The Knowledge - Velocette Owners Club

piston or up the valve guides, AND there are no air leaks, the engine will remain below atmospheric
pressure for all but a few degrees of the crank shaft near BDC. The vast majority of the crank angle will
create a partial vacuum, helping to reduce oil leaks, air pumping, and the loss of power from the added
work load. Obviously, the engine will need to rotate rapidly a few complete cycles to pump out the air in
the rank case via the small copper tube on the crank case breather. However, in only a few seconds
this will occur and the engine will essentially remain below atmospheric pressure inside the crank case.
Having a crank case breather keep the air out, the engine will show far fewer oil leaks, and the best
part, there will be no loss of energy by having to pump air in and out of the crank case. Less air
churning around, less oil being blown out into the primary chain case or the breather. There are
numerous one way valves available at any decent auto parts house. Ask to look thru their assortment of
PCV valves, or use a check valve like used on most autos in the brake servo hose. These are generally
about 1/2 inch spiggoted check valves and fit very nicely in the vent line going up to the tank. Cost is
under ten bucks US. Almost all modern engine employ a breather. Ride oil weep free.
# 6267 Wasn't Percy Goodman clever to devise a timed breather on the early K engines? This allows
the down going piston to breath out and restricts it's inhaling. Same device is still there on the Mk VIII -
and no other breather is required. Not bad for a 1925 design is it? For those poor souls who are
ignorant of the ways of a 'K', this timed breather is a cast on section that overlaps the drilling in the
drive side mainshaft and effectively blanks off the hole as the piston rises. No additional pipes, valves,
reed valves or other wot-nots.
# 6271 You have made some interesting points, however a lot depends on the design of the engine
&/or modifications, & the placement of the NRV, there is a case for claiming that in some cases, the
addition of an NRV can make the underside of the piston an excellent device for pumping oil out of the
internals & on to the road unless some sort of baffle chamber is fitted to separate & drain surplus oil
back into the system. However, -- my old mate Les Diener had a terse saying for this sort of thing, & he
had designed & tried timed breathers as well as various NRV's. And you are dead right about the
simple KSS design, --- maybe that is why I never worried about engine breathing in all my years of
riding Velocettes daily, -- they were all KSS's ! - apart from Les Diener's MOV which I rode at odd times
during the WW2 years, - mainly for petrol economy, ( my cammy had a sidecar attached & was
registered, -- but only for the petrol ration that it was entitled to ! ) it wasn't until the late 70's that I finally
got around to riding a MSS & it never seemed to need a breather!
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