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Triumph 3hw primary chain case (one that dosnt bloody leak)


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It a nut shell I'm getting rather bored of my primary chain case leaking. I've tried all sorts, currently there is 4" (slight exageration) 3mm cork gasket on there and it still chooses to leak.......the joys. Does any one have one lying around that they can confidently say 'dosnt leak'


thanks alot


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  • 3 weeks later...

Hi ..I have just seen a advert for an original Triumph 3sw Primary chain case cover on e bay ..both outer and inner


item no 111591403597 outer


Item no 111591411213 inner


original so whether they would provide the holy grail of a leak free bike is a different issue




Good luck






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The 3HW has a tin chain case, like the one sown on Ebay. A 3SW has an ally chain case. Another source of oil leak on either type is at the back of the inner chain case where the gearbox/clutch is. Both mine have leaked here despite how good a seal I get at the main jointing faces. Ron

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glue the cork gasket onto the inner chaincase with an automotive sealant.

and a thick smear of grease between the outside case and the cork .

in this way it's easy to remove the outer cover in a later stadium.

turn in the screws equally! ;)

it works for me.

Edited by Marnic
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In general, there was not the concern in the 1930s with vehicles that dripped oil. Oil sealing was not high on the list of priorities. This was not solely an aspect of British machinery. Motorcycles of this period will have only leather, felt or occasionally rubber to reduce the flow and any breathers etc. will normally vent to atmosphere.


Anyone running a post-1960 British motorcycle with oil leaks is not trying hard enough but on earlier vehicles, some re-engineering will be required.


I've never worked on Triumph transmissions from this period, but Ron mentions leaks behind the case. If the Triumph, as was common at the time, has a sleeve gear which stops short of the case, any wear in the bushes will allow oil to escape here, especially if the box is full and the field stand used.


Any leaks between crank and chaincase will be made worse by cylinder wear as the pressure has to find a way out somewhere.


It's quite straightforward to glue a cover on with silicone but getting it off again afterwards can be interesting. If the lower edge is sealed together with the lowest inch or two of what is only an oil bath, the case itself shouldn't be leaking. The edges do need to be fairly flat and a thick gasket can cause more bowing. If you're not confident in your own tin-bashing skills then it might be worthwhile giving both halves to a specialist to have them true the flanges.

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  • 4 weeks later...

trick I've used to seal things M20, B31 tin primary case etc.... line the sealing surface of one half of the case with a layer of cling film... glue the gasket with silicone ( polymer is even stronger.. read the label for suitability} .. rub fairly generously on all faces with fingers round and round till satisfied.... assemble tightening screws in stages evenly to about 2/3 torque leave to set... finish tightening. this should mean that one side of the gasket definitely won't leak..cling film allows for disassembly and if there is any leak can be reassembled with weaker sealant Hermatite do a range.

Introducing a breather pipe connected to the air filter on the manifold side creates a negative pressure and helps reduce leaks

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