Author Topic: Plunger Bolt size to gearbox  (Read 1634 times)


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Plunger Bolt size to gearbox
« on: 18.01. 2009 17:27 »
Hi all
Can anyone tell me what size and thread the two bolts are that go into the gearbox through the two lugs on the rear frame downtube.
Many thanks

Online RichardL

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Re: Plunger Bolt size to gearbox
« Reply #1 on: 18.01. 2009 17:58 »

Not that I have a plunger (outside of the loo), but sometimes I enjoy this kind of research. (The thought that I could have the soul of a research librarian is sacring me.) My parts book includes plungers as early as 1954 and this is what I see there:

Facing upwards through the bottom crossmember, 2 x 67-4028 Gearbox foot set screw; hex head with locking-wire hole; 13/16" x 5/16" 18TPI BSW

Through the down tube, 1 x 67-4027 Gearbox top fixing bolt; hex head; 2-1/4" x 7/16" 20 TPI CEI

Dimensions coutesy of

Hope this helps.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020. This year it's a solo or pillion ride in dapper attire. Watch website at for details.


  • Guest
Re: Plunger Bolt size to gearbox
« Reply #2 on: 18.01. 2009 20:04 »
Thank you Richard I have found that the two bolts are missing from my bike which may explain the excessive vibration I get.
However I tried a 5/16th bolt only to find that the frame holes do not line up with the gearbox holes.
Why I do not know yet --my thoughts are that the frame is ok and if I slacken all the engine plate bolts along with the head steady ones I may be able to shift the engine unit back enough to line the holes up , or should i just open the frame holes up a bit?
The easy way out

Offline dpaddock

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Re: Plunger Bolt size to gearbox
« Reply #3 on: 20.01. 2009 15:53 »
By all means try loosening things up first - you'll probably do fine. If not, the frame might be bent slightly and this would need attending. Enlarging holes leads to other, permanent problems, and should be your very last resort.
'57 Spitfire


  • Guest
Re: Plunger Bolt size to gearbox
« Reply #4 on: 20.01. 2009 18:04 »
Hi, All!  These fixings for the gearbox to frame are actually studs not bolts,The thread size given by Richard is correct for gearbox end of stud other end is 26 tpi BSF. Important that these are fitted to minimise vibration.If they dont cure your vibe problem then could be main bush *problem* *problem*.G/F Dave.


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Re: Plunger Bolt size to gearbox
« Reply #5 on: 20.01. 2009 19:06 »
Thank chaps
I will loosen things up first and I am sure the bolts will go in, it will have to be bolts as I have no studs and now the unit is in the frame bolts may be easier
G/F dave main bush ok has been checked and it is a different vibration to that as I had some noise and vibration come on slowly to the Super rocket and Steve Mcfarland ( Mr.SRM )suggested I look at the bush and he was spot on so ok on that one

Offline beezalex

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Re: Plunger Bolt size to gearbox
« Reply #6 on: 29.01. 2009 21:45 »
Richard L. : Great link, thanks for that one.

G/F Dave: With all due respect, while it would be better engineering practice to make these bolts into studs, the parts books refer to them and show them as bolts.  I am guessing this was done to ease engine/gearbox unit removal.

Richard: My golden flash was originally missing the same bolts and actually cracked one of the front motor mounts AND a head steady bolt (I thought it was SUPPOSED to vibrate like that *eek*).  The vibration actually got better when the front motor mount broke- go figure, but then I found the gearbox bolts missing and replaced them and vibration was much improved with them in and the other mounts replaced.  Let us know how yours fares.

Too many BSA's