ET: Engine (Twin) engine breather

erik

Well Known and Active Forum User
VOC Member
what is different on a D head with the valve guide and breather to a B or C head. I dont understand this Detail.regards Erik
 

timetraveller

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VOC Member
If you look vertically down along the line of the valve stem you will see a crescent shaped cut out between the head and the top guide. This is where air pressure from lower down the engine has to pass through to get to the top of the breather cap. This cut out provides is larger on the 'D's but I do not have dimensions. It is easily increased by increasing the size of the cut out in the head. Just don't use this type of breather along with the timed breather as air can pass both ways and be sucked into the engine when the pistons are moving upwards.
 

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Erik my handsome there is no difference between B, C & D heads, breathing via a "D" can help but the ones that have been most effective are the the ones with a non return valve of some description, the ball types sound as if a steam engine is running, reed valves are silent, ideally the vapour should pass through a course gauze to remove oil droplets or via a larger chamber so vapour velocity is reduced and droplets fall or the most effective is the breather fed into a round a round chamber "tangentially" so the vapour gets centrifuged to the outer surface and clear vapour goes up the centre via a non return valve (NRV) then this chamber must have a drain on it BUT via a liquid trap with sufficient height that the crankcase pressure will not overcome the trap height. The drain to be lead to a low point of the engine with about six inches (15cm) trap height.
Mr Dyson has made a fortune with (patents in place) his vacuum cleaners using this method yet it was invented in the 1800's to separate water droplets from steam before it left the boiler through "cyclone separators".
 
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erik

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VOC Member
is it recommendet to convert a B and C breather to a series D breather.I would not like to Mount an elephant trunk because of a rev-Counter.
 

Bill Thomas

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VOC Member
I don't think a "D" Breather will work, Unless you mod' the heads, Like T.T says.
But you can always change a tappet cap for a "D" cap, With a bit of pipe and an oil separator.
But as T.T. says, Block the standard one off if you do.
comet grass (2).JPG
 

erik

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VOC Member
what i want to ask,if the d-System is superior to the b and c System ,especially on atwin?
 

macvette

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Non-VOC Member
My D has the standard breather arrangement with the the outlet on the front inlet valve cap. You can't fit the D breather in this location because the C Ufm is in the way so it has to go on another valve cap. I think that the oil drain grooves in ET26/1 (rocker bearing) are bigger than on a C but I'm not sure about that. There is no timed breather on a D, the pipe is routed over the top of the motor and down in front of the rear wheel. When my motor was rebuilt I had valve seals fitted and have a catch tank made from an aluminium drinks flask beside the battery tray into which I piped the breather. Inside the drinks bottle is a dip pipe which stops about1/2"above the bottom and I keep enough oil in the bottle about an inch above the bottom of dip pipe. There is also a vent pipe on the top of the bottle. I found that I only need to check the bottle once a year and there is very little carry over. In theory the dip pipe can create about 6" water guage vacuum.
Mac
 

stu spalding

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VOC Member
You can fit a"D" cap on the rear inlet valve on a "C" but on some bikes it'll need a couple of threads removed from the neck and the banjo bolt. Cheers, Stu.
 

erik

Well Known and Active Forum User
VOC Member
i have seen flat caps on the exhaust of the rear and the inlet of the front.they both can be fitted under the ufm of a b and c. who makes this breathers?
 
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