-18, 06:12
jouhar jouhar is offline
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Quote:
Originally Posted by Lolat492 View Post
I'll take it part first and measure what space I have.
...
This

Until you see exactly what is in there, and measure them accurately to see what the alternatives are, the discussion is premature IMO - I replaced similar seals when I rebuilt a BSA Bantam. Good luck with it - its a lovely looking bike !
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