650 Crank sprocket valve-bolt issue

[dave16mct]

Hi Ian,
first of all, don't panic. Take time to think it out and explore all ways to solve your problem.
I'm no expert (who is?) but;
a) you could replace the bolt and put a crankcase breather somewhere else. Do a search, someone added a breather under the magneto into the timing chest, fitted a pipe and used a Bunn one-way valve. The crankcase breathed into the timing chest, then out through the valve.
b) could you modify a similar bolt to accommodate a one-way valve? Could the valve be machined to fit in a recess machined in the bolt? ie join the 2 to make a new relief valve.
c) could the crank thread be cleaned with a tap and the correct valve/nut fitted? Not sure if it would be a loose fit.
d) maybe the manufacturer of the valve would sell one without the thread, you could then thread it 18tpi.

Just a few ramblings to think about. Cheers Dave

[lostinsdf]

Thanks Dave. I am busy at work for a few days. My next move is to visit my local machine shop and see if they have a solution. I will post what, if any, they can do.

[zwarts]

Ian, assuming that the18 tpi bolt is a good fit in the crank then you could just continue to use it and fit a breather to the timing cover. I have done this on my racing outfit and it works very well with no oil loss There is a baffle plate inside the timing cover to prevent oil passing straight out into the breather pipe. The pipe is 1/2" bore flexible tube and leads into a catch tank. I can supply photos but don't know how to put them on here!

[ajsph]

how can you modify the thread to another pitch, but with the same diameter, I am sure the thread in the crankshaft is destroyed, by this modification
Best regards
Poul

[ajscomboman]

I have been pondering how a change of thread can come about without wrecking the crank and the obvious answer is that it could have originally been an alternator crank cut down and re-threaded with what was at hand to make it fit a dynamo case. As Bob has stated the simplest solution is to fit a breather in the timing side.

[lostinsdf]

Hi Zwarts. Thank you for the suggestion about fitting a breather. I believe that is the best and easiest way to go. I would appreciate some pictures. You can PM me but it would be nice to get them on the Forum for others to see.
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IMG_3084.jpg

[zwarts]

Ian - you have a p.m. Or at least I hope you have - not sure if it went or not!

[lostinsdf]

Nothing there. I sent one to you as well.

Thank you.

[zwarts]

Sorry. Having a bad day with the computer today. I received your p.m. and you now have an email.

[lostinsdf]

Hi Ian,
I have attached some photos of the timing cover breather which has been in use on my 650 M31 engine for a good many years now.
I will give you the background to how this came about.
The engine is from my 'Vintage Racing' sidecar outfit. Over the winter of '89/'90 I spent considerable time and effort to cure the many minor engine oil leaks which had annoyed me for the previous 5 years. This was entirely successful in that I then had an engine which leaked no oil whatever, no matter how hard it was worked. The only fly in the ointment was the crankshaft breather which would discharge small amounts of oil into the open primary chaincase. The key word here is 'open'. From there the oil would become distributed over the drive side of the machine - largely spoiling the success of the work to make the engine oiltight. To cure this, I plugged the 1/4" bore of the crank with silicon sealant and fitted an oil seal which bears on the engine sprocket spacer (normally a hardened scrolled ring, but I softened mine and turned the dia, down to suit an easily available lipped seal). In the process the (now unnecessary )scroll was removed.
A hole approx 3/4" dia was machined in the timing cover, located over the magneto drive pinion. Into this I fitted a brass 90 deg. elbow with a short length of 1/2" bore copper pipe as you can see in the first 2 photos. To prevent oil being able to escape directly out of the timing chest into this elbow, a baffle plate was made to cover the exit, as shown in photos 3 and 4. Thus pressure can easily be vented, but oil cannot pass directly out.
In practice it all works well. The copper pipe has a 1/2"bore flexible hose leading to a catch tank, as required by the racing regulations. There are rarely more than a few drops of oil in the catch tank after a full weekend's racing and no more oil is lost into the open chaincase.
I have read many learned discussions about the advantages of maintaining crankcase pressure below atmospheric, which clearly my breather cannot do as there is no valve to prevent backflow into the timing chest. All I will say about that is that if I put my hand over the exhaust from the breather pipe, at tick-over I can feel the pressure pulsations due to the rise/fall of the pistons causing changes in crankcase volume and therefore pressure. However as soon as the engine is above tick-over speed, the pulsations cease and neither pressure nor suction can be felt i.e. there is no net change in pressure above atmospheric. I have a plausible theory why this is so, but it is irrelevant to the outcome of the breather in use!

breather 1.jpg

engine1.jpg

breather2.jpg