one way valve in breather

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Aug 20, 2005
Has anybody done this mod, idea is to run with negative crankcase pressure. Does it cause any problems?

I first experimented with this concept some 12 months ago. To me it is logical that if I reduce the amount of intake air to the crankcase on the 'upstroke' .. then pressure on the down stroke will mean less pressure within the crankcase. ( The valve is fitted to allow air to be pushed out but limits air on the intake cycle.)

I fitted a PVC from a GM six cylinder engine .

I have found the results to be excellent. Whereas I found a oil mist deposit on the front of the lower crankcases and the engine required a wash and degrease regularly .. every <100 - 200 miles> to maintain a clean appearance.

Now I only wash off the crankcase once in a while ~

I have seen these valve now sold as kits .

I did some research and the recommendation is that you fit the valve as close to the oil tank as practical ~ given that you have the standard coupling set up!

Seems common sense to me really!
Since posting original message I have fitted a temporoay valve intended for pneumatic air lines which is a ball and spring type and gone for a short thrash, no ill effects seen but too early to tell about good effects! I will get a larger bore valve of the same type & fit it in properly. When I hold the output from the valve against my hand, I can feel a small amount of gas coming out gently, which must be what gets past the rings, compared to the crazy chuffing & oil mist that spews from the standard setup! Looks good!
BMW bikes have a nice little reed valve arrangement for the crankcase. It would be more complicated to fit than an in line one, but could be a neat way of doing it.
I have fitted the one way valve just behind my crankcase breather (73 850) This seems to retain negative pressure in the crankcase. All of a sudden the Norton has the cleanest parking spot. Of the 4 bikes in my garage, the Brit is the most oil tight. Now that's just weird.

-Chris in Chicago

73 Commando 850
95 MZ Skorpion Sport
75 CB400f

one way valve in breather
now theres a sight- a perfectly dry brit bike, are you sure there's any oil in the engine?
I fitted a Norvil 1/2" several years ago and other than a major blow up (nothing to do with the breathing!) have had no problems. It solves most breathing issues except on 72/3 Combats with the original front oil pickup but with an 850 style converted timing breather. I run the froth tower pipe into a small bottle next to the battery and in the spring & summer I get a little water and emulsified oil collecting but it only needs emptying every 1000 miles. It's the best mod you can do for oiltightness and really eases the pressure on the top end where my rocker covers and shaft covers used to weep a lot.
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