Triumph Gearbox Plunger Conversion

A load of posts that show you how to do various projects, step by step.
Posts: 113
Joined: Sun Nov 29, 2009 11:05 pm

Triumph Gearbox Plunger Conversion

Unread post by Webby » Mon Feb 22, 2010 2:14 pm

Hi All, by popular request, here is how I converted my 71 Bonnie Gearbox Leafspring to a earlier (and later) spec. plunger. Feel free to ask any questions you like. The leaf springs were fitted to the T120s from late 69 till early 73 if my memory serves me correct.
But first, a bit of background, I bought this bike as a box of bits to give me something to do and keep me out of trouble. I'm not a bike mechanic, nor do I have a fully equipped workshop. I'm an aircraft engineer and in order to do this job correctly you need to be able to drill a straight hole, you need nothing more than a hand held electric drill, a few drill bits, a couple of reamers and some taps. There is very little room for error, if you get the pilot hole off centre by 1mm your final hole will be about 6mm out! You have been warned :)

The plunger assembly consists of three parts, the body, the spring and (of course) the plunger. You will also need the sealing washer. Here are the part numbers below.

Plunger Housing: 57-4400
Plunger Spring: 57-4459
Plunger: 57-7020
Sealing Washer: 57-3978

That lot should cost you about 10 quid.

I found it easier to do all the work on the bench with the crank cases split, but you could just remove the engine from the bike and remove the gearbox internals. You will need to slide the cam plate into position each time to check the alignment of the hole (I just left mine in place, but be sure not to scar the edge of the cam plate) Also get a plastic bag and place it inside the gearbox casing, you don't want any swarf getting into your bearings (I recommend you do this even with a bare case, it makes it easier to periodically clean out the swarf)

1) The angle of the face of the casting (the plunger boss) (on the bottom of the gearbox housing) for the plunger housing is already set at the angle required for the plunger to sit straight on the edge cam plate.

2) Very carefully file the casting marks off the face of the casting, make sure that it is flat using a straight edge and a light.

3) Next, double check that the centre of the casting will line up with the centre of the cam plate, use a straight edge to do this and centre punch the point where you would start to drill the pilot hole (I used a 1/8" dia. drill bit) you want to keep the pilot hole small, this way if you find you are slightly off centre you can drag the hole slightly to get it where it should be.

4) Start the hole off by hand at approx. the correct angle (90 perpendicular to the casting)

5) After checking and double checking that your hole is in the correct position, drill down about 1/8".

6) Next using a steel bush as a guide (with mine the I/D was a little too large so I wrapped some tape around the drill bit to make a snug fit) Drill the rest of the hole, checking the angle now and again.

7) Using the drill bit as a guide you can see if the pilot hole lines up perfectly centre with the cam plate.

That's the difficult bit over and done with, now it's just a matter of opening the hole up, cutting a thread and then opening up the bottom of the hole to accept the shoulder of the plunger housing.

8) Open the hole up slowly with whatever size drill bits you have lying around, checking each time that the end if the drill bit lines up central to the edge of the cam plate.

9) The plunger housing is 5/8" UNF, the correct size for this according to my Zeus book is 0.5709" (14.5mm) so open the hole up to about 14mm with the drill bits and then finish off with a 0.507 reamer (I used an adjustable one).
Plunger 1.jpg

10) Now is a good time to double check the face of the plunger boss, (I didn't do this as I don't have a lathe) turn down a piece of aluminum bar with one end about 14.5mm diameter so its a snug sliding fit in the hole, a section slightly larger than the diameter of the boss and the third section a good diameter to fit into the chuck of your drill. Stick a small piece of emery paper to the face of the bar (the one that will touch the boss) and give it a turn, you want to make sure that the face is completely flat to prevent oil leaks.

11) Tap the thread, you will need a 5/8" UNF 18 TPI tap. Take it slowly, you need quite a lot of force to cut a thread in a hole this diameter! I used a first and second (intermediate) tap just to make sure I got it right.
Plunger 2.jpg

12) You will notice that the plunger body has a shoulder (I can't remember what size it is) but you need a drill about the same diameter and slowly cut away the threads in the hole for a depth of approx. 1/8".

That's it now it's finished you will have no more false neutrals and no more problems trying to index the gearbox quadrant.
Plunger 3.jpg

Here's another site with some pretty pics of how to do it, ... ph+plunger"
I hope this is of some use to you.

Webby (Brian)

User avatar
Posts: 10183
Joined: Thu Feb 19, 2009 11:52 pm
Location: Doncaster

Re: Triumph Gearbox Plunger Conversion

Unread post by Hooli » Mon Feb 22, 2010 2:34 pm

Excellent thread, brilliant info thanks. Added to masterclasses for you.

Oh btw, I like how the first pic is upside down (as you'd be working on it) to confuse those like me who are a bit slow looking at such things! :lol:
Classics ain't built in Metric

Posts: 113
Joined: Sun Nov 29, 2009 11:05 pm

Re: Triumph Gearbox Plunger Conversion

Unread post by Webby » Mon Feb 22, 2010 2:41 pm

Thanks Hooli :)
I forgot to mention that I found it easier to turn the case upside down :)


User avatar
Posts: 2022
Joined: Tue Feb 17, 2009 11:31 pm
Location: Gourock

Re: Triumph Gearbox Plunger Conversion

Unread post by jimmy » Fri Feb 26, 2010 1:16 am

Well done mate.
Well written and v. interesting

Post Reply