Question for those who know more then me
On a different forum there was a discussion of the timing covers particularly is respect to the extra thickness around the magneto pinion which was used latter on for the Gold Star tacho drive
Now I was sure I had seen a few M20 with a cap / plug over the timing pinion which I had assumed was there to allow for timing setting without the mess of removing the entire cover .
However when I searched the parts books I could find no mention of one anywhere
Then yeaterday a photo of a captured impressed M20 was posted showing both the timing plug and the oil pressure button

So does any one know when they were abandoned ?
What models or variants they were fitted to?
Part number ?

As my bike is nothing like standard it is some thing I would like to fit.
much the same as the feeler gauge plugs on the long stroke A 7's a very handy modification.

I think it was the economy measures that fazed out the plug along with lots of other Deluxe features during the early part of the war. I have one on my 1940 bike, but it was something I added along with the front brake rod. Ron

email (option): ronpier@talk21.com

27-209 is the part number for the plug....The matching cover is 66-1927...I think Mark Cook made some of the plugs...Ian

email (option): ian@wright52.plus.com

C7287, the large (17,000 machines) contract doesn't seem to have had them. C6126 definitely did, so the change is likely to have been mid-1940, and probably occurred somewhere between WM20 21000 and 25000, the same time as the loss of the valanced rear mudguard and oil tell-tale.

I've often thought I'd like one of those covers to save the bother of taking the whole thing off. But would it be harder to keep oil off the magneto taper with just that small access space?

I thank you greatly gentlemen
So would I be correct in thinking they were a Deluxe extra ?
If so I shall ammend my parts books
Slowly the mud clears to reveal murky waters
27-209 is also the primary chain case inspection plug so BSA used the same plug for both purposes .
However 27-209 is also listed as timing cover plug for Y 13 " and the parts book depicts it slotted. not hex headed
According to my wm20 IPL the timing cover is 66-1918
The all models parts book shows both 66-1918 & 66-1925 but 66-1925 does not appear in the text atributed to any bike .
The price lists shows a 66-1925 as "timimg cover with fixed fittings" and last used in 1939 so would it be safe to assume that the 66-1925 is the cover with the plug in it as the only other fixed fittings would be the breather tube that was not yet fitted .

Well strangely the cover with the mag bung is not shown in either the 1939 or 1940 civy parts lists. Both show only the standard cover #66-1918. It's also not shown in my WD parts lists. Does that mean it was for the Deluxe WD models only or an optional extra? Ron

email (option): ronpier@talk21.com

Just to be helpful.

https://pesltd.uk/?product=27-0209

Mark

email (option): pes.sales@btconnect.com

It's the same plug as in the primary case....Except it's brass. Ron

email (option): ronpier@talk21.com

The timing cover number I quoted, 66-1927, is the part number for the cover without the bung...As Ron notes the primary case bung is brass and the timing cover one is aluminium...Both parts (cover and bung) can be found in the WD parts list for 1940 contract no. C6126....Ian

email (option): ian@wright52.plus.com

Not trying to start a fight in any way shape or form as I appeciate any & all information that I can get from others who know more than me ( which is not hard )
I have 2 BSA price lists and neither show a 66-1927 but it does make good sense that there would be a separate number for just the cover with a threaded hole.
When I make an ammendment to my files I like to make sure it is correct as I will be using it future should questions arise and it will end up in the BSAMCC of NSW library and I for one do not want to spread misinformation as there is enough of tha out there already .

Good to know about the Brass / alloy bit .
When I got my M20 it had the alloy plug in the primary that was pretty well chewed up ( and glued in with paint ) so it got a brass one, then a brass one with O ring behind it as the first one had a habbit of dropping off at the slightest provocation ( well to be truthful I was probably over reving at the time trying to keep up with the A 65's )nd it is amazing just how far they can bounce off the road .

Now I imagine there would have been a suffix to distinguis the brass from the alloy but what about the slotted Y 13 one ?

The cover only part number is correct and quoted from the WD parts list noted in the previous post....

Regarding the primary chaincase plug 1946 and '47 girder fork models had the alloy one fitted, not the brass one...This was superceded in '48 with the introduction of many other changes, by the smaller post war 'oil tank' cap type...Ian

email (option): ian@wright52.plus.com

I can't help with the part numbers as mine tend to get updated as knowledge improves.
Don't think my face mounted O-ring caps are like any other, they are very good at staying put. They were designed that way.
As for your slotted caps, do you mean these? If so they have been made.

Mark

email (option): pes.sales@btconnect.com

And thank you again Ian
As this looks to be something of a mine field we are considering doing an M & B series timing cover page on the clubs web site
Hopefully it will also include th Gold Star variants and casting numbers
Despite most of them looking almost the same it is now apparent they are a lot more complicated than just the absence or presesnce of BSA with or without the wing

There is also the timing cover with the cast lump for a breather to be added. Some people refer to it as late/post war? But my 1942 WB30 should have one fitted. Ian has prepared one for me (drilled and tapped with breather pipe fitting) I haven't received it yet. Maybe Ian could post a picture. The part number of this cover is 65-2143. Ron

email (option): ronpier@talk21.com

I wonder why a B30 needs 4 engine breathers?

Rob

email (option): robmiller11(a)yahoo.co.uk

Not only did BSA vary the number of breathers on OHV engines (1, 3, 4) but they also moved the locations, changed the design of the valve three times and the timing cover twice....Even post war they were still having problems... From what I have observed and read the problem seemed to be the conflict between having sufficient breather capacity without an excessive loss of oil...
This also doesn't include the later Gold Star that revved to a point where the flap valve didn't know whether it was coming or going....That ultimately led to the driven and timed breather to solve that problem...Ian

email (option): ian@wright52.plus.com

Here's Rons WB30 timing cover with the early pattern (pre 48) BSA script and breather assembly...Despite the late B30s having a total of 4 breathers (2 flap valve breathers, 2 plain breathers) the post war B31, which was a direct developement of the B30, had only one and it wasn't in the timing cover!!..However, it returned to that location in 1948 and remained there on SV and OHV types until the last M21s were produced in 1963...Ian

email (option): ian@wright52.plus.com

BEcause air has mass there fore it also has inertia
So the movement of the air inside the crankcase lags behind the piston movement as the revs increase to the point that it becomes 100% out of phase with the piston movement and then there is resonance of the flapper valve on simple M series types and an even bigger problem with spring loaded cheack valves .
The original flapper was OK when engines had no oil seals were long stroked and reved to 4000 rpm flat out
Add to that most had very heavy flywheels so they accelerated slowly.
As strokes shortened , flywheels got lighter , engines accelerated faster, crankcases became sealed ( sort of ) the flow of air bcame a big problem
More splash allowed way too much oil to pass through the breathers .
Thus it was found necessary to move them, modify the acual outlet hole and it's orientation within the crank case

When Rex Bunn did the original research which led to the Bunn Breathers he found that very little actual published research had been done on single cylinder crank case breathing

Thus HD & RE now use a system he designed
Now when it comes to splash and oil flow inside a crankcase it was and largely still is guess work .
The Aukland University actually did a Bert Hopewood and fitted some crankcases with windows and cameras to try & verify theories of internal oil flow
When we tried fitting a flow through system to A 65's it was a major problem trying to place the outlet where it did not take 3/4 of the oil flow to the head with it but the A 50, essentially the same engine worked fine .