Gearbox info

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worntorn

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I'm looking at some Commando gearboxes for sale on EBay and also a couple that are for sale privately. One of the best looking and most expensive boxes is an 850 box which the seller says has the high second gear ratio.
On checking part numbers at various parts retailers, it appear the gearbox shell changed in 73. The pre 73 shell is part # 06-0808 while the 73 and 74 shells are part # 06-4126.
This info is from the net, I don't have the actual parts books for these years.
If the shell was changed in 73, was it made stronger? Aside from the high 2nd gear, which is nice but not necessary, are there any other advantages to the later box? I recall reading something about an added circlip.
Also, I'm wondering at what serial number did the high second change occur. I found an old Access Norton post on this subject. One owner with a gearbox 307xxx says his gearbox has the high second.


Glen
 
The high 2nd was to pass a noise emission test ie to drop the revs as it passed a DB meter, to a rider it is not a factor to decide which gearbox is better. Don't know the answers to the rest but any gearbox will need close inspection of the layshaft bearing and the ball bearing (with brass or steel cage, I know LAB says its brass but the cage pieces I fished out of the gearbox looked like steel but lots of debris may have disguised the metal) upgraded to a roller or a Mick Hemmings ball bearing with specialist plastic cage.
 
worntorn said:
This info is from the net, I don't have the actual parts books for these years.
If the shell was changed in 73, was it made stronger?

As 06-4126 appears to supersede 06-0828 and no other case parts were changed at that time then it seems 06-4126 was improved in some way, so it probably is stronger unless somebody knows of another reason?
https://andover-norton.co.uk/en/shop-de ... 3-06-0808-


worntorn said:
Aside from the high 2nd gear, which is nice but not necessary, are there any other advantages to the later box? I recall reading something about an added circlip.

A circlip was added inside the outer end of the sleeve gear on Mk2/2A (so from 307311) to prevent the outer bush working out of position and wearing against the clutch circlip, this single circlip sleeve gear assembly is 06-4991. Eventually superseded by the Mk3 sleeve gear assembly 06-5954 which has shorter bushes and a circlip at each end.

https://andover-norton.co.uk/en/shop-de ... 7-06-4991-


worntorn said:
Also, I'm wondering at what serial number did the high second change occur. I found an old Access Norton post on this subject. One owner with a gearbox 307xxx says his gearbox has the high second.

Originally introduced on the 850 Mk1A model (from 306591) and to all by Mk2/2A (307311) as the standard ratio 2nd. gear parts do not appear in the Mk2/2A supplement.
 
kommando said:
but any gearbox will need close inspection of the layshaft bearing and the ball bearing (with brass or steel cage, I know LAB says its brass but the cage pieces I fished out of the gearbox looked like steel but lots of debris may have disguised the metal)

Earlier (pre-FAG) 6203 bearings certainly had steel cages.
Some Portuguese FAG 6203 bearings could well have had steel cages but the majority seem to have brass cages.
 
Thanks Les and Kommando, your info about covers it.
I do like the high second for riding mountain switchbacks, both climbing and descending as the gear down from 3rd to 2 nd is a little less abrupt with the high second, easier to avoid rear wheel slide. I've got the earlier ratio on the 650ss and although it's not a big problem, the MK3 2nd ratio suits that kind of riding, especially with the extra torque of the 850 for climbing.
This gearbox is for a 920 project, so there should be some additional torque there.
All the more reason to use the stronger shell, if that was the change.

Glen
 
kommando said:
...upgraded to a roller or a Mick Hemmings ball bearing with specialist plastic cage.
Same here, but a different supplier. My Portuguese bearing passes the magnet test on the cage; not magnetic, so must be brass. No matter the material, it was an eye-opener to find it in there after 10,000 miles, followed by an immediate sigh of relief that it was still intact.
As for the higher second gear, yes, the extra torque of the 850 helps cover the increased 1-2 gearshift spread. That's even with a 22 tooth front sprocket.

Gearbox info


Nathan
 
The later 850 box housing is like the MKIII. It has the neutral switch boss (unmachined).
It has the upper shifter boss full height, this was cut back for the MKIII cross shaft clearance.
A big strengthening factor is a fair amount more meat around the sleeve gear bearing and layshaft bearing housing...
This casting then recieved the MKIII mods in 75.
I think a side by side exam with earlier boxes will show a bit more beef all around.

old britts 06-4126 Gearbox Shell, Pre-MK3 $465.63

AN part-no: 06.4126 273.91 £
 
I'd be more interested in the internal gears / shafts condition. The shell would be much less important as a new and stronger AN shell is a relatively cheap way to increase peace of mind IMHO.

A stronger shell with light clutch and primary takes a lot of that strain off of the box. A Maney outrigger is an option too of course.
 
I successfully used an early AMC box fitted with an American close ratio gear set, behind my 850. First gear was higher than second in a standard box. If ever the casing was going to fail, that would have been the situation where it was most likely to happen. Off the start in races, I was using a heap of revs and riding the clutch out very rapidly. The torque of that 850 motor is massive. When I fitted the TTI box, I didn't realise the gearshift was reversed. The 850 pulled 5th gear from a standstill and I almost stuck the bike through a fence.
 
I wonder what the percentage of layshaft bearing issues is related to lack of NORMAL MAINTENANCE (lube changes), what percentage is due to inferior materials, what percentage is due to ABUSE, and what percent is due to accumulated mileage wear?
 
| think you can order the TTI box to shift either way. Keep that in mind when purchasing and if you use
rear sets with a reverse shifter.
 
Nigel, I wish it was possible to have a look inside the gearbox prior to purchase. Unfortunately it is a couple of thousand miles away and at a retail bike shop. They aren't going to want to pay labour to disassemble , take photos and reassemble.
So like most things with old bikes it's buying a pig in a poke.
I've been lucky buying from afar in the past, will see if that continues

Glen
 
Nater_Potater said:
kommando said:
...upgraded to a roller or a Mick Hemmings ball bearing with specialist plastic cage.
Same here, but a different supplier. My Portuguese bearing passes the magnet test on the cage; not magnetic, so must be brass. No matter the material, it was an eye-opener to find it in there after 10,000 miles, followed by an immediate sigh of relief that it was still intact.
As for the higher second gear, yes, the extra torque of the 850 helps cover the increased 1-2 gearshift spread. That's even with a 22 tooth front sprocket.

Gearbox info


Nathan

That's exactly what mine looked like. I'm pretty sure I was the first person inside the box since the factory. If I'm right, the bearing had done 20,000 miles and was still in one piece.
 
Using a 22-tooth drive sprocket must put a bit of stress on that high second gear, because I broke two teeth off the SS clone's 2nd gear years ago. While replacing the 2nd gearset, I also replaced the layshaft bearing, due to it's cheesy reputation. Mk III box converted to rhs.

Recently, while doing the bearing replacement on the flood bike's box, I dug the old layshaft bearing out of the bonepile just for comparison. It had a patina of rust, including the cage, so it was definitely not brass-caged. The old bearing in fv's box ('73 850) also appears to be steel-caged.

I think the later shells were strengthened to prevent the "taint crack" between the bearing seats. I have seen the crack in 750 gb shells, but neither of these 850s have it.
 
Danno said:
Using a 22-tooth drive sprocket must put a bit of stress on that high second gear, because I broke two teeth off the SS clone's 2nd gear years ago. While replacing the 2nd gearset, I also replaced the layshaft bearing, due to it's cheesy reputation. Mk III box converted to rhs.

Recently, while doing the bearing replacement on the flood bike's box, I dug the old layshaft bearing out of the bonepile just for comparison. It had a patina of rust, including the cage, so it was definitely not brass-caged. The old bearing in fv's box ('73 850) also appears to be steel-caged.

I think the later shells were strengthened to prevent the "taint crack" between the bearing seats. I have seen the crack in 750 gb shells, but neither of these 850s have it.

My '74 850 gearbox shell had a crack between the bearings, with only 20,000 miles on the clock too. Although the belt had been run WAY too tight by the PO, maybe this contributed.
 
Fast Eddie said:
My '74 850 gearbox she'll had a crack between the bearings, with only 20,000 miles on the clock too. Although the belt had been run WAY too tight by the PO, maybe this contributed.
Not likely from the belt. My '74 was also cracked at 10,000 miles with a floppy primary chain, and it had a pretty easy life up to that point. It's just too dang thin right there. Should have upped the gear diameters just a bit to get some more meat there. Really no reason to push the shafts that close together.

Nathan
 
grandpaul said:
I wonder what the percentage of layshaft bearing issues is related to lack of NORMAL MAINTENANCE (lube changes), what percentage is due to inferior materials, what percentage is due to ABUSE, and what percent is due to accumulated mileage wear?

An old racing friend of mine broke 16 gearboxes in one season back in the '70s.

As I understand it the failure mechanism is associated with the long overhung moment of the mainshaft causing flex which is then transmitted to the layshaft. The displacement causes the layshaft bearing to become an axial pivot point and the bearing cage is duly prised apart, resulting in the bearings collecting at the bottom of the race and the inevitable locked gearbox.

As a percentage? Anyone's guess :wink:
 
Any second hand gearbox expect the gears to be shot - allow for this.
As before MK3 gearbox shell is stronger than the earlier versions, don't know why, as the earlier cases seem to last in normal use, may be at the time they thought it maybe required for the MK3 or another purpose. For a road bike a sound secondhand early case would not worry me.
A case cracked between the bearings was most probably like that when it was originally fitted - many are still in use, though rarer to here of someone finding one these days.
Less teeth on your gearbox sprocket = makes life easier for the gearbox, but if you are not going to being doing the miles and not racing then no real issue in what you prefer to use.
Well put together and maintained these boxes will 10's of thousands of miles each year, there are riders out there who do these sort of miles.
 
Onder said:
| think you can order the TTI box to shift either way. Keep that in mind when purchasing and if you use
rear sets with a reverse shifter.

The shells are interchangeable. The difference in shift mechanism is in the inner and outer covers, which is all you need to swap sides.
 
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