1972 Oil Breather Problems

Joined
Apr 15, 2009
Messages
10,728
Country flag
I have an 850 crankcase with PCV reed valve issuing from the rear timing side (standard position) into a small catch bottle.
I have fitted the barrel which has been lined to 750 (for racing purposes), with new Omega pistons, rings and big end shells, along with a new oil pump. The engine was rigorously cleaned including the centre of the crankshaft, before re-assembly.
Fired her up, and all ran sweet as, no oil issuing into the catch bottle.
Having got the engine warmed up, I revved it sporadically to 4500 - 5000 rpm, and when I turned round, the catch bottle was full of foam, and oil pouring out the breather hole in the bottle. I re-routed the main breather pipe after the PCV valve directly into the swirl pot oil tank, and managed to race without further issues.
The unit has an oil cooler, and I heard someone say that this can cause issues with the breather pumping oil out. It certainly was OK as an 850 with no cooler!
Any ideas?
Which specific PCV valve are you using?
 

L.A.B.

Moderator
VIP MEMBER
Joined
Nov 20, 2004
Messages
15,828
Country flag
I have an 850 crankcase with PCV reed valve issuing from the rear timing side (standard position) into a small catch bottle.
I have fitted the barrel which has been lined to 750 (for racing purposes), with new Omega pistons, rings and big end shells, along with a new oil pump. The engine was rigorously cleaned including the centre of the crankshaft, before re-assembly.
Fired her up, and all ran sweet as, no oil issuing into the catch bottle.
Having got the engine warmed up, I revved it sporadically to 4500 - 5000 rpm, and when I turned round, the catch bottle was full of foam, and oil pouring out the breather hole in the bottle. I re-routed the main breather pipe after the PCV valve directly into the swirl pot oil tank, and managed to race without further issues.
The unit has an oil cooler, and I heard someone say that this can cause issues with the breather pumping oil out. It certainly was OK as an 850 with no cooler!
Any ideas?
 
Joined
Apr 27, 2015
Messages
1,630
Country flag
I have a timing side reed breather on my commando. When I did the modification, I drilled 2 extra holes from my timing chest into the crankcase. I drilled a 3/8" sized hole at about 9:00 in relation to the intermediate shaft bearing as being center of the clock, being carefull to drill between the webbing of the casting so I didn't weaken the case. The other hole I drilled was a 1/4" sized hole below the oil pump. The second hole is to drain the height of the oil down somewhat in the timing case to offset the tendency of the vacuum being applied by the reed valve to pull oil into the timing chest. I have it on good authority that I've done no harm to my engine with these two holes... If you are getting a lot of blow by, perhaps your oil level in the timing chest is so high that you are getting a huge amount of oil discharge at higher RPM's... That's my guess

Here's my modification below...



With that single timing chest breather it helps to eliminate leaks, but it's not the most efficient breather because the pulse is generated in the crankcase and then travel through the timing case too, so the volume of compressed air is greater than just the crankcase if you used a crankcase mount. Moving a smaller volume of more compressed air would be more efficient.... which is why the direct crankcase breathers actually block off the timing chest holes to reduce the volume of air that is pressurized.

I went with the timing side reed valve modification alone for a few years, but I could tell by the occasional drips of oil around the tachometer drive that I could still improve crankcase pressure issues, so since I have a '70 commando I was able just to add Jim Schmidt's camshaft reed breather to the other side of my bike to assist my timing chest breather modification, and there's virtually no oil leaking anywhere... dual breathers...
 
Last edited:
Joined
Apr 15, 2009
Messages
10,728
Country flag
I have a timing side reed breather on my commando. When I did the modification, I drilled 2 extra holes from my timing chest into the crankcase. I drilled a 3/8" sized hole at about 9:00 in relation to the intermediate shaft bearing as being center of the clock, being carefull to drill between the webbing of the casting so I didn't weaken the case. The other hole I drilled was a 1/4" sized hole below the oil pump. The second hole is to drain the height of the oil down somewhat in the timing case to offset the tendency of the vacuum being applied by the reed valve to pull oil into the timing chest. I have it on good authority that I've done no harm to my engine with these two holes... If you are getting a lot of blow by, perhaps your oil level in the timing chest is so high that you are getting a huge amount of oil discharge at higher RPM's... That's my guess

Here's my modification below...



With that single timing chest breather it helps to eliminate leaks, but it's not the most efficient breather because the pulse is generated in the crankcase and then travel through the timing case too, so the volume of compressed air is greater than just the crankcase if you used a crankcase mount. Moving a smaller volume of more compressed air would be more efficient.... which is why the direct crankcase breathers actually block off the timing chest holes to reduce the volume of air that is pressurized.

I went with the timing side reed valve modification alone for a few years, but I could tell by the occasional drips of oil around the tachometer drive that I could still improve crankcase pressure issues, so since I have a '70 commando I was able just to add Jim Schmidt's camshaft reed breather to the other side of my bike to assist my timing chest breather modification, and there's virtually no oil leaking anywhere... dual breathers...
On my 72 I did the opposite since I was going to be breathing through the crankcase and not the timing case. I plugged as many holes as I could although I did drill the lower hole to keep the oil level down.
 
Joined
Nov 30, 2018
Messages
88
Country flag
I have also seen the Combat engine mod drawing for the extra holes in the crankcases. Checked mine, and the drive side mod is not relevant as the scavenge hole on the 850 is in the right position. On the timing side case, only one of the recommended holes was not present - a 3mm dia hole under the crankshaft bearing, just right of the lowest timing cover mounting hole. I added this on rebuild. This should have the same effect as the hole modification under the oil pump mounting?
However, having now read previous posts, should the upper holes be blanked off due to the PCV valve?
 
Joined
Apr 15, 2009
Messages
10,728
Country flag
Having got the engine warmed up, I revved it sporadically to 4500 - 5000 rpm, and when I turned round, the catch bottle was full of foam, and oil pouring out the breather hole in the bottle. I re-routed the main breather pipe after the PCV valve directly into the swirl pot oil tank, and managed to race without further issues.
Catch bottle I get, but what is a swirl pot oil tank?

If you go back to a catch bottle now does it still fill up rapidly?
 
Joined
Apr 27, 2015
Messages
1,630
Country flag
I don't see where an oil cooler, or no oil cooler, should change the way your breather works. Could it be raising the level of oil in the timing chest? or lowering it so it whips the oil into a froth? Hard to know exactly without experimenting a little bit. I think you might be on your own with that aspect.

Personally, A catch can makes no sense to me. Why not just run the breather back to the oil tank and then have the oil tank breather separate the air from the oil. This way the air leaves,... and the oil is where it is needed, back in the oil tank instead of a catch can.

It sounds like you are building a race bike, so maybe my experience and suggestions don't apply. I have played with reed breathers and mounted a translucent tube on one to see how it works. What I saw was a marching column of oil bubbles going up the tube when the engine was running. When I goosed the throttle those bubbles moved faster. When I turned the engine off, all the bubbles popped and ran down to the breather. There was about 3" of oil in that tube, so oil is ejected with the air... I don't think there's a breather that works well that doesn't have that characteristic...
 
Joined
Jul 16, 2020
Messages
126
Country flag
This is a PCV valve we used on our race bikes a long time ago.
It is a Ford part, quite large. I think you can see the part number in the picture.
inlet and outlet sizes are .75 inches
 
Last edited by a moderator:
Joined
Apr 15, 2009
Messages
10,728
Country flag
This is a PCV valve we used on our race bikes a long time ago.
It is a Ford part, quite large. I think you can see the part number in the picture.
inlet and outlet sizes are .75 inches
PCV valves are typically just plastic flappers. 4 cylinders (and more) typically don’t have the pumping pressures like a 360 twin. If you use one you may want to take it off periodically and shake it.
 
Last edited by a moderator:
Joined
Nov 30, 2018
Messages
88
Country flag
Catch bottle I get, but what is a swirl pot oil tank?

If you go back to a catch bottle now does it still fill up rapidly?
A swirl pot tank is a cylindrical oil tank with a 'swirl' helix ridge that funnels the oil down to the outlet just that bit faster. Mainly used on engines converted into dry sump versions. I know the Commando is just that, but on the racing sidecar, it is mounted under the seat unit, and the revs used require some pretty fast oil movement!
As catch bottle filled up rapidly at the last race meet, I haven't (daren't) retry the catch bottle as the engine hasn't had any changes since then...
 
Joined
Nov 30, 2018
Messages
88
Country flag
I don't see where an oil cooler, or no oil cooler, should change the way your breather works. Could it be raising the level of oil in the timing chest? or lowering it so it whips the oil into a froth? Hard to know exactly without experimenting a little bit. I think you might be on your own with that aspect.

Personally, A catch can makes no sense to me. Why not just run the breather back to the oil tank and then have the oil tank breather separate the air from the oil. This way the air leaves,... and the oil is where it is needed, back in the oil tank instead of a catch can.

It sounds like you are building a race bike, so maybe my experience and suggestions don't apply. I have played with reed breathers and mounted a translucent tube on one to see how it works. What I saw was a marching column of oil bubbles going up the tube when the engine was running. When I goosed the throttle those bubbles moved faster. When I turned the engine off, all the bubbles popped and ran down to the breather. There was about 3" of oil in that tube, so oil is ejected with the air... I don't think there's a breather that works well that doesn't have that characteristic...
I believe the theory behind the oil cooler maybe not helping with breathing, is that the return feed to the tank travels a fairly long path out to the cooler, then has to run through the radiator fins before travelling back to the tank. Maybe causing extra pressure in the return line? I really don't know to be honest. Catch cans were used predominantly on water cooled engines as air, oil and water breathed out of the engine, and definitely not good to feed that back into the oil tank. I guess that a catch bottle on the Commando let's you see how the engine is breathing and how much oil is driven out. I thought that the reed valve (apart from improving engine run) would also reduce oil in the breather! Not so as it happens...
I am running the breather directly into the oil tank now, and as no water is involved, feel happier with that solution, especially as the oil tank has a vented cap.
All new stuff happening, and just trying to keep up with observations to make improvements! My thanks to all who contribute to my learning curve...
 
Joined
Nov 30, 2018
Messages
88
Country flag
PCV valves are typically just plastic flappers. 4 cylinders (and more) typically don’t have the pumping pressures like a 360 twin. If you use one you may want to take it off periodically and shake it.
It seems that the XS 650 reed valve is the recommended part for Nortons/Triumphs and the like. They are on sale in USA, but shipping price is higher than part price! Found Holland Norton offer them at a good rate and much cheaper shipping.
 
Joined
Apr 15, 2009
Messages
10,728
Country flag
A swirl pot tank is a cylindrical oil tank with a 'swirl' helix ridge that funnels the oil down to the outlet just that bit faster. Mainly used on engines converted into dry sump versions. I know the Commando is just that, but on the racing sidecar, it is mounted under the seat unit, and the revs used require some pretty fast oil movement!
As catch bottle filled up rapidly at the last race meet, I haven't (daren't) retry the catch bottle as the engine hasn't had any changes since then...
If you feel like testing it would be interesting to see what happens now if you run it to a catch bottle. Obviously not for a race. :oops:
 
Joined
Apr 15, 2009
Messages
10,728
Country flag
It seems that the XS 650 reed valve is the recommended part for Nortons/Triumphs and the like. They are on sale in USA, but shipping price is higher than part price! Found Holland Norton offer them at a good rate and much cheaper shipping.
As have been said before the part isn't actually from an XS650, but some sort of valve that is sold through Mike's XS650 site. I wonder if Holland Norton Works buys them in bulk from Mike's or found a better source.
 

illf8ed

VIP MEMBER
Joined
Dec 30, 2003
Messages
2,873
Country flag
This is where I located Mike’s XS reed type engine breather on my combat 201881. Fits nicely under the seat. Large breather hose from the crankcase to the reed valve then smaller hose to the oil tank. Just checked. I bought this Aug 2009. It is not affected by oil pumping through back to the oil tank when the engine has wet sump (ed).
5432D54D-3A99-448D-983A-4B72FBEED94F.jpeg
 
Joined
Jul 16, 2020
Messages
126
Country flag
PCV valves are typically just plastic flappers. 4 cylinders (and more) typically don’t have the pumping pressures like a 360 twin. If you use one you may want to take it off periodically and shake it.
The good news is the openings are so large you can see inside, and that the internals are metal.
I should have pointed out in my first post, I found these , I think, in a book that included tips from Grumpy Jenkins, this is what they were using in there 500 Cu In drag race motors, 1000 + CC per cylinder.
 
Joined
Apr 27, 2015
Messages
1,630
Country flag
I believe the theory behind the oil cooler maybe not helping with breathing, is that the return feed to the tank travels a fairly long path out to the cooler, then has to run through the radiator fins before travelling back to the tank. Maybe causing extra pressure in the return line? I really don't know to be honest. Catch cans were used predominantly on water cooled engines as air, oil and water breathed out of the engine, and definitely not good to feed that back into the oil tank. I guess that a catch bottle on the Commando let's you see how the engine is breathing and how much oil is driven out. I thought that the reed valve (apart from improving engine run) would also reduce oil in the breather! Not so as it happens...
I am running the breather directly into the oil tank now, and as no water is involved, feel happier with that solution, especially as the oil tank has a vented cap.
All new stuff happening, and just trying to keep up with observations to make improvements! My thanks to all who contribute to my learning curve...
The way it works in my bike is the dual breathers pump air (and oil) into a common hose that dumps out into a port on the roof of my oil tank. That puff of "air" goes into the tank causes the tank to pressurize. In my tank there's a built in "top hat" on the roof of the tank which the tank's breather tube runs up into in an attempt to draw the "cleanest" air out of the tank to releive the tank pressure from the breather hose "puff".

It works, but still some very small amounts of oil make it into the tank breather tube and dribble out that breather hole exit. The hole's exit is positioned on the forward face of the oil tank, so the air and oil mix would be drawn into the engine with the intake air and be harmlessly burned with the fuel..

Some people put a tube and a catch can on that oil tank breather port and only tiny amounts of oil are collected, which kind of shows that it works pretty well getting the oil out of the pulses of air. Below is a picture of the early tank... When you see the image with the "top hat", it's a little easier to visualize how the design works...

 
Joined
Nov 30, 2018
Messages
88
Country flag
If you feel like testing it would be interesting to see what happens now if you run it to a catch bottle. Obviously not for a race. :oops:
It is easy enough to try out before I run it in earnest, but as there have been no changes in the set up since the 'disaster', I am not confident I will see any difference. I am now looking at fitting an oil/air separator catch can (the previous owner of the frame had one when he raced), just working out how and where to fit it, and if I can possibly buy on that is not made in China!
 
Joined
Nov 30, 2018
Messages
88
Country flag
The way it works in my bike is the dual breathers pump air (and oil) into a common hose that dumps out into a port on the roof of my oil tank. That puff of "air" goes into the tank causes the tank to pressurize. In my tank there's a built in "top hat" on the roof of the tank which the tank's breather tube runs up into in an attempt to draw the "cleanest" air out of the tank to releive the tank pressure from the breather hose "puff".

It works, but still some very small amounts of oil make it into the tank breather tube and dribble out that breather hole exit. The hole's exit is positioned on the forward face of the oil tank, so the air and oil mix would be drawn into the engine with the intake air and be harmlessly burned with the fuel..

Some people put a tube and a catch can on that oil tank breather port and only tiny amounts of oil are collected, which kind of shows that it works pretty well getting the oil out of the pulses of air. Below is a picture of the early tank... When you see the image with the "top hat", it's a little easier to visualize how the design works...

Looks good! Unfortunately I don't have the space to fit something like this on the kneeler sidecar! I may look at improving the oil tank breather from a vented cap to something more substantial. Also considering an oil/air separator...
 
Top