Engine breathing (Oil drip)

[LJ.]

Is the brass tube just a press fit in the hole?

Can be, Yes! If you can find the perfect fitting tube. I remember looking around for the ideal fitting brass tube and found that a good model shop will sell this. I seem to remember that the hole was 8mm in diameter, I maybe wrong as brain cells struggle to hold data nowadays. It seems that BSA may well had fitted these tubes as Brian suggests but with heat and vibration they would eventually be prone to falling out. However a good rubber hose fixed with a jubilee clip and securing to the frame should ensure it stays put.

I found it interesting during my research on this, that by looking at crank cases on ebay and at autojumbles, a number of people had tried to overcome the problem of the tube slipping. I had seen a number of punch marks around the hole and various cements and glues etc. This might all seem very petty but I do agree with Brian about parking up on someones neat and tidy paved drive way. Plus the old BSA/Oil leak jokes get a little tiresome after a while.

[dpaddock]

Ah, the eternal breather problem!
The root cause is blowby gases from the combustion chambers during the power strokes. This phenomenon is multiplied by the 180-degree crank arrangement, whereby both pistons pressurize the crankcase in unison. More importantly is the fact that the crankcase has very little volume to absorb the pressure rise from blowby, and when the breather valve opens the crankcase pressure is initially quite high - especially in "non-new" engines.
A simple fix would be to increase the case volume by adding a bottle of sufficient volume to the crankcase to reduce the pressure rise at the breather valve. Wet sump engines have large crankcase volumes which greatly minimize this problem which, when coupled with multiple cylinders, create blowby pressures hardly much more than atmospheric.
Routing the engine breather outlet to the primary drive case is a step in the right direction as long as one drains the case regularly. I did this on a racing Gold Star with good effect, at least sufficient to complete a race card.
In any case, make sure there are no external openings to the engine that could increase the amount of gases the breather has to handle.

[Brian]

Hi dpaddock, I put a small bottle on the breather to see how much oil was coming out and after 200 miles I had about half a teaspoon 3 or 4 ml so its not a major problem, just an annoying one. What I had in mind for the primary case was to run the breather into the case and then put another breather in the case to vent it. All this would not be seen [forgot to say this is on the swing arm model] The small amount would only require the case to be checked for level every thousand mile or so, maybe less. Its a simple job on the s/arm models as they have a level pipe as part of the drain. I am in Australia so the climate is warmer and there is no condensation to deal with. My plunger had the same problem for about the first 1000 miles but once it had settled down it only very rarely drops any oil. This one has only done 700 miles so far and will hopefully improve as time goes on. I hope to ride both bikes so it will be interesting to see which one ends up being my favorite.     Brian.

[LJ.]

With the amount of oil loss that Brian reports, then I don't see any problem in draining into the Primary case. But for anyone thinking of going down this route having a considerable amount of oil loss then I think problems can arise within the clutch arrangement. Whilst an excessive amount of oil is good for the clutch bearings etc, I don't think it would be long before a slipping clutch problem arises. Anyone with a slack primary chain would be amazed to see how much splash there is in the case, especially if the level is above the reccomended hight.

By the way Brian... I have seen small breather holes in the clutch spring adjusting screw, thats if you have that sort of primary cover.

(Have a good weekend everyone! Safe riding out there.)

[dpaddock]

First of all, let me correct my post: I meant 360-degree crank, not 180-degree.
Anyway, there's no sense adding a bottle to the primary case to reduce excessive crankcase pressure. That phenomenon occurs in the crankcase, not the primary case.
In a perfect engine there would be no blowby and no external openings to the outside. Thus, there would be no need for breathing. If you visualize this, you can make your non-ideal engine less leaky - button it up and seal the combustion chambers from the crankcase.

[RichardL]

Gents,

I am returning to this topic in reponse to Pim's question asking, "...what this 'breather' is everyone is talking about?"

In this thread, we've talked a lot about the oil drip from the breather, but I don't think we answered the question as to why the breather  is really there and what the timing implies. Pim's question made me think a little harder about this. With quite a bit more more than a little repect for dpaddock, I'd like to suggest a different reason than blow-by or pressurization by the pistons. While it is true that pressure in the crankcase due to blowby gets out through the breather, the breather is not timed for this, as there are two ignitions for each breath from the breather.  With regard to pressure from the pistons when they come down, that pressure is relieved when they go back up.

Even though I wanted to, I did not try to calculate the pistons' position when the breather port is open. That would be interesting, but take too much time, also, my crankcase is now out for machine work. If someone knows, please share.

Anyway, my opinion is that the breather is serving two purposes, the most important being to achieve near-atmospheric pressure in the crankcase so that oil can be pumped in and sucked out. In a worn engine with enough blowby to presurize the crankcase, oil could probabaly, be sucked out, but it might not get back in. The second purpose is the exhausting of blowby, for the reason mentioned. While there may be something to the timing of the breather, I don't see it and I think it may not matter. Instead, I think it is just a convenience of the  architecture of the crankcase.

The vent on the oil tank serves a similar purpose as the breather (except for the blowby issue) in that it allows oil to be sucked out and pumped back. 

As always, I am quite interested in contrary and abundant more expert opinion.

Richard

[RichardL]

OK, I'm going to argue with myself. If all they wanted was atmospheric pressure, they could have just had a hole into the crankcase and forgotten about the rotating breather. So, now I think It must inhale on every other upstroke to prevent excessive oil (mist or otherwise) from being blown out on the down stroke.

Richard

[RichardL]

In honor of being completely obsessed, the arguement with myself continues (since no one else has told me I am wrong). Since my engine is disassembled, I decided to take the gears in question, the breather and the inside timing cover and look at the relationship of the breather hole with respect to TDC. My crankshaft is off to machining, so I had to look at the parts book to see the relationship of the woodruff key with the crank journals. From what I can see, the keyway points directly toward the journals. Based on this, and contrary to my previous assumption, it appears that breathing occurs on the downstroke approximately 90 degrees ATDC. To me, the 90 degrees part makes sense, because it should be the point of balance between compression and vacuum in the crankcase. I still like my idea of doing the breathing in the upstroke, but I guess BSA had another idea, yet to be revealed to me.

Richard

[Pim]

Sorry for being a complete noob, I understand what the purpose is now, but i can't picture it where this breather is, and where the oil leaks out. In time when the engine will be running I'm likely to find out I guess. But since you have got your engine apart, is it possible cou could take a picture so I can visualise it a bit better? Thank you for the detailed post(s) :)

Pim.

[RichardL]

I want to steer all to the following web page and the other pages connected with this one. I just discovered it while searching for breather timing topics. I don't know if some of you have seen this, but it includes the firsthand stories of engineers at BSA. On the page I am providing, Roland Pike describes designing the rotating breather for the Gold Star and discusses its timing. I don't know if breather timing for the Gold Star is similar to A7/10s. If so, I got it wrong. However, in honor of my current obsession, I will be reviewing it again.

http://www.restorenik.com/daytona/RP_chp_23.htm

[a10 gf]

(off topic)

Pim, don't be sorry, ask for any help and information, and soon you will answer others question. Everyone with an A have had lot's to learn, discover and ask or read about. It will be very interesting to read about your progress on the bike.

e.

[Pim]

(also off topic)
Ok:)


Manosound, I'm from the Netherlands (Enschede/Rotterdam)

[Rocket Racer]

I was recently reading an article on a racing A10 side car built in NZ in the mid fifties for which the owner had written to BSA due the indifference of his local BSA agent. the owner states David Munro from the technical department at BSA sent him a response dated 17th March 1954 in which he recommended the use of the famous 67-358 cam and a number of other mods including...drilling a new location hole in the mechanical breather to give the thirty degree retard that DM wrote was most important.
Thought you might be interested...

[brackenfel]

Brian,
This photo has been on the Forum before but I can't remember how to find it so I'll post it again...
My bike has never been remotely oil tight in the short time I've owned it and it's in bits to try and fix this.. I had a mysterious leak on the primary side - loads of oil dropped but I couldn't see where from.

When I removed the primary cases I found this on the engine block :-
You will see a small brass tube pointing downwards - this had no other hose attached, something I'll rectify on re-assembly and direct it towards the rear chain!
Experts here on the Forum said it was a not uncommon mod, maybe your bike has one too..

HTH,

Adrian

[MikeN]

When I removed the primary cases I found this on the engine block :-
You will see a small brass tube pointing downwards - this had no other hose attached, something I'll rectify on re-assembly and direct it towards the rear chain!
Experts here on the Forum said it was a not uncommon mod, maybe your bike has one too..

Er, Yes,Mine has one too.But I think everybody who owns an "A" does. Although your original steel one appears to have been replaced with a shiny brass one. Its originally listed as BSA part number 67-992 "Breather".
MN