Airhead Gearboxes

[Voyager]

Drive shaft shock absorber slipping? Broken spring on same?

As 'Packer' says, it could be the shock absorber cams if the spring is broken:

There are two different designs, depending on whether you have a 6-spline or a 12-spline input shaft.

Alternatively its possible that the output shaft drive flange is slipping on its taper under high torque loadings - as in 1st gear.

The output flange nut is torqued to 162 Lbs/Ft, I would be inclined to check that (if possible) with the box still attached. If its at the right torque then I'm afraid its a box strip .

Bob.

 

[Padmei]

Thanks for the suggestions guys. I think you may be onto something. When taking the flange off for the rebuild last time the flange came off relatively easily. The time before that it was actually quite loose.
When trying to torque it up this time I couldn't manage to get it to full torque (mine only goes to 150) as I was doing it off the bike on the ground & it kept wanting to judo flip me out of the workshop.
I will check that this morning. Actually torquing it down may cause a simulation of the fault which would point towards the spring broken perhaps.

The other thing is that I went for another 10min ride up & down our long steep driveway & it wouldn't jump out again which perhaps points to the spring being realigned or jammed somehow?


I'll let you know what i find anyway.

 

[Padmei]

I took the driveshaft off & gave the flange a good nudge - def wasn't loose which points to the spring &/or cams.

Has anyone had a broken spring before? Is there any reason why the spring should break apart from metal fatigue? I don't recall any particular catastrophic occurence happening while riding.
Could it be something that was caused by the reassembly of the gearbox - shimming perhaps? it was reassembled by a good bike mechanic that hadn't done an airhead box before however I left him the shim plate & shims & he sorted it out.

Next question- what would/ could happen if i just left it as it & rode it? nuclear meltdown?

 

[Rob Farmer]

If you look at post 2, picture 4 there's a picture of a broken spring..depending where it breaks it could wind into the other part to the point that you loose drive. All speculation though. The only way to be sure is to pull it apart.

 

[Box’a’bits]

Just open it up Padmei. Not too hard to do, & then the input shaft will just slide out. You won't need to replace bearings / reshim etc unless you need to break apart one of the other shafts. You'll need to replace the gasket on reassembly. I have a spare trans input shock spring if you need it, & I can send down tools if you need them...

 

[Voyager]

I suppose its just possible that the circlip that locks the kickstart gear to the input shaft wasn't located properly and has jumped out of its groove - releasing the spring.

Its a long shot though as the design traps the circlip effectively.

Its most likely to be a broken spring - due to old age & fatigue.

Strip it out and show us the pics .

Bob.

 

[Padmei]

OK split it open & had a looksie.
It all looked ok to me. The spring was intact & there were no missing teeth etc.





The only thing that may be suspect is when I went to shift it with the arse end of the box off it didn't want to sit easily in the correct notch for 1st gear. I put this down to the shaft the forks slide on wasn't secure with the arse end of the box off & twisting a bit. A good 'stomp" & twist got it in usually





There seemed to be a little bit of sideways movement in the cam changer things. Not sure if this is usual?

Unless you can see or think of anything that should be addressed i will button it back up & see if it happens again.

 

[Voyager]

OK split it open & had a looksie. It all looked ok to me. The spring was intact & there were no missing teeth etc.

Unless you can see or think of anything that should be addressed i will button it back up & see if it happens again.

Hmmmmm Very strange .

It looks very good in there, I can't see anything wrong with that.

The gear selector never works cleanly when the shafts aren't properly supported.

So, what could it be ????

While the box is out, check the clutch - oil on the plate ???

Rubber bonding in the drive shaft starting to fail ???

Just guessing now .

Put it together and see what happens next .


Bob.

 

[DrFarkoff]

Thinking about it earlier any friction material in base of clutch area?

 

[(RIP) Arkwright]

I wonder if the ramp cams are worn, there's no gap between the cam and the gear unlike the one in post 81?

 

[Paul Rochdale]

I have an oil leak which drips from below the bevel box gaiter. Would I be right in assuming this is a failure of the gearbox ouput shaft oil seal and the oil then runs down the driveshaft tube and drips out of the bevel box gaiter? Is this a DIY job to replace (the oil seal) as this thread does suggest that special tools are required?

 

[Paul Rochdale]

OK, I have resorted to reading what Haynes says about the gearbox output shaft oil seal and it seems it's of an unconventional design, made from Teflon and requires pre-shaping using a special mandrel. As I don't have one it sounds like another visit to Jim Cray........... :-(

 

[Voyager]

It seems it's of an unconventional design, made from Teflon and requires pre-shaping using a special mandrel. As I don't have one it sounds like another visit to Jim Cray........... :-(

I don't use a special mandrel - I have a suitable socket with a rounded end that gives an easy entry and pre-forms the seal lip effectively (its from a 3/4" drive set and is different to a normal square-ended 1/2" drive set). I start with one that's just too small and once the seal has started to form change it for the correct size. I then leave it in place for 30 mins or so.

Your biggest problem will be in removing the output drive flange which is on a taper and torqued-up to 162 lbs/ft on assembly. There is a special extractor tool for this - though I have used a large hydraulic bearing splitter with a heavy adaptor plate in the past (the special tool is better).

Check that the 6mm set screw that holds the battery earth and locks the speedo drive into place has a hole down its centre and that the hole is clear - if the gearbox can't breath it may leak.

There is a notch in the rear oil seal housing that should be sealed with silicone (or similar) on Paralever boxes once the new seal has been installed.

Bob.

 

[Paul Rochdale]

Well I tried to book the bike in with Jim Cray but he's up to his eyes with work and asked me to wait a couple of weeks. A fortnight passed and he's still busy so I thought I'd have a go at removing the gearbox myself - 1 3/4hr later, it was out. I hadn't appreciated that there's another oil seal on the front of the gearbox, close to the splines, and this shows signs of weeping too. I am determined to have an non-dripping bike at the end of the day. Jim mentioned that just doing the rear seal in situ and ignoring the front seal would not be advisable.

Interesting story about the four M6 bolts on the flange under the gaiter. I had (wrongly) re-used the old bolts which I know are stretch bolts, and they've been perfectly OK for the 1200 miles I've done since I got the bike back on the road. However I thought that as I've a big trip coming up, better safe than sorry, and I'd replace them with new. I even found a website on BMW Airhead tools so made up a special tool using half an M10 ring spanner and an old 1/2" drive socket, so I could (just for once) torque the bolts down. Speaking to Motorworks yesterday I was strongly advised AGAINST using a torque wrench as these bolts seem to be made of monkey metal, and that simple hand pressure using an M10 ring spanner would be enough (which is what I'd done previously).

The gearbox was a tight fit taking it out - I removed it through the nearside - and used foam pipe lagging to protect the frame. The drained oil was surprisingly jet black, odd as it was replaced in October/November.

 

[Beemerboff]

Jet black oil usually means it has been getting hot, and getting hot usually means the box has been shimmed too tight - 5 to 15 thou clearance is the current spec, or 15 to 25 thou if you measure it by crushing solder.

But it might just be that you are using Castrol or some other crap oil, so just change it to Mobil 1 synth gear oil if you are.

 

[Garry H]

I use a decent quality GL5 spec in my boxes
The fancy fully synthetic stuff is wasted in a airhead box
I change it once a year and it comes out just as clean as when i put it in
This is the best way to monitor the condition of the gearbox
Same thing with the bevel box

 

[Paul Rochdale]

Halfords Gear Oil EP80W/90 GL5. I am collecting the gearbox from Jim in the morning and will mention the jet black oil to him. Clearly a specialist's job to check the shimming?

 

[Paul Rochdale]

Right, good news. I collected the gearbox from Jim this morning and took a sample of the black oil along to show him. He agreed it was not ideal although not unusual. The bike had been standing idle for some six years and perhaps when I drained the oil, sludge remained in the gearbox. There's no binding of the gears and everything spins round nicely. He was able to tell that the gearbox had been dismantled and rebuilt in the past and that everything seemed to be OK. He also recommended NOT to use synthetic oil. So the box is nicely cleaned up and oiltight for the first time in years.