Author Topic: B-M series plunger rear tank mount  (Read 123 times)

Offline mikeb

  • Resident Legend
  • *****
  • Join Date: Dec 2014
  • Posts: 679
  • Karma: 13
B-M series plunger rear tank mount
« on: 11.04. 2021 12:09 »
Hi
can a B or M or C series owner help cast some light on this please? I need to make the stepped bolts pn 66-8089 that is the rear mount bolt for B and M series tanks. And the rubber bits. But I've only seen pictures (mine was someone else's improvised solution). the question is about dimensions... have i got this right?...

the bolt is described as 5/16CEI thread on the stepped 3/8", and overall length 1 3/8". the un-threaded section is 1" under the head.

I think the un-threaded section is 7/16" dia?? as the washer 30-660 is 7/16" ID
the rubber part 68-8032 (the square bit) - I think this has a 7/16" hole and sits around the unthreaded section above the frame mount?
the rubber part 66-8091 (round bit) - I think has a 5/16" centre hole and sit over just the threaded part of the bolt (on the step)?
and the threads go through the tank mount plate 2 full turns and do not hit the bottom of the tank.

... is that right?

pics below to scope this in (not to scale)
thanks!

EDIT - second pic added - appears to confirm sizes
New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline Sav

  • A's Good Friend
  • ***
  • Join Date: Jul 2011
  • Posts: 176
  • Karma: 2
  • VMCC Men of Kent and BSAOC member
    • Ashley, Ripple & Studdal Enthusiasts
Re: B-M series plunger rear tank mount
« Reply #1 on: 11.04. 2021 14:28 »
Sounds a bit different to 65-8034 on my M23 but that is prewar

Unthreaded is 3/8
2/3 to top of shoulder from under head.

I don't have the rectangular pieces but the 'penny' washers are the full width of the rubbers

1961 A10SR, spent a fortune at SRM
1961 A7SS, finally the right green
2011 New addition 1937 Empire Star, twin port, high pipes. Turned out to be the most unreliable bike I have handled.
2017 finally found the liner/barrel were flexing and causing all the overheating/nipping up. Early B33 barrel fitted and it's reliable at last!

Offline mikeb

  • Resident Legend
  • *****
  • Join Date: Dec 2014
  • Posts: 679
  • Karma: 13
Re: B-M series plunger rear tank mount
« Reply #2 on: 12.04. 2021 11:42 »
Thanks Sav - I'd noticed from googling that some of these look shorter especially re pre-war bikes and maybe also narrower. tho have the same pn. feked says 1 3/4" length and that looks about right in the pics of later ones.

does the round rubber 66-8091 have a 5/16" hole and sit on the step?

I think I'll get the rubber parts (have found a local supplier) and then see what is going to fit and then make the bolts. this is a b33 tank probably off a rigid on a plunger m20 frame. the tank is currently getting some repairs after a long trip on unsealed roads.


New Zealand
'61 Super Rocket  - '47 B33 -  '18 Triumph Street Triple RS

Offline Sav

  • A's Good Friend
  • ***
  • Join Date: Jul 2011
  • Posts: 176
  • Karma: 2
  • VMCC Men of Kent and BSAOC member
    • Ashley, Ripple & Studdal Enthusiasts
Re: B-M series plunger rear tank mount
« Reply #3 on: 13.04. 2021 12:01 »
I made my own rubbers out of rubber sheet of a thickness to have two shock absorbers insted of one, which I put one above and one below the frame member to cover both bases.
1961 A10SR, spent a fortune at SRM
1961 A7SS, finally the right green
2011 New addition 1937 Empire Star, twin port, high pipes. Turned out to be the most unreliable bike I have handled.
2017 finally found the liner/barrel were flexing and causing all the overheating/nipping up. Early B33 barrel fitted and it's reliable at last!