What is this???

[jachenbach]

Apparently some PO put an extra hole in this left engine case, and I'm trying to figure out why. It opens into a passage that then passes through a spigot to the right case, which then opens into the breather passage. The hole was plugged with a screw shown in the picture. Anyone know why this would have been done?

[RichardL]

This  the actual breather hole for the crankcase. It puffs  air (I think, a balance of in and out)according to the timing of the rotating breather. You need to take out the bolt. Also, there might have been a pipe in it at one time (part number 67-922), but I think few of our bikes have that pipe.

Richard L.

[trevinoz]

And if you leave it there you will have many oil leaks due to high crankcase pressure.

[RichardL]

The bolt, he means.

[duTch]

  First thought is to stop oil coming out. ...
 Dodgy, but maybe the bolt was put there if the bike was put out of service, to keep mud wasps from building in there. ... Not recommended though, a bit of pipe as Richard said would be better, I've added a length of clear tube to run into a bottle(when I find one suitable)

[jachenbach]

So if I understand correctly, the hole was factory made and the bolt added by someone. I looked at my other engine and didn't see the hole, so I thought the hole was an owner addition. I'll have to take another look. It did puzzle me trying to figure out how the engine could breath without an exit. It will be interesting to see the differences when I take the other engine apart.

[Topdad]

Dutch, for a supply of small bottles try your friendly pharmacist ,I nick ..er borrow, all sorts from my lads place , great for small amounts ,I have 2 for the forks (great for balancing the amount ) and another for the gearbox  which I fill and have on the shelf when I'm about to drain it ,gearbox that is not the shelf !!  but you could hook up a small bottle that wouldn't be to standoutty ,  Bob

[Greybeard]

That's Kwality workmanship; with a capital K!

[bsa-bill]

it's an interesting thread (not this one but the one on the screw in the photo) and the head of the bolt/stud is reminiscent of what we used to call "tech screw" when we fastened down profile sheets on roofs

[Greybeard]

it's an interesting thread (not this one but the one on the screw in the photo) and the head of the bolt/stud is reminiscent of what we used to call "tech screw" when we fastened down profile sheets on roofs

The thread is similar to the screws that hold down the floor panels on an old Land Rover. They have a spring steel 'U' shaped clip that fits over a steel bracket and acts as the nut.

[jachenbach]

Well, duh.... I took a better look at the other engine and yup, it has the breather hole, just no screw plugging it. Probably should have taken it out of the box to look.  It is an odd screw. I don't recognize the thread at all.

[duTch]

  It also looks a bit like a screw from a hose clamp....?

 Yo Bob I have a small collection, but nothing yet that is 'just right' , and I also keep forgetting. ..!

[BSA_54A10]

Actually it is a self drilling masonary bolt and it looks like a little masonry ws left in the thread roots.

It is very common.
Clowns witn no understanding of the oiling / breathing systems buy a BSA, find oil leaking from this hole so they plug it up.
then they spend endles hours trying to fix all of the oil leaks from thsi " Pile of Pommie Shit " and finally sell it and go back to trying to look cool on a Honda.
I have found untold numbers of bike, particularly A 65's with the breather blocked off.

[Rocket Racer]

I've tapped this hole on my race bike and put a fitting on it so I can run a hose off it and feed it to a catch bottle via a filter.
On a road bike this wouldnt be necessary. The breather timing is set on the cam end where a flanged tube with two holes permits limited breathing to reduce crankcase pressure.  I recall cake street classics recommend extending the holes to extend the breathing.
I also run bunn breathers but thats another story.
For road use just take out the bolt.