Crankcase relief valve shape G9

[Trefor]

In the probably futile attempt to stop the dynamo oil seal lubricating my boot, I checked the crankcase relief valve of my '53 G9. The diaphragm in my valve is a disc, whereas the spare part 016676 has 4 circular cut outs. Am I right in thinking I have the wrong diaphragm and should cut my disc to match the spare?

[clive]

Trefor I doubt the crankcase relief valve is the cause of a leak around the Dynamo. This is more likely to be coming from the seal of the Dynamo to the inner timing side cover. The gasket there is fraught with problems not least being the pinch caused by the stud. However the leak may be caused from the gap in filter tunnel where the crankcase halves join and appears to be coming from the Dynamo as runs out from behind. There should be a small paper gasket there (no other gasket between the halves). If it is missing the very oil high pressure on starting can start a leak which then continues. On my G11 the PO had not fitted the gasket and doing so, along with fitting an oil pressure relief valve (not the same as the crankcase relief valve) and use of 20/50 instead of straight 50 and all leaks stopped, until the Dynamo came loose! The oil pressure relief valve is on the timing side and came and went according to the year of the bike!

[Trefor]

Thanks for the reply, Clive. I think I'm clutching at straws as I have tried half a dozen times to fit the dynamo with new seals and cure the leak. I noticed other threads mention the possibility of pressure build up in the timing case, but I agree this seems unlikely in an unmodified engine in good condition. I did modify the relief valve anyway and will apply a generous amount of silicon to the dynamo and crankcase- next step a mobile drip tray!

[clive]

Trefor, I don't think you will stop the leak if the gasket between the crankcase halves is missing. As I described this allows oil to leak from the oil filter housing, along the line between the crankcase halves. This leak is immediately behind the dynamo body and appears from all around the back of the dynamo giving all the appearance that it is the dynamo to crankcase seal that is failing. Unfortunately the only way you will know if the gasket is fitted is by splitting the crankcases. The part I am referring to is listed under the oil filter in engine on page 10 of your parts list.
"015034 Gasket, paper, for oil filter chamber (fits between the two crankcase halves)". I mucked about trying to stop a leak for ages and was unsuccessful until I split the crankcases on one twin. Of course this turned out to be expensive, not for the gasket, but for the crank regrind, new parts for the main bearings and conrods all of which needed doing whilst it was apart! Did stop the leak though.

It seems counter to normal practice to have a small gasket in an otherwise large area of seal without one but that is what should be fitted. If a previous owner was not aware of the requirement it may have been left out. Looking at the parts list your engine should have the oil pressure relief valve fitted it is the annoying bit with a spring which tends to fall out when the timing side cover is removed. If the gasket is missing you will not have any luck stopping the leak and may well have to use the "park places where a series of drips will be less noticed" method until you have been able to fit it. A number of my bikes, although mostly the singles, work on this principle! I definitely avoid friends brick covered drives.
One way you could test this idea for the cause of your leak, without splitting the crankcases, would be to manufacture a blanking plate of some sort to cover the hole in the back of the crankcase where the dynamo fits. You might even get away with using high quality duct tape. Then run the engine, or take it for a quick spin without the dynamo fitted. If oil appears along the crankcase joint the case is proved.

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[Trefor]

Thanks for the reply Clive. I should have mentioned I had eliminated the dreaded crankcase seal as a culprit. I cleaned the front of the motor and took the bike for a short run, before the whole front of the crankcase was covered in oil and was relived to find the crankcase joint clean and the oil coming from around the dynamo. I think as you described before the problem with the dynamo seal is just poor design and I'm working towards getting the oil leaking down to a level I can live with and parking where it doesn't show.

[les ward]

For many years, on numerous twins, I never had a problem with oil leaking from the dynamo. Then a couple of years ago like you I just couldn't get it to seal.
By chance, I had an old gasket set with the cork seal left over. The cork is a fine grain compared to the course grain cork of the modern version. Like magic, it cured the leak. And I have had the dynamo off several times reusing the same gasket and it is still leak free.

I also help out a young chap who has inherited his grandad's bike and uses it every day for work. We couldn't get his dynamo to seal with modern gaskets. So I found a part used old gasket set on eBay and again it cured the leak.

I also use a small round file on the gasket so it fits round the draw stud.

[les ward]

The one on the left is the modern version, which didn't seal well.

[Trefor]

Thanks for the tip, Les. The 'modern' gasket with the coarse grain on the left looks like the ones I'm using. I've encased the dynamo-crankcase gap in clear silicon and have crossed my fingers. If that doesn't work, I'll start chasing one of the old gaskets or some fine-grained cork.