Royal Enfield Classic Bullet
Some ideas from Pete Fletcher
There seems to be a lot of confusion about the crankcase breathers fitted to the
later RE classic Bullets (after about 2004)
I'm hoping this article will be useful to those who are trying to understand the system.
These are, of course, just my personal thoughts on the subject.
Why do crankcases need to breath?
Think about an engine with the piston at TDC (at the top of it's stroke). There is a
certain volume of air in the crankcase underneath the piston.
Now think of the piston at BDC (bottom of the stroke) There is obviously a smaller volume
of air in the crankcase so the air will be pressurised.
However good your piston ring seal is there will always be some blow-by from the very high
pressure in the combustion space above the piston. This blow-by will also raise the
pressure in the crankcase.
If this pressure is not vented it will to try to leak out any way it can - like through
your oil seals or any poor seal at a gasket. The leak will take oil with it and cause a
messy oil leak.
(I also suspect that high crankcase pressure will tend to stop oil from the oil pump
reaching the big end. I've no proof of this but pressure in the crankcase will certainly
not help flow of oil from the oil pump)
What does an engine breath out?
1. A certain amount of oil vapour,
2. Blow-by gases from the combustion space (CO and lots of other stuff)
3. Quite a lot of water vapour especially when the engine is cold.
How do the later Bullets breath?
The later bullet models do it like this;
1. There is a hole in the wall between the crankcase and the oil tank so that pressure
from the crankcase passes to the oil tank (this is not really an oil tank as such, but the
rear part of the main engine casing which is a separate chamber to the crankcase)
2. There is a vent out of the top of the oil tank, which relieves pressure via a black
rubber hose into the catch can, which is situated behind the battery.
3. Pressure is vented to the catch can through a duckbill (non return valve) which is
inside the catch can.
4. Excess pressure in the catch can is vented through another tube to the air filter in
the RH toolbox.
5. Oil (and water) caught in the catch
can is returned to the engine via another tube to the back of the timing
chest. There is another NRV in this tube so that oil can't blow up from
the timing chest into the catch can. (I suspect that oil drains down to
the timing side mainly when the engine is stopped) Some people
think this is a timing side breather but it's not
What's this duckbill thing?
The duckbill is a neoprene tube with a flattened end, which acts as a non-return valve.
This valve helps to preserve any negative pressure in the crankcase. (generally a good
thing for various reasons but I'll keep it simple here)
Problems with this system.
The main problem with this
system (even when it's working properly) is that water vapour from the
breather will condense in the catch can and be returned to the timing
Another problem is that the drain pipe can suck air into the engine which destroys any negative pressure in the crankcase.
Now, there's and old saying that "oil and water don't mix" but that isn't quite
true. If you whisk oil and water together vigorously they will form an emulsion, which has
the consistency of mayonnaise and is about as useful as a lubricant. Any water returned
from the catch can to the timing side will be vigorously mixed with oil by the timing
gears to form this emulsion.
This is not what I want in my engine!
If enough of this goo forms in the catch can and breather pipes it can bung up the drain
to the timing side so the catch can fills up and dumps all the crap into your air filter
(not good for the filter)
Emulsion forms more in a cold engine and will tend to separate back out if the engine gets
to full operating temperature so short runs make the problem worse.
If you do lots of short runs then frequent oil changes will help.
Why does RE make them like that?
I can only assume that it's something to do with emissions control and is an attempt to
not dump any oil on the road.
What can you do?
(This is just my opinion remember)
Remove the catch can together with all the piping.
Seal off the drain to the timing side. (some people say a plastic number plate screw does
the job with a bit of silicon to seal it. It wouldn't fit mine so I sealed off the pipe
and left it in place for now)
Fit a length of 1/2" ID plastic tubing to the vent in top of the oil tank (00 jubilee
Lead the pipe up behind the battery to terminate under the LHS rear footrest and fit a
duckbill in the end. (you can take the one out of the catch can or get a new one)
If you prefer, you can extend the tube all the way to the back of the bike over the rear
As the engine vents from the crankcase via the oil tank, you should not get more than a
slight oil mist out of the back - most of what comes out should be water. If you get any
significant amount of oil out the back you may have other problems with the engine.
Some people terminate the vent over the chain so that any oil vapour will help to
lubricate it but don't forget that water will be vented on to your chain as well.
What can't you do anything about?
As the engine breathes via the oil tank you will get water vapour in to the oil tank and
some of it will condense in there (especially when the engine/oil is cold) It will not
form emulsion with the oil too readily as it is not getting thrashed around in there (not
like in the timing chest) but it will do to an extent. Most of it should be driven off
again when the engine warms up.
The only way I can think to cure this is to fit the breather back where it used to be on
earlier models (primary side just below the bottom of the cylinder)
This would require you to drill the crankcase and fit some sort of spigot.
It's something I may do if I ever have the case split but probably not till then.
You could also seal up the hole through to the oil tank (but you would definitely need the
case split to do that)
One slight advantage to venting via the oil tank is that it should catch any
"globs" of oil which are blown out of the crankcase, so you should only get oil
vapour out of the breather.
I say again, this is just my opinion (after looking at lots
of other people's ideas on various forums)
It works for me!
Since this picture was taken I have
re-routed the pipe to run further up behind the battery.
discourages oil to escape as it tends to fall back into the oil