Triumphrat

bum · 06-29-2009, 05:43 PM

I'm going to be changing my 1971 Tigers' gearbox cam from leaf spring to plunger style. I would like to do this with the engine still in the frame. If I raise it up on a fork lift, I can get under it to drill and tap it for the new plunger. I would appreciate any tips you can give me. Please be gentle, this is my first thread.

Webby · 06-29-2009, 06:48 PM

Hi Bum,

Welcome to the forum

I guess it would be possible if you remove the gearbox inner cover & all the parts inside the box. If you do try it I would suggest that you clean the inside of the box and cover the output shaft with some plastic sheeting and tape. (you don't want any swarf in your bearings)
Also I would like to think that you are pretty handy with a drill, it's not always easy to drill a straight hole lying upside down (I know, I used to be an aircraft mechanic!)
Before you start I would suggest that you make up a few drilling bushes with an outside diameter of approx. 1" with the inside diameters the same size as your drill bits, the most important would be the pilot (I suggest 1/8" dia)
Provided you make sure the bushes are dead flat and you start your pilot hole in the centre, I see no reason why it would not work. Make sure you clean out the box before you assemble it.

Webby

PS: I'm thinking of doing the same on mine (72 Bonnie) but with the cases out on the bench.

thirdbike · 06-29-2009, 10:51 PM

Welcome and your initiative has to be recognized.
Before you go and drill a hole in a perfectly good gear case, did you have a look at the the underside of a later bike with the plunger style.
The Inder plunger housing is threaded into a fairly subtantial buildup of the gearbox case around the plunger to support this threaded piece. I can see a bump in the manual for your model but do you have as much metal there to support the plunger?
The plunger itself is a pin so you have to be dead accurate to catch the camplate dead center or it may push off.
Assuming that you do have enough casting to do the job, you may need more than a bush and a hand held drill.
Do let us know how you make out and good luck.

valiantbultaco · 06-29-2009, 11:47 PM

Hi bum,

I did this on my 71 t120R. I used a standard bultaco plunger. Works perfectly and positvely. loose the detent spring. Its a nightmare.
Make sure when you drill out the boss hole in cases you dont make too much mess with aluminium filings and tap her out!!!

Cheers!

Webby · 06-30-2009, 05:29 AM

Quote:

Originally Posted by thirdbike

Welcome and your initiative has to be recognized.
Before you go and drill a hole in a perfectly good gear case, did you have a look at the the underside of a later bike with the plunger style.
The Inder plunger housing is threaded into a fairly subtantial buildup of the gearbox case around the plunger to support this threaded piece. I can see a bump in the manual for your model but do you have as much metal there to support the plunger?
The plunger itself is a pin so you have to be dead accurate to catch the camplate dead center or it may push off.
Assuming that you do have enough casting to do the job, you may need more than a bush and a hand held drill.
Do let us know how you make out and good luck.

Hi Thirdbike,
Before I replied to Bum, I had a look at my own case (72 Bonnie) The build up in the case is still there, so all that is missing is the hole. The accuracy is the problem, if you start your pilot hole off centre you're scr*wed! As you rightfully say you need to be dead centre to the camplate and that's the tricky bit!

Webby

68TR6R · 06-30-2009, 10:06 AM

When I did it (last winter), with the cases split, I careful measurements where the plunger would be perfectly aligned and transferred them to the outside of the case. I then clamped the case to the table of my mill (which was also an ordeal) and used an end mill of the correct size for the tap (cannot remember size off hand). Then, since there was a little shoulder on the plunger holder, I used another end mill of the next size up and drilled just enough to sink the shoulder. In the end it worked great, but I would not recommend doing it by hand, as it needs to be spot on in order to work correctly.

thirdbike · 06-30-2009, 02:29 PM

Thanks Webby,
I wasn't sure if the castings were the same as I couldn't tel from the manual. It also doesn't go over too well if your seen crawling underneath some other guy's bike. Anyway, I hated to see the guy drill a hole into a case that wouldn't have had enough thread to support the plunger.

68TR6R tackled the problem but even he describes the task as more than laying on your back with a tape measure and picking up the cordless drill. I assume that bum can now appreciate what it entails.

Out of idle curiosity, what is the problem with the leaf spring design?

Webby · 06-30-2009, 03:12 PM

Hi Thirdbike,

The problem with the leafspring is it does not exert enough force on the camplate, very often you can find the box will change 1 1/2 gears at a time (Not a good idea!) Also after a rebuild it is very difficult to refit the inner gearbox case (the leafspring is attached to the case) without moving the camplate. I know the design was modified in 73 or 75 but I have no idea what the later was, did they just revert to the pre 70 design with the plunger or did they try something different?

Webby

thirdbike · 06-30-2009, 08:39 PM

Webby, the T140V - '76 and on have the plunger style.

Webby · 06-30-2009, 08:42 PM

Thanks Thirdbike,
I wasn't sure if they had gone back to a good idea, or come up with new stupid one!

Webby