A Series Crank Case Breather

[Terryb]

Guy's, I always thought that the crank case breathing system via the cam shaft was to provide an oily mist to be emitted when the pistons came to the bottom of their stroke. So if that is the case why when reading through previous threads regarding this subject, some are experiencing substantial quantities of oil being discharged?

The reason for this question is that I have a deceased friend's 1961 A10SR to dispose of, which is experiencing regular oily discharges from the breather, which I've been asked to resolve. I myself have a 1955 A7SS and I've only seen a very small dribble from it's breather. Can I assume this Crank case pressure or would there be another reason?

[muskrat]

G'day Terry.
9/10 times it's wet sumping after it's been stood for a few days/weeks and will clear after the sump has been emptied. Depending on how much oil is in the sump the bigger the mess and time to clear.
If blowing oil out on a ride the oil pump may not be scavenging enough. The timed breather cork is too thin and the breather has jumped off it's peg. The rings are shot and allowing too much blow by increasing crank case pressure.
Just a few things to check.
Good luck.
Cheers

[KiwiGF]

Guy's, I always thought that the crank case breathing system via the cam shaft was to provide an oily mist to be emitted when the pistons came to the bottom of their stroke. So if that is the case why when reading through previous threads regarding this subject, some are experiencing substantial quantities of oil being discharged?

The reason for this question is that I have a deceased friend's 1961 A10SR to dispose of, which is experiencing regular oily discharges from the breather, which I've been asked to resolve. I myself have a 1955 A7SS and I've only seen a very small dribble from it's breather. Can I assume this Crank case pressure or would there be another reason?

There should be a vacuum in the crankcase, caused by the timed breather during the few seconds after start up, the breather output should then after start up (theoretically) be equal to the volume of blow by (from the pistons). It should not pick up much oil on its way out.

Having said that, some experience success with “breathers” on the rocker box that would in theory work against the oem breather, and not allow a vacuum to be created 

[limeyrob]

My understating is that the BSA timed breathers were intended to maintain a slight vacuum in the crankcase to reduce leaks.  Ideally all that would some out it oily air.  My guess is its wet sumped and the rings are sticking.  You could idle the engine for a while and see if it improves with some running time but I would want to know how much of the oil is int e case before trying that.

[Terryb]

OK thanks, I’m getting a slightly better understanding of the breather workings.

The bike has had a history of dumping oil from the breather since 2012, after its first refurb. The previous owner has tried a number of different ways to solve it, including dumping to ground, over the rear suspension and into the oil tank. The oil tank mod worked, but didn’t resolve the quantity of oil.

So, now I’ve volunteered to try and resolve it 🥴 with respect in 2015 the engine was stripped angain and restored including new pistons, rings and top end new item. So I think it’s most likely wet sumping or the breather assembly is suspect. Also I’m told that most of the oil is coming out is on startup or on tick over, don’t know about when riding.

Anyway, I have my A7SS to finish and then we will begin, so thanks for now, I will be back when I find out more. Cheers guys 👍

[Swarfcut]

 Working correctly there will be some element of oil vapour from the breather, but a continuous flow indicates a sump full of oil for the reasons mentioned. Couple of things to check.......sump plate off, see what comes out, big bowl handy, and give the little ball valve down there a prod to make sure it has not stuck shut. There's a whole lot on the Forum about failure to scavenge, from  leaks in the pick up pipe, oilpump gaskets, cracked castings and blocked oil return to the tank.

 A good oil return to the tank soon after start up is a visual indication that the oilpump is working to scavenge the sump.

 As for how the breather works......opened for a mere instant....defies all logic, but it does. Maintaining a vacuum against a twin cylinder air pump is a clever trick. Some say a bloody miracle.

 Swarfy.

[chaterlea25]

Hi Terry and All,
A couple of other things to check..

Remove the timing outer cover and  start the engine briefly, while you look to see if oil is leaking from the pump, spindle, body or gasket area, or if oil is leaking out between the main bush and crankcase..
I have come across  one or two instances where excess leakage  causes the timing gears to carry enough oil up and flood the breather area..
There were some crap quality rings about some years ago, that were either too hard or irregular shaped to seal properly

John

[berger]

John a yes on the bad  rings i had some , they were useless and then i discovered gandini , perfect

[Slymo]

Second or third the rings thing. The rings that come with JP pistons are too hard and never seem to bed in. I too swapped them for Gandini rings and never had a issue since. I also assembled the bores dry which took a leap of faith but it was recommended to me by the engineer that bored a Ducati bevel motor for me and it seems to be a very successful ploy.

[Rex]

I've been assembling pistons and rings dry (actually the merest smear of oil) for many years now, ever since reading that Hartley the pre-war tuner always did it.
Modern oil is just "too slippery" to allow rings to quickly bed in before becoming work-hardened, and there's nothing worse than seeing some old duffer still running in his bike at 30mph over 500 miles while leaving a smoke-screen that a WW1 sub-chasing destroyer would be proud of.

[Triton Thrasher]

On my 650 twin from another Midlands factory, that dry assembly and immediate acceleration made a noticeable difference in ring sealing.

Bloody hard to kick over, in fact!

I also made the second ring gap about 25% bigger than the top ring gap.  That’s another trick I got off the internet.

[Terryb]

Ok guy’s, Shooter back on the road 🤪

The SR is wet sumping, obviously when the engine is hot and the oil is thinner. She’s incontinent initially until the oil thickens and after say 4-6 weeks she’ll have about a pint in the sump. So that’s why oil is being dumped out of the breather at startup.

Many thanks guys for your input. This forum is invaluable to us classic fiddlers, cheers👍

[Swarfcut]

 How do you folks reckon a simple plug on the oiltank breather would work to lessen draindown into the sump on standing?

  The alternatives of stripping out and replacing the anti-drain valve, or further modification as on A series unit engines (ball behind pump) or the maligned and some say risky external flow valve have all been championed, but I've never heard of  simply restricting air into the oiltank to prevent the sump filling on longer periods of storage.

 Seems a simple idea, would it work? Easy to find out, no need to reach for the spanners every time it comes out of the shed. Just finish your ride, park up, put in the plug and add your reminder for next time.

 Swarfy.

[berger]

what about the oil cap? clingfilm ?

[muskrat]

How do you folks reckon a simple plug on the oiltank breather would work to lessen draindown into the sump on standing?

  The alternatives of stripping out and replacing the anti-drain valve, or further modification as on A series unit engines (ball behind pump) or the maligned and some say risky external flow valve have all been championed, but I've never heard of  simply restricting air into the oiltank to prevent the sump filling on longer periods of storage.

 Seems a simple idea, would it work? Easy to find out, no need to reach for the spanners every time it comes out of the shed. Just finish your ride, park up, put in the plug and add your reminder for next time.

 Swarfy.

G'day Swarfy.
That has merit, especially if capped off after a ride while still hot. As it all cools it would create a vacuum in the tank.
Cheers