HT crankshaft oil seal

[Dave.Thomas]

I`m returning my HT to actual HT spec, rather than the NH spec it came in. I`m confused about the need for a crankshaft oil seal, all else being similar?
I`m sure I`ve had it explained before, but , like my school reports said, I have a short attention span and often fail to listen !

[brenton.roy]

Hi Dave, I understand that the seal is there to keep the possible water and mud out when you are fording those creeks and find a deep bit...
Brenton

[Dave.Thomas]

Thanks Brenton, I was thinking possibly the reverse, i.e the seal was there to keep excess crankcase pressure,and therefore involuntary oil movement in check, ( more in HS mode than HT)?

[nevhunter]

The whole breather set up should be reviewed, as it's restrictive on most models. Is there much room for a seal? I think I would be content with making the hardened spacer fit well. It will stay a neat fit till the mainbearing develops play and that shouldn't happen too soon.. The breather system is probably more important on high speed work like outfit racing where blowby becomes a factor. Nev

[brenton.roy]

Hi Dave, I don't think they (factory) would worry too much about oil breathing from competition dirt bikes? I think they take a far dimmer view of oil on bitumen though.
As you say, the seal is likely to add a little to crankcase pressure and my experience is that rather than keeping oil in check, this generally leads to oil being delivered nicely out of every other orifice...
There is nothing physically stopping fine particles getting into the primary via unsealed area behind the clutch and making their way through the (admittedly small) gap between the spacer and the rollers - although you would need to be in a middle eastern dust storm or a couple of feet of muddy water to get them there.
Also, you'd have to expect that positive crankcase pressure would make this all pretty difficult.
If you don't have them, the dimensions and numbers (via Ben M) for the seal and shim are here http://www.barkshire.co.uk/AOMCC/Knowle ... spx?id=484
Are you going trialling or restoring the bike?
regards, Brenton

[dave.moffat]

I'd certainly use a seal to keep water and muck out; a rubber sealed main bearing would help too. You could remove the inner seal on the bearing to allow oil in to it if you choose.
Add breathers elsewhere if necessary to allow escape of excess pressure.
Even if you can seal up the primary chaincase to keep water out I'd still do this as there is inevitably condensation in there.
Bitter experience years ago when I had to strip my trialling WNG to change the worn out drive side mains due to muck getting in.
The crankshaft sludge trap was just short of full so the big end had a VERY narrow escape.
A one-side-sealed main bearing cured this completely.
Good luck,
Dave

[Dave.Thomas]

Thanks for the answers chaps, I`ll mainly be using the bike on tarmac but hopefully will do the odd LDT so will take the advice on board.

[nevhunter]

the sealed ballrace is a good idea. I wouldn't even bother removing the inner seal Those sealed bearings last a long time in multivee belt drives at the front of car engines where they are in harsh condiions. nevhunter.