Gearbox Teardown with plenty of pictures

The wife finally gave me enough time to make it back out the garage. I spent the first bit sorting the frame parts to be powdercoated and tapping the threads and adding bolts to protect said threads. Next for the frame is to document and print out the pictures of how to mask the frame for the powdercoater. Maybe next weekend for that.

Back to the gearbox...

The box does not match the serial number of the engine, in fact the gearbox is a wee bit earlier (149xxx or so) for a 72 Combat.




Cover off.


Tapping the sleeve shaft lightly was all it took to loosen the inner cover.



Sorry about the blurry picture.


This is the one area I'm stuck on. Can't get this nut off. Manual doesn't help. It says something about locking the rear brake, not much good on the bench. Heated it, Liquid Wrench, air impact wrench... Nothing. It sure look like it isn't a reverse thread. Is it? Sprayed some more Liquid Wrench on it. Maybe next time it'll come off.


Next out was the layshaft bearing. It's behind the shaft on the left. Just heated the case on the outside for a few minutes and that's all it took. Wiggled the shaft and the bearing and all popped out.


It's stuck to the shaft so more heat on the bearing and tapped on the gears which popped the bearing off.


Here's the bearing. Is this the infamous kind that likes to explode? It was a little rough.

<I don't know where this photo went>
Then the sleeve gear bearing. Again a little heat (handheld propane) on the case and a few light taps and out she came. This bearing was rusty and NFG. Would barely turn.


All empty. Camplate and gear selector came out with no problems. Helps to have to some Whitworth wrenches as the bolts holding these two items in were Whitworth. Funny thing was that the drain plug wasn't.


All the parts. Hope I can remember where they all go. :mrgreen:

Next is to give everything a good cleaning. Gears all looked really good. Perhaps it was from a low mileage bike? Or the earlier bikes were just made better? Dunno.

I have a complete gearbox rebuild kit, bearings, gaskets, bushes, etc. I'm going to have some questions about replacing the bushes. The selector and camplate bushes felt really good. Not sure if I'll replace those.

So: Get that mainshaft nut off, replace any bushes I can, and put it back together.

It's RH thread. I'd Kroil it for a day or two, then chuck the mainshaft in a vise (softjaws preferably, but it's hardened steel so don't sweat it) and go at it with a breaker bar, or if that fails, your impact wrench. It'll let go.

serendipity dave! just got the gearbox and motor outta my 72. start teardown tommorrow. where did you get your kit

I'm sure you guys know this, but just in case, there are terrifically detailed teardown/rebuild instructions on Old Britts website.

SwooshD
Use your Clutch Center and place it in a soft jaw vice then drop your shaft into it. That should hold things well enough to turn that nut off.
CNN :wink:

Wow! look how blue your mainshaft looks. What's the sleeve gear look like inside?

rgrigutis said:

serendipity dave! just got the gearbox and motor outta my 72. start teardown tommorrow. where did you get your kit

Click to expand...

Don't know where the kit came from. I can try to find out.

BrianK said:

I'm sure you guys know this, but just in case, there are terrifically detailed teardown/rebuild instructions on Old Britts website.

Click to expand...

Yeah, but everything in those pictures is too clean. I wanted to show what a gearbox really looks like. :mrgreen:

Old Britts "pretty" teardown

RennieK said:

Wow! look how blue your mainshaft looks. What's the sleeve gear look like inside?

Click to expand...

Dunno, I'll have to look.

I hear you Dave.

Better luck than me, I wish you. I had three gear sets show significant spalling. Fortunately, I was able to buy a decent gearbox off ebay for well under the price of the gears, so I held on to my original box (to keep numbers matching) and mixed and matched all internal parts. Then managed to misstime the camplate so I got to dig in again....word to the wise!

Looking at the pictures a little more closely there must have been some moisture in the box. Check your inspection cover if it has a hole in it or it may be plugged up with polish paste. Or you could modify your box to the 75 breather. which involves drilling /tapping. Like the above comment OldBrits is a good sourse for info.
CNN

This is a good time to check for a crack between mainshaft and layshaft bearing bores. Many gearboxes have these cracks. It can be felt by scratching a fingernail there in the thinnest area between the bores. Nothing to prevent the box from working perfectly for the next 50,000 miles, but something to keep an eye on, and put your new bearings in carefully

Good luck.

CanukNortonNut said:

Looking at the pictures a little more closely there must have been some moisture in the box. Check your inspection cover if it has a hole in it or it may be plugged up with polish paste. Or you could modify your box to the 75 breather. which involves drilling /tapping. Like the above comment OldBrits is a good sourse for info.
CNN

Click to expand...

Yeah, a little moisture and old oil and stuff. It hasn't been use "in a loooooong" time. Not sure the origin of the gearbox. I think my brother pulled it from his pile. Or it was the one that came with the bike in a bucket so many years ago.

I'll just put a hole in the inspection cover... when I get one.

More backstory on the bike here.

Dont forget to put your mainshaft in a lathe to check for trueness.

The mainshaft might need to be hardened, it looks like it got a little hot.

This looks like an awesome bike! thanks for the photos.

Hey, the more pics the better , thanks 4 this

pelican said:

Hey, the more pics the better , thanks 4 this

Click to expand...

I was out in the garage with a dead battery in the camera. I would stick the battery in the charger and every couple minutes put the battery in the camera and snap a couple shots. And... repeat.

Next time it will be better. :mrgreen:

Mick Hemmings has an excellent step by step instruction video showing gearbox strip and rebuild.

JAYMAC said:

Mick Hemmings has an excellent step by step instruction video showing gearbox strip and rebuild.

Click to expand...

I went in search of that video and didn't find it. Do you have a link?
Thanx
Russ

You can get the video here:

http://www.inoanorton.com/Merchandise/index.html