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#861525 10/23/21 9:49 pm
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dose using a vent tube and hose help at all on relieving crankcase pressure in Pre-Unit 650 6t 1951. thought I saw that once on a Cafe' bike?

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scott garland #861542 10/24/21 6:27 am
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In my experience, a big extra breather reduces oil leaks.


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scott garland #861545 10/24/21 9:24 am
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There are two main schools of thought on crankcase breathing and they can't be mixed. One is to try to pump down the pressure in the case by a timed breather or a one way valve. These two can be combined as they in essence do the same thing.
The other principal is to let the under piston volume escape through one or several big outlets into the open air as on the ca 72-on Nortons or via a plenum chamber as on the 70-on Triumph twins.
The former system can't be mixed with the latter, as it would let air in when you try to create a low pressure, which is the opposite of what you want. I see this frequently, vented rocker box caps on Triumph twins is a favorite.
The latter system doesn't really care though, the timed breather can be retained if you want, though most people will blank them off.

The main disadvantage of the timed breather is its convoluted and rather small path to the outside. It often struggles with high rpm use and/or excessive blow by, and the addition of a one way valve somewhere can be beneficial. It has to be understood that the process of achieving low pressure, or even negative pressure, happens over several revolutions after start up, it isn't immediate. A spot of oil from the breather after a cold start is common, especially on engines that tends to let oil through to the sump while left standing for a period of time. Less of a problem on Triumphs than some others.

The main problem with the "open" system is that you can't really make it big enough, though in practice the Triumph system in particular seems to work well enough. Norton guys these days seems to favor a reed valve system in the sump, which reportedly works well. I converted the non-timed Combat breather to a one way valve with basically a disc and a spring and it solved (with other mods) a bad scavenging problem. I've used one way valves on the existing 850 breather hoses and it does work, but one needs to keep in mind that by doing this, you convert your system from an open system to a timed one, so any additional breathers needs to be blanked off or fitted with one way valves too.

SR

Stein Roger #861625 10/25/21 4:30 am
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thanks for the reply

scott garland #861665 10/25/21 6:43 pm
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The accepted practice in racing engines is to use a vacuum pump or exhaust ejector to create a partial vacuum in the crankcase


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scott garland #861670 10/25/21 7:44 pm
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What is on my dual engine land speed racer...The only vents are what you see ,open to the atmosphere... They blow out no oil, so not catch can needed. My single engine racer is the same .I don't see in LSR that vacuum devices add any speed on Brit bikes..But I don't see everything

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thanks everyone for the reply's, my friends bike is a stock 1951 6T and seems to gain about a 1/4 inch rise in oil level on a 50 mile or so ride. We are having trouble trying to find out how it is getting there.

scott garland #861982 10/30/21 4:16 am
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On my roadrace 750 T140 I used a belt drive so I had to make alternate provision for a breather. I welded the bolt-on T140 breather fitting from behind the clutch onto the timing cover directly over the end of the exhaust cam so the cam gears wouldn’t fling oil into the breather and ran a 3/4 inch clear vinyl tube up and over the intake manifolds to a catch can behind the left shock— pretty much what Hillbilly Bike shows. I drilled 1/2 inch holes in the crankcase in a couple locations and used the timing case as a small breathing space or plenum, and of course this still air space then vented up the tube. On startup it would visibly pump a little oil up and down in the vertical portion of the breather (without pumping it out) but as the engine got hot and the revs came up it reached a pressure balance—a thin oil film would just hang in the vertical without any pumping oil up and down and just about nothing ended up in the can even after extended running above 7000 rpm. I did use gapless second rings so that probably helped a lot on blow by. This is more or less the same crankcase volume as a preunit, with lots of revs, and it worked quite well. And it worked better after I blanked off the valve cover breathers I had added. Good point, Stein, I forgot I did that until you mentioned that. Other guys I raced with drilled vent holes directly from the crankcase into the hollow of the front motor mount then vented that space out a reed valve up a tube paralleling the frame down tube but I think my arrangement worked better. But again, this was a fairly extreme setting and a street bike doesn’t need this sort of breather.

Not sure what to make of your oil level increasing after running. LOL I’ve never had that “problem.”

Last edited by linker48x; 10/30/21 4:21 am.
linker48x #862113 10/31/21 8:28 pm
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If it works it works, that's what matters. One can always theorize on it afterwards, if one wants to. Norton arrived at the same solution as linker and Hillbilly for their 850 twin, and I'm not arguing with any of the above. Many singles, including the mighty Panther, breathed through a simple drilling and a rudimentary unsprung disc flapper in the drive side mainshaft, which suggests that that approach may work well too.

However, whenever I see fancy breathers all over an engine, something is usually amiss. The latest example from my garage is a 68 Bonnie with an added breather with a one way valve connected to the timing hole behind the cylinders. This is possibly the worst place for a breather on a Triumph twin, but the reason they even tried was badly scored pistons and cylinders combined with a wrong primary sprocket without a boss for the crankcase seal to seal on. The seal itself was there, but installed the wrong way around. Any of these faults would have compromised the timed breather, which was incidentally correctly fitted.

SR

Andy Higham #862122 10/31/21 10:07 pm
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Originally Posted by Andy Higham
The accepted practice in racing engines is to use a vacuum pump or exhaust ejector to create a partial vacuum in the crankcase

My understanding is that venting into the exhaust works well until you start adding mufflers. The necessity for mufflers (mandated by the people against everything) spawned the use of vacuum pumps. Again…. if I understand correctly, initially the plan was to keep oil (and seals) in the engine, but a side benefit was a small hp gain. Sometimes I wonder if the pumps are overdone and end up sucking out too much of the precious oil mist. Some use a pop off valve to limit vacuum. Aircraft Spruce sells kits for venting into the exhaust, but it also uses a check valve. They mention that lower pressure means a lower boiling point which is an added benefit.

https://www.aircraftspruce.com/catalog/eppages/crankcase05-12429.php

scott garland #864260 11/26/21 7:49 am
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Originally Posted by scott garland
thanks everyone for the reply's, my friends bike is a stock 1951 6T and seems to gain about a 1/4 inch rise in oil level on a 50 mile or so ride. We are having trouble trying to find out how it is getting there.

Probably a combination of oil which has drained into the crankcases and pumped back into the oil tank plus a slight expansion in volume of oil due to heat.

The ideal engine breather (on a 360 deg. twin or single) creates minimal resistance as the pistons rise and minimal resistance when the pistons descend and clears the crankcase of blow by gases. One way check breathers do the latter but not the former because the pistons are working against a significant vacuum as they rise. Open breathers create resistance on the up and down strokes and the gases and oil mist shuttle back and forth along the breather tube.

Norton did a pretty good job on the Mk3 using piston vacuum to scavenge crankcase gases via the oil tank free space.

Last edited by Chris the camper.; 11/26/21 7:51 am.
scott garland #864264 11/26/21 9:44 am
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1/4” isn’t much. I wouldn’t have noticed that.

What happens when you ignore it and do some more riding?


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