Re: Commando primary drive problems

Hi Dave - I've had a RGM belt drive on my 920 for circa 15-20 years and the clutch etc. has been fine (until now). When you state that the 'gearbox is fixed in place' what do you mean? The gearbx on all but the Mk.III is adjustable (or should be!) to set the chain/belt tension. How is it 'fixed'?

Previously dave_lee wrote:

....noticed in Haynes manual the clutch locating circlip on gearbox mainshaft has shims behind it , mine has clutch spacer and shim on outside . ....

Hi Dave!

Just reassembled a friend?s primary drive with triplex chain on a 69 Commando today.

There are no shims behind the circlip.

Circlip- spacer- shims- clutch body is the sequence to my knowledge.

During reassembly I needed two circlips.

The first was warped after I tighten the clutch nut.

I had to open it again because the crankshaft sprocket was not aligned to the woodruff key on the shaft.

I sanded down the second circlip a little so it fits better in the recess on the gearbox shaft.

The rounded side of the circlip faces towards the clutch body.

If it were mine I'd put the chain back on and buy some new modern clutch plates and use ATF in the chain case. Works for me.

Neil +1, you can go to duplex chain Iwis merc camchain and the chain will out live you.

Do you have the second gearbox adjuster fitted, seems to be a requirement to allow you to get the gearbox shafts in alignment with the crank on belt drive systems.

Dear Dave,

With regard to clutch slip. if you are still using original bronze plates I would change them for new ones with more traditional friction material. This will restore combined thickness of plates back to original spec giving you a lighter clutch as a bonus - just make sure of thickness you need before ordering as they are options,

Your belt tension issue is interesting,could be belt itself or maybe more serious like bent gearbox mainshaft. If you have access to a lathe and dial gauge I would check this while apart. If not reassemble it, adjust gearbox / belt tension and turn till you find a tight spot; now pull clutch in and use kickstart to turn clutch centre without engine moving and see if tension changes

good luck

Martin

Are you folowing the correct sequence for setting the tension so the rear drive cannot pull the gearbox backwards,, the factory manual covers this.

When I first read this it sounded to me like the rear chain was adjusted too tight. The first bump you encounter will be enough to pull the gearbox back if there's any slack in the system. It might only be a few thou but that's enough to tighten the belt.

Hi Dave, when adjusting the belt tension when cold, you should have at least 30 mm up and downmovement 'cause in action, alloy clutch drum expands a lot and tightens your belt.

I first fitted a belt drive to my 650 about 18 years ago. The idea mainly being to finally cure the dreaded primary case drip that curses mainy Dominators. After a month of running the bike it was very clear that the conversion had not worked in this respect. The damn bike still caused significant embarrassment wherever I parked. I must add here that my primary case had a Norvil upgrade that adds a spacer on the footrest shaft that sits between the inner and outer cases. This slightly spacesthem apart to allow ventilation through the working bits. It also allowed water in and oil out.

On removing the outer, it became obvious that oil was leaking past the crankshaft seal and also through the screws holding the inner to the cases. Plus horror of horrors the 30mm wide belt was now down to 25mm due to grinding itself against the keeper plates on each side of the pulley.

After replacing the oil seal and gooing up the screws with Silicone I then re-assembled the big chunks. At the time 30mm of play was advised for the belt plus the tip that if you could just push it across the clutch pulley with your little finger then it was probably tensioned correctly.

Being curious as to why the belt was grinding itself to death I then left the outer primary case off and took the bike for a run. The answer became instantly apparent as when accelerating hard the belt tracked outwards across clutch pulley and inwards when changing down gears. It even did this, to some extent when the bikes was stationary and the engine revved up and down.

The cause of the dancing belt was simply worn gearbox bushes andbearings . The other fact that hit home hard was the change in the belt tension. Despite all the moving parts being exposed to the air, both pulleys had become too hot to touch and the belt play reduced down to zero as a result of the alloy pulleys expanding.

Manuals available here

http://britmoto.com/

From memory, so could be wrong so check the manual, the sequence is that you loosen the chain (and the belt will be the same) so it is slacker than the spec and then tighten until its the required tension. The objective is to be moving the gearbox rearward as the last action, so its tight against the tensioners and the rear chain will not have slack to move it back further, if your last action is to loosen the chain/belt then there is enough slack left in the mountings so the gearbox can move backwards.

There's around 12 thou clearance between the top gearbox mount bolt and the hole in the adjusters and the same between the bolt and gearbox mount hole. If the gearbox is dragged rearwards on the adjusters to adjust the primary, there will be potentially 24 thou 'slack' that could be taken up if a significant rearward load is experienced on the gearbox main shaft by the rear chain.

You can easily test this by just nipping up the top bolt after adjusting the primary then applying pressure on the bottom run of the rear chain with your foot or whatever. The primary drive will tighten significantly.

Unfortunately, I'm not sure how you would stop this happening apart from making sure the rear chain is slack enough on compression of the rear shocks.

Previously Jim Brierley wrote:

There's around 12 thou clearance between the top gearbox mount bolt and the hole in the adjusters and the same between the bolt and gearbox mount hole. If the gearbox is dragged rearwards on the adjusters to adjust the primary, there will be potentially 24 thou 'slack' that could be taken up if a significant rearward load is experienced on the gearbox main shaft by the rear chain.

You can easily test this by just nipping up the top bolt after adjusting the primary then applying pressure on the bottom run of the rear chain with your foot or whatever. The primary drive will tighten significantly.

Unfortunately, I'm not sure how you would stop this happening apart from making sure the rear chain is slack enough on compression of the rear shocks.

I think you will find that the correct procedure is to adjust the gearbox so the belt is bar tight. Which involves pulling the gearbox rearward as far as possible and then pushing or pulling it forward until the belt is the correct tension using the adjusters. Doing it this way takes all the free play out of the gearbox attachment points. That way the rear chain cannot pull the gearbox rearward under use. There is a technical reason for why this happens - explanation available if required.