My tank mountings have never been right or had the right bits, so I'm trying to get it sorted. But I can't work out how the front tank mounts work. Here's a parts drawing, so where does the front tank 'ear' go? Is it position 1, 2, 3, or 4? The front mounting plates on the tank have great big holes in them that do not seem to be right.
I'm reasonably sure the tank ear goes at 2. So 27-8447 goes behind the ear, then the rubber, then the cap then the bolt. When you offer up the tank to the frame is there a gap that allows you to fit 27-8447 behind the ear?
Quote from: limeyrob on 12 Apr, 2026, 21:06 I'm reasonably sure the tank ear goes at 2. So 27-8447 goes behind the ear, then the rubber, then the cap then the bolt. When you offer up the tank to the frame is there a gap that allows you to fit 27-8447 behind the ear?
There is a gap on each side but the ears are nowhere near straight so I can't tell what it should be like, plus I suspect this tank is a pattern part. Apart from other things the rear tank mounts do not sit flat on the frame mounts, (another issue to solve) and I'd have to raise the front of the tank a couple of inches to get them to sit flat.
What year s it? Rigid or plunger? Could it be a mix of parts?
It's a 1954 model year plunger frame machine, and nearly all of the original bodywork has been changed. This isn't the original tank as it has earlier badges and also lacks the balance pipe an original tank should have. The left and right halves of the tank do not quite line up; one side is a little forward of the other, which is why I suspected it was a pattern part. Unless they came from Small Heath like that...
Can you post a picture of the tank?
If you do a google search of M33 and Andy Tiernan you will see he's posted several videos on YouTube of M33s. One is and outfit and there's a good shot of the tank and mountings.
Quote from: limeyrob on 12 Apr, 2026, 22:33 What year s it? Rigid or plunger? Could it be a mix of parts?
The rigid and plunger front frames were the same for the M series. There was a difference between M type telescopic and girder fork front frames, but that's another subject! The rear frame (aka Chainstays) were bolted onto to the front frame to make either rigid or plunger rear.
Regarding the make up of the front tank mounting, BSA designed this to act as a shouldered arrangement, so the tank 'ears' should just about make contact with the threaded bosses on the headstock. Now you place the cup and rubber washer to the tank ear open side out, then screw the setscrew with steel washer through the top hat which is pushed through the rubber washer. I made the mistake of assuming a different arrangement for my M33 restoration so had the tank ears welded wider on the Indian repro tank I bought, so the whole of the cup, rubber washer and top hat are behind the tank ear, but with time I've come to learn of the way I described, so my later restored WM20 tank is set up as described too. The link below may help too.. HTH
https://pesltd.uk/?product=27-8449
Now that is interesting and confusing. It makes sense but the one on the Andy Tiernan video looks like it has the cup out side covering the rubber. The bike in the video could be wrong but it matches the parts book with the bolt head up against the cup on the outside. Perhaps both ways work?
I wouldn't rely too much on the parts book illustration (or Draganfly drawings which are based on condensed/amalgamated BSA originals) for correct assembly, I have come across several instances (for all BSA model types) where the drawing depicts a part back to front or incorrect assembly sequence.
In some cases the parts are not depicted at all, just a listing of part numbers and description.
This subject was discussed at length on the M20 forum.
https://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1489523&cmd=show
Quote from: limeyrob on 13 Apr, 2026, 08:03 Can you post a picture of the tank?
If you do a google search of M33 and Andy Tiernan you will see he's posted several videos on YouTube of M33s. One is and outfit and there's a good shot of the tank and mountings.
I had look at the Andy Teirnan video. Here's a few pictures. If things look out of line it's because they are; this is why I suspect it's a pattern tank. Yes, the holes in the ears are a bit on the big side. limeyrob, the tank ears are a quarter of an inch wider on each side (where they are joined to the tank) than the threaded frame bosses. But can anyone identify this tank?
We're only allowed to post four photos per comment, so here's one more. As you can see, the mounts on the tank are not right. The bolt is just a temporary measure to keep the tank in place while the bike's in the shed.
Quote from: Ian C on 13 Apr, 2026, 13:13 This subject was discussed at length on the M20 forum.
https://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1489523&cmd=show
This seems logical; the tank is gently held at the front so that it can pivot and the rear can move up and down.The rear mounts suppress vibration, at least in part to stop the tank making a noise when the engine's running.
It could be a pattern tank, it could also be an old one that's been repaired. Hard to know. Underside should give the game away but I'm in a similar situation with a Matchless tank and I can't work out if its original or pattern.
That mil thread is interesting but I ended up with the impression that several ways got used, which is where all the searches seem to end up ;D
I don't think the rubber is about vibration as much as its about stopping the tank cracking when the frame flexes. The frame has a tiny bit of flex but the tank is very rigid so something has to give. I'm coming to the view that the rubber my not be to isolate it but to limit the clamping force so it can move slightly on the frame, especially when ridden on rough roads.
Quote from: limeyrob on 13 Apr, 2026, 20:11 That mil thread is interesting but I ended up with the impression that several ways got used, which is where all the searches seem to end up ;D
I don't think the rubber is about vibration as much as its about stopping the tank cracking when the frame flexes. The frame has a tiny bit of flex but the tank is very rigid so something has to give. I'm coming to the view that the rubber my not be to isolate it but to limit the clamping force so it can move slightly on the frame, especially when ridden on rough roads.
I think you are right. about the clamping force.The drawing in that thread shows the top hat bearing against the tank ear, clamping it firmly against the frame. That means that the cup and rubber don't do anything. It would seem to make more sense if the top hat instead passed through the tank ear, bearing instead on the frame.
But I agree with Ian Wright in that thread, in his reply dated Nov 14, 2023 - 11:29AM.