Bertha's Box - What Can Go Wrong in an Airhead Gearbox


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Nov 11, 2006
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Kirkliston, Scotland
As most of you will know, ‘Flipfly’ (Phil) fell heir to a 1990 R100GS when his father, ‘Pastyman’, sadly passed away earlier this year.

This 100GS had a hard life before Pastyman acquired it, having done over 100,000 miles on a world tour. It was not in good health and let Pastyman down occasionally. In particular the gearbox packed up when he was attempting a tour of Spain, so the tour was never completed.

The box was repaired, but Phil wanted it checked over as he intends to take it to Southern Spain later this year. I said I would “have a look at it”.

This is an account of what I found and gives an indication of what can go wrong in an Airhead Gearbox.

On first examination the box didn’t seem bad. It turned smoothly, the gear change worked, there was no excessive backlash, nasty feelings or noises.

The lugs that the clutch operating arm pivots on had been broken off and welded back on at some point. It wasn’t a brilliant job, but it worked.

I went to remove the output drive flange and noted that the nut had been abused – add that to the ‘replacement parts’ list.

The output flange was removed in my time-honoured way with my home-made spacers, flange-holding bar and the Sykes hydraulic puller. I remembered to remove the Speedometer Drive Gear first. I have documented all this before and these pictures are not of Bertha’s box.




With the flange removed I undid the nine 6mm socket-headed Set Screws that hold the rear cover on.


These Sets are supposed to have washers underneath – one under eight and two under the top centre one.

No washers . . Hmmmm, signs of things to come ??

I heated the cover and tapped it free with a rubber mallet.

There wasn’t a gasket on the rear face, just the remnants of some sealing compound.

Normally, that reveals the shims sitting on top of the bearings, but there were no shims in there at all. Not a good sign.

I removed the gear selector mechanism and that looked OK.


The Indexing Roller looked a bit past it however . .


I always replace old, discoloured rollers. There are two options.

Either replace it with a new nylon roller.
Replace it with a stainless sealed bearing.

The bearing gives a very positive ‘clacky’ change but can be a bit difficult sometimes – particularly when selecting neutral.


I replaced it with a nylon roller.

On removing the long oil baffle, I found that it was bent and its mounting had been torn out at some point. The bent threaded collar was held in with a screw on both sides - which were loose, making it a bit tricky to remove.

It was replaced with a good used one - £3 well spent.



I then heated the front of the box and removed the three shafts. It looked as though the three helical gears were toast as the teeth were chewed.

However, on closer examination it was only the outer faces of the teeth that were damaged, as if something had been rubbing along all three. The contact surfaces weren’t too bad meaning that I didn’t have to replace the complete Intermediate Gear cluster.

The damage can be seen here:


First I stripped the Input shaft.

The front roller bearing was OK (but I replaced it with a tighter one from ‘used stock’). However the sleeve it runs on was pitted and scored. The bearing had been replaced but the sleeve hadn’t.

I put a new sleeve on the shaft and it and the worn old one (discoloured by heat from when I removed it) are seen here.


The input helical gear incorporates the ‘male’ side of the shock absorber. The front of the gear was badly worn and the lobes were worn down.

The ‘female’ side of the shock absorber cam was gouged out. The wear on both components is seen here:





Motorworks came up trumps with replacement used units (these later ones have the ‘X’ mark on the front face to indicate they have the later tooth angle). The replacement pair cost £22.00. There isn’t a mark on the ‘male’ side and only a small degree of shine on the ‘female’ side - excellent:




I put in a new Shock Absorber Spring as well – The new one is slightly longer than the old one, though I suspect the old one isn’t very old from the amount of ‘Optimol TE’ paste under the sliding components.

New one on the right:


The Input Shaft was then re-assembled. I used a small ‘Clarke’ bearing splitter to compress the shock absorber components


The spring is compressed until the circlip groove in the shaft is accessible.


Then the circlip is worked over the splines and into the groove. Its held tight when the puller is removed.


The ‘Top Hat’ that the rear bearing mounts on is a tight fit on the end of the shaft. It incorporates the small Clutch Pushrod Oil Seal which is very thin and a bit of a devil to fit. I find it easier to insert it from the ‘wrong’ side, gently tap it until its all level, then press it flush in a vice.


Then I use a small ¼” drive socket to press it up to the other end.


If one tries to knock it in to the other end the seal lips get bent out of shape. I have ruined a few – at £12.00 each.

The rear Input Shaft bearing is normally an ‘open’ one which seems a bit odd as dirt entering from the pushrod tube will go straight into the track. I use a ‘half-sealed’ 6304 C3 bearing to keep dirt out.

I remove one seal from the new bearing and also replace the oil baffle plate. The one that came out was quite bent (though I managed to draw the bearing off without touching it !).


Then, with the ‘Top Hat’ pulled on the bearing & oil baffle are drawn on.


The Intermediate Shaft cannot be taken apart (without a 30-ton press) so the bearings were just removed and checked.

The front one was fine and will be re-used.

However the rear one is a double-sealed bearing and the one that came off looked very good, clean and tight. I then looked closely at its Part Reference . . .


It’s a type 6304–2RS1 which is not a ‘C3’ self-aligning bearing. The ‘C3s’ have greater running clearance than other bearings and are better able to handle rapid changes in temperature.

I looked the bearing up on ‘Simply Bearings’ and found this:

It appears to be an incredibly expensive stainless bearing for use in the food industry (although it also mentions Agriculture). Stainless has lower load-bearing properties than conventional bearings and, although this is a fine bearing, its not right – so I replaced it with a conventional sealed SKF ‘C3’ bearing.

Next I examined the Output Shaft and found that the front bearing was slack on the shaft. It would just slide up and down, yet its supposed to be tight to prevent the shaft moving and upsetting the gear selectors and the rear bearing.

To prevent the possibility of bearing movement, BMW fitted a circlip to the front of the Output Shaft, then stopped the practice, then re-introduced it. The shaft that came out didn’t have a circlip groove.

The box has seized at some point in the past and the bearing has spun on the shaft – ruining it.


The bearing that came out was fairly new and was good – I put it back in (eventually).

The shaft was toast so Motorworks produced a replacement - £55.00.


The replacement had a circlip groove (Jammy B !):


I replaced the flange nut while I was at it, replacement on the right:


I also found that the sleeve that Second Gear runs on was worn. Old one above the replacement here:


This probably meant that Second gear was worn as well. I compared it to a good one in my ‘used stock box’ and mine was better – so that went on the new shaft.

A bigger problem was at the front. The helical drive gear is fifth gear and it is locked to the output shaft by Third Gear that is splined to the shaft. The engagement webs on third gear were badly worn with the corners missing – as seen here:


However, the ‘dogs’ on fifth gear weren’t worn . . So fifth gear has been replaced at some time . . But the helical teeth are worn so that damage was later . . Curious.


Anyway, Third gear had to be replaced for two reasons and Motorworks provided a good used one for £20.00.

Reason 1 – the worn engagement webs can be seen here on the old pinion which is compared to the replacement on the right:


Reason 2 – The loose bearing on the front and the lack of shims has allowed the shaft & gear cluster to move about – putting great strain on the selector forks and the pinions that they control.

This has worn both the Selector Forks and the pinions. The wear on third gear can be seen here when compared with the replacement pinion:


The Selector forks were also toast. Look at the wear on the forks here:




Now compare it with the £22.00 replacement from Motorworks:


Similarly, the 1st / 2nd gear selector was also worn, but wasn’t quite as bad & the 4th gear pinion that does the engaging had survived. That is probably because Second gear is locked to the shaft by circlips - limiting its movement when the pinions nearer the front are loose:




Although fifth gear wasn’t too bad, its front face was scored so I replaced it with a better one from my ‘used spares’.

The replacement fifth gear and Input gear are compared with the original Intermediate Gear here:


Next, the Output shaft was assembled and the bearings pulled on. I don’t like pulling a bearing on by its outer track, but First Gear is so wide this is as close as I can get. I must make up an adaptor for future use:


When ‘pulling’ the rear bearing on, I use a ‘dolly’ in the end of the hollow shaft to avoid damaging it. I once belled the end slightly by pulling off a very tight bearing so I turned up some spacers and they have proved to be very useful.


The new circlip can be seen here:


The Gear Selector mechanism was OK, but I replaced all three springs as well as the Indexing Roller.

I won’t go through this step by step, but these pics should give an indication of what’s involved for those who haven’t done it.








So – on to assembly . . :D

The shafts and selectors were put in with the case hot. Then the gear selector mechanism was bolted in – while carefully aligning the selector forks with the slots in the cam plates. I use a length of bent wire to adjust the selectors to fit.

I also replaced the front bolts as the heads on the old ones were a bit rounded. I coat the bolt shafts with jointing compound as the lower bolt is below oil level.


The long oil baffle has to be put in and screwed up at this point.



The inner faces of the case were quite marked where the cover has been levered off in the past. Fortunately the mating faces were OK. A new gasket with a light smear of ‘Hylomar’ should prevent any leakage.


Getting there now. The ‘Shimming Plate’ is bolted down over the gasket.


I have been through all this before but here goes again. The height of each bearing above the shimming plate is determined with a ‘Depth Micrometer’. I take five or six readings around each bearing.



Then the depth of the bearing housings in the cover plate are measured. Again, I take five or six readings for each.


The Intermediate Shaft has an oil baffle on the rear and this has to be measured as well – as does the thickness of the bearing plate.


All this is written down and the thickness of the shims required to give 0.05mm clearance to each shaft is determined.


With each shim cluster in place, I put four studs into the case to guide the cover down. A smear of grease on the shims holds them in place while the cover is fitted.


The cover is then heated and carefully replaced so as to not dislodge the shims. Then, while still hot the cover screws are applied and torqued down.

The original cover screws were fairly worn and rusty so I donated a set of stainless screws to the cause.
Problem . . The screws supplied by MotoBins come with washers which are too big for the recesses in the case. I spent at least an hour searching out smaller washers and then opening each out to 6mm with a small round file.

Fortunately I did this before replacing the cover.

With the cover in place, I tapped in a new output flange seal, remembering to block the small triangular vent with gasket sealant. I use a 50mm socket from my ¾” drive set to put the seal in place squarely.

Once in, the seal has to be ‘conditioned’ before the flange is fitted. I use a 27mm socket from my ¾” drive set.

Also, as the lugs that support the clutch operating lever are quite vulnerable I turned up some alloy spacers and bolt one between the lugs to strengthen them in transit.


When cold, the box is quite stiff, so I use an old clutch plate to turn the shafts when checking that the gear change works as intended.


Then fit the drain and level plugs plus a new Neutral Switch – all with new sealing washers.


Evidence of ‘Bertha’s’ hard life can be seen on the alloy sleeve that surrounds the drive flange. When the box packed up the bearings broke up and the flange was able to thrash about. It has cracked and distorted the sleeve.
Crack seen here:


Compare the output flange clearance around the circumference – lots of space here:


I didn’t take pictures of the flange being replaced but its not difficult. I used the same locking bar shown at the beginning, then torqued the nut up to 161 Ft./lbs. with a large ‘certified’ wrench.

And that’s it. Hopefully the box will get Phil & Bertha to Spain and back without trouble. It was an interesting exercise and I hope everyone found this exposition useful.

The story doesn’t stop there though. Phil and Simon intend to take Dad’s ashes for one final run on his bikes. Bertha had better not let him down again :rob.

Enjoy the trip Pastyman :thumb :thumb.

Excellent! & really good photos :thumb2

Great to see what actually makes those clunking, grinding, clanking noises... :blast

Yup, gotta love clapped out Airhead gearboxes :D
Thanks Bob, nice to get a look inside berthas box :D

So glad that there are guys like you doing this stuff. I've looked at the pics and the write up and .... I'm still none the wiser. I can see the wear issues though.

As I have said before, I thought the box was actually the bike finest point. It was positive and never missed a gear :nenau but it was obviously fairly shagged.

Thanks again Bob, you're a genius :thumb
Good write up Bob, with excellent pictures, that box was properly poorly!
Excellent. Thanks for taking the time to document this - fascinating to us that would love to have a go but know better:D
I've never quite understood the shimming thing until now. It reinforces to me that it's one thing to take the gearbox apart but another to know what looks right & what doesn't.
Judging by this writeup & Rob's the future of Britains neglected gearboxes are in good hands
I only understood about 10% of what you described there but this is one of the most interesting reads on here for a long while :thumb2
Excellent Read Bob thanks for taking the time

You may have saved an innocent from the slaughter in the future

Would it be worth mentioning that all the bearings apart from the front one are available from bearing suppliers BUT Must be "C3" clearance

and that once the box is installed and you're about to add the oil to check the oil that it is GL5 specification no matter what grade

Thanks Again

P.s. My biggest tip is to check the neutral switch is there before fitting and leave the level bung unscrewed so that it is a visual that the oil isn;t done

It was a lesson hard learned but learned it was Eh Rob? :blast (p.s. Rob sorted the damage I made the balls up!)
Fantastic write up Bob - many thanks for taking the time to photo it on the way and then write up the work done.

Tell me, as a numpty at this depth, is the 'damage' the result of harsh gear changing, lack of oil changing or just gold old wear & tear?

Nice one Bob!
One of these days I might get around to doing my own box, thanks to your excellent write-ups. Be warned though, I may end up sending you a pile of bits to put back together for me when I've gone out of my depth! :D
Tell me, as a numpty at this depth, is the 'damage' the result of harsh gear changing, lack of oil changing or just good old wear & tear?

The box seems to have been apart a couple of times.

It appears to have had a catastrophic bearing failure which has resulted in the front bearing on the output shaft seizing and spinning on the shaft. Then either the front or rear (or both) output shaft bearings have broken up.

There are teeth marks on the inside of the case near the front of the output shaft and the output drive flange has moved around, distorting the alloy sleeve that surrounds it.

I'd hazard a guess that it ran out of oil in some far distant place and was badly repaired.

The bad repair caused the wear on the selectors and gear pinions.

I don't think harsh gear changes would damage it.

Hopefully its now good for another 100,000 miles :thumb.

Be warned though, I may end up sending you a pile of bits to put back together for me when I've gone out of my depth! :D

No probs :thumb

I now have Andy Frizzel's R100R box sitting on the workbench. Plus, he made me a prezzie of a Cycleworks output flange puller :bounce1 Top bloke :clap.

Two different oil seals

I'm hopefully replacing the gearbox output seal next week...ordered and received the brown seal from Moto-Bins ( excellant service btw ) and then found this blue seal in my box of bits. Brown seal needs conditioning but the blue seal has the traditional spring around the centre. I intend to use the brown seal as this appears the later version......just asking opinions re the seals ?


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The brown seal is for a Paralever - it is fitted open side inwards (and needs conditioning as you say). It keeps all the oil in the gearbox and the small triangular vent has to be sealed.

The blue seal is for a Monolever and is fitted open side outwards. It lets the gearbox breath oil into the leg - which has oil in it anyway.

I take it you acquired a flange puller (as I couldn't find your request when I looked a second time :nenau).

I have just stripped Andy Frizzel's gearbox . . .

Its from a 1992 R100R (the one with the 'K-Series' forks & brembo four-pot calipers). Its a Paralever (naturally). It has 50K miles on the clock.

So; why is the gearbox apparently dated 1981, has a Monolever rear oil seal yet has the late 17.5 degree gear cluster with the 'X' marks :nenau :nenau :nenau

Wonderful things airheads :blast.

Good luck getting the flange off Kenny :thumb :thumb


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