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Crankcase breather pipe

Submitted by tony_wood on Fri, 11/06/2021
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Heavy Twins

Hi. Is there an optimum length for the metal breather tube that exits from the back of a Dommie 99 crankcase? I bought a new one and the metal tube is about 9 inches long. The old one that I took off the engine was only about an inch long so why do the suppliers provide all that extra pipe work if it’s not needed? Should I just cut it back to an inch or two and then use rubber pipe from there?

Thanks

Regards

Tony

Submitted by robert_tuck on Fri, 11/06/2021

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The copper tube was resilient and reliable  and could be bent to lube the chain if you wished. Rubber would perish and would need to be restrained to avoid getting chopped up in the chain. Your choice.

Submitted by tony_wood on Sat, 12/06/2021

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Thanks Robert. I hadn’t realised it was intended to be a chain lubricator pipe - that now makes sense of the long length. I didn’t realise it was copper either. All of the Dommies I’ve seen have a rubber tube on the pipe and that rubber tubing is routed to the back of the bike where it can happily deposit any oil into the atmosphere/onto the road (or garage floor). I’m not keen to lubricate the chain with ‘waste’ engine oil as I’ve read that it will be carrying acids dissolved in the oil, which can’t be good for the chain. I think I’ll cut it short and route a rubber pipe out of the bike’s back end.

Regards

Tony

Submitted by robert_tuck on Sat, 12/06/2021

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I have had the pipe in a few positions ,Best to get rid of engine fumes  , Mine currently runs to the oil tank as I was  fed up with oil pools . But thats not ideal  , may try a reed valve as it can help reduce internal pressure and leaks.

Submitted by bob_matthews on Sat, 12/06/2021

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Oil pipe exitI cut mine short and then used oil line with a PCV (positive crankcase ventilation) valve in-line, fitted behind the chaincase. I exit the oil line on-top of the rear mudguard but behind the number plate so that any drips are well away from the rear tyre.  It works very well.

PCV valve

Submitted by tony_wood on Sat, 12/06/2021

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Thanks guys. Are those valves (PCV and reed) in addition to the rotary disc breather valve on the end of the camshaft in the crankcase or do you use them instead of that one? I’m also considering putting an oil catch-can on the bike and feeding the rubber breather tube into that so the bike doesn’t dump oil on the floor - any views on doing that?

Regards

Tony

 

Hi Tony. The photo shows how my catch tank is fitted. The tank is just an aerosol tin.

The tank is an aerosol tin donated by my wife, the clips were from a hardware shop.  With this system the catch-tank can easily be removed for emptying.  As already stated in this thread, the timing disc must be left in situ otherwise the camshaft could move sideways.  For it to be really effective you need to drill some holes on the inside of the crankcase into the tunnel between the disc valve and the crankcase breather outlet.  The holes allow the gases to escape via the one-way valve to the catch-tank on the piston down stroke.  On the up stroke a vacuum is produced, in the crankcase, helping to keep the oil inside the engine.  Since doing this mod my engine has been practically oil free on the outside..

Cheers,  Peter J Maple  

Submitted by bob_matthews on Sat, 12/06/2021

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My PCV valve is in addition to the rotary disc on the end of the cam.  I was going to fit an oil catch can too, but couldn't find one I liked or find a good place to fit it - so I just dump on the road :)

Submitted by gordon_johnston on Sat, 12/06/2021

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Far better to have the breather pipe sending oil vapour to your rear chain than just letting is suffer from rain/road dirt and whatever. Almost as good as a Scottoiler and more reliable and much cheaper. Don't believe what's said about dissolved acids being bad for your chain. If the oil vapour was that corrosive, your engine wouldn't last 5 minutes.

Submitted by david_cooper on Sat, 12/06/2021

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By the early 60's it went to the "froth tower" on top of the oil tank. Tidier than running it along the bike. Doesn't seem to lead to problems.

Submitted by robert_tuck on Sun, 13/06/2021

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I put mine into a "Froth Tower"  ,but since then have  struggled  to keep the oil tank clear of mayonase ,  Short running time probably the big issue.

Submitted by bob_matthews on Sun, 13/06/2021

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My breather emits mayonnaise after a short run and personally I wouldn't want that either in the oil tank or on the rear chain as it obviously contains water.

Submitted by roger_johnson on Fri, 18/06/2021

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I thought I had had a brilliant idea when I routed my breather to very close to the rear chain, wrong.

I was shocked to find half the oil in the tank had disappeared in a few days due to the depression felt around the hole in the re-positioned breather pipe. It certainly did a good job of oiling the chain the rear wheel and ... .

 

 

Submitted by gordon_johnston on Sun, 20/06/2021

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My 99 has the later oil tank with the oil separator tower. Norton obviously listened to the owners. The oil tank breather/overflow is ducted to the top of the chainguard, giving the rear chain some welcome light lubrication on long, hot runs. It has the other advantage that if your bike has wet sumped at all, on start up the oil goes to the oil tank and not over the garage floor. Worth looking out for a later oil tank if you get the chance. 

Submitted by carl_james on Tue, 22/06/2021

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ive modified mine and routed mine into the oil tank then moved tank breather and routed over the chain . works well. i have 3 breathers into my oil tank (1 rear crank 2 rear rocker cap 3 timing cover ) .

 



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