crankcase breather

[Derby Rob]

hi,
     after just over 500 miles since the restoration of my 61 a7 the number of oil leaks seems to be getting greater.(my dad told me if it dont leak oil theres none in it!) ,but i am sure it never had all these leaks in the first 100 miles!.i have had the timing cover off a number of times to reseal it,also put sealant around the dynamo to timing case gasket many times ,but it still leaks.

     So one time i cleaned all the oil off the engine and frame and went for a quick ride,just enough to warm it all up.got home put it on the center stand and checked for leaks ,just saw two one from the dynamo gasket again! and the other one seem to come from some where middle of the crankcase under the dynamo,or at least i thought so.

    These leaks soon stopped within a few minutes? why i thought the oil would still be warm and thin? Oh by the way i forgot to say it never leaks when cold and parked in the shed.

    So i came to the conclusion it must be forced out by excessive crankcase pressure,so reading the old bsa service sheets i read some where that if the crankcase breather had any play it would result in a very oily engine.

   So this weekend off came the timing covers and checked the breather .......yes there was some play in the breather ,about 0.4mm sorry for the metric guys .looked in my gasket set and found the next thickness of cork washer ,fitted that but was so tight i could hardly turn over the engine!

  Well i had no gasket material of the thickness i needed ,measured the cardboard that my full gasket set came on 0.4mm hummmm worth a try, so cut a ring of cardboard to fit under the old cork gasket ,refitted everything and went for 10 mile local run ,stopped half way to check for leaks.....none! and when i rev the engine a slight mist could be seen coming from the vent hole in the crankcase just behind the primary case.

   When i got home checked again no leaks! no sure how long the cardboard will last but at least it proved it was a problem with the crankcase breather .Although still trying to work out how the bloody thing does what it does and why the little play can make all the difference!

   If any one could explain the workings of the crankcase venting (in simple terms please) just for my own piece of mind it would appreciated!!!.
 
 
   

[duTch]

 If it makes you feel better, that's what I did- but with gasket paper..!

 my understanding of the workings is;- the hole in the side of the breather 'top hat' aligns with the hole in the aperture in the case every time the pistons are at the bottom of the stroke, to allow the excess built up C/case pressure to be released through hole in top of top- hat via this gallery to atmosphere

 comments welcome...?

[KiwiGF]

like Dutch says, and the top hat /rotary valve shuts when the pistons go up meaning you get a vacuum in the cases...theoretically at least......

The idea is that even if there is a bit of blow by the pistons when the engine fires near tdc there will be a vacuum in the cases to offset the pressuring effect of the  blow by gases, and as the pistons go down any positive pressure caused by the blow by gases will be let out when the pistons are near the bottom of the stroke, hence one can generally feel a slight puffing out of the breather hole on tickover which I guess is equal to the amount of blow by gases.

If the top hat/valve leaks due to having too thin a cork washer it will stop the valve being able to cause a vacuum in the cases as the pistons go up and oil leaks may result!

Simple eh?

[Greybeard]

How does the plunger engine breath?

[muskrat]

Same as S/A

[Greybeard]

I don't think I have any kind of breather pipe on my engine. Is the pressure released into the return oil pipe to the tank and out onto the chain sprocket maybe?

[bsa-bill]

Top hat breather releases the pressure into a gallery at the rear of the camshaft, it exits out of a hole that points downward and situated at the drive side of the engine, near the top of the crankcase but behind the primary case, you can just feel it with a finger poked down behind the engine and between the magneto (phew - running out of breath)
I believe some engines had pipes on there but most don't unless fitted by the rider, it was a popular mod at one time, the pipe being taken to feed onto the rear chain

[Greybeard]

Thanks guys; I'll have a go at finding the breather when I get a chance.

[Derby Rob]

hi,
    thanks for the comments guys,things have become a little clearer now.And in reply to Greybeard,i never thought i had a vent hole in my crankcase until i looked for it,needed one of those small inspection mirrors on a flexible shaft and a torch to find it though!

[BSA_54A10]

big snip behind the primary case, you can just feel it with a finger poked down behind the engine and between the magneto (phew - running out of breath)
I believe some engines had pipes on there but most don't unless fitted by the rider, it was a popular mod at one time, the pipe being taken to feed onto the rear chain

The long strokes had a factory fitted copper pipe and a P clip which ftted to the back of the chain case .
At some point in time BSA stopped doing this and there is a dealer service bulliten advising that they be removed due to the p Clip coming loose & getting picked up by the chain following which the pipe did the same thing.

[duTch]

 

,needed one of those small inspection mirrors on a flexible shaft and a torch to find it though!

That was going to be my suggestion to Gray-B, great tools, but spend half my time looking for it/ them...
least it can't be lost inside the engine 

 I attached a length of clear hose to the pipe in mine intending to run it into a small bottle, and/or venting to near the air intake/filter, so that any fumes would either be collected or sucked up, kinda like the PCV's on Holdens '60's-> (PCV= primary crankcase ventilation)- got halfway there...!

 I think not all engines breathe the same way, the Gutzzi, has a external breather box, integral with,and vents to the air box(850T), but later ones have a box with a 'valve' and then vents to the road behind

[muskrat]

My '51 short stroke had one of those fitted Trevor. It looks like it had been eaten by the chain a couple of times.
Cheers

[Brian]

The one thing you can not do is restrict the air flow out of the breather.

A few years ago I thought I would put a tube on the breather and run it back to the oil tank, seemed like a good idea at the time ! What happened was after a few miles the oil mist built up to the point of collecting in the tube and the engine was not able to blow the oil up the tube to the tank, effectively restricting the breather. The result was a engine that started leaking oil everywhere.

[bsa-bill]

The one thing you can not do is restrict the air flow out of the breather.

Good advice Brian.
I'm wondering where you attached the pipe from the breather to the oil tank as the pipe exiting the oil tank is a breather for the oil tank, and sort of nearly one way.
The return side of the pump pumping oil plus air from the engine makes for the requirement of a breather in the tank so if you used this orfice you'd have pressure at both ends

[Brian]

What I did was put a new fitting in the very top of the tank so the breather tube went to that, then the tank could vent through its normal system.

Like a lot of things it seemed like a good idea but didnt work.