Bevel tube Exploded

[lloydy1]

Hi,
I have just got my Ducati 350 single running and it seemed to be running not to bad maybe a little rich, when I blipped the throttle the bevel tube exploded Upon removing the the head and tube the top cir-clip on the transmission shaft above joining tube was not in its grove and the sleeve high in the tube.

Has anyone had this happen to them before?

Could the shimming be incorrect and as it has heated up pushed the clip out of its seat?

There was a little oil in the barrel also could this have any relevance?

[graeme]

Bummer!
I haven't seen that before, it sounds like the oldham coupling tube has moved up due to the circlip not being home, then allowing the un supported lower end of the shaft to flail around.

[lloydy1]

Hi Graeme,

I think your right! but my issue is how did that happen. I never removed the tube or shaft from the head itself, so thinking that there must be a shim missing. There is a shim in place at the bottom of the tube, I made sure that I was careful when I removed the head with the tube attached and what shims where there went back.

Should there be any free play up and down at all in the oldham coupling, because the only thing I can think of that unseated the cir-clip was that oldham coupling was to tight between the clips and the top one became unseated from its grove.

I have managed to get a new tube etc so if any one has any ideas before I refit it that would be great.

[ducwiz]

Hi,

possibly the upper circlip was set under axial force during assembly, being pushed out of it's groove, by the sleeve being too long. Also, the cylinder face or head recess could have been machined down by somebody (to increase compression, or restore pressure tightness?), which would have the same effect on the circlip after reassembly.
I experienced once, that circlips in our engines loose their tension over the years, which makes them easily rotatable in their grooves. Possibly they loose resilience against centrifugal forces, too? You should check their condition, and in doubt install new items.
Circlips are stamped from sheet steel, thus comprising a sharp and a rounded ridge on their circumference. Mounting the ring's sharp ridge against the thrusted side of the groove improves resistance against push-out forces.
At times of production, sleeves with at least three different inner diameters were available, in order to adjust the right radial play of the shaft couplings. Too much play here may have contributed to the connection's failure.
Btw, the broken tube now makes a perfect tool to check the sleeve's axial play between it's stop positions; just cut the damaged end off in order to gain sight/access to sleeve and circlip.

rgds and good luck

Hans

[lloydy1]

Hi Hans,

I think your correct in your diagnosis. I cut the bevel tube and replaced all parts together and the head was being held off the pot by about 1.5mm and when I tightened the head down a little the cir-clips where under pressure.

I also noticed that the top cog was not meshed correctly, it was to low in the mesh when the oldham coupling tube was not in place, so I presume the one and only shim underside of the top bearing is not adequate. it needs to be shimmed up slightly.

I will machine the oldham coupling tube down 1.5mm so there is a little slack between the cir-clips up and down and shim the top bearing to re-align the mash and fingers crossed that should do the trick.

Thanks, Dave

[Bevel bob]

Don't do that. It is likely that the top bevel shims are wrong as you say, it is also more likely that the bottom bevel shims are very wrong and have been removed / moved in a futile attempt to compensate for a different problem. This raises the bottom bevel assembly and takes away the clearance at the shaft joint. You will also find that the angular contact bearings at the top bevel have been damaged by this bad work. Start with the crank end float ,if there is none ,ok, the crank bevel gear should have a thick shim behind it ,I bet its missing ,your problem started here.If you want to progress without getting into the bottom end you could fit a compression plate under the cylinder to raise the shaft.

[ducwiz]

Hi,

a good How-To about mesh adjustment was published in DUCATI International Owners Club Newsletter, issue No. 40: http://ducwiz.minus.com/mAbKNEkv8/1g.

..

[Jordan]

I like the last sentence.

[lloydy1]

Thanks for the info!
I contacted a guy here in the UK called Nigel who owns a company called Lacey Ducati. I had a chat with him about what went on.
He gave me some good guidance also. Here is one to watch out for, on the bottom of the bevel shaft there are two groves, even though there are two groves there should only be one cir-clip on the lowest grove. I had another look last night at my shaft and there was two clips one in each grove, this seems to be the reason that the oldham coupling was tight. Nigel explained that all shafts have these two groves, but the top one should never be used. has anyone come across this issue?

I have checked the cogs for wear and damage and there is non. There is no play in the bottom bevel and the shims and washers all seem to be in order.
The only issue I have now is the top bevel is not quite meshed correctly, there are two shims on top of the bearing underside of the top bevel and there is free play where it meets the bevel on the end of the cam, so I can only think it needs addition shims to hold it in its correct position, as there is play in the oldham coupling which is not forcing the shaft up into the mesh which is why the cir-clip was forced out of its grove and the reason for the oldham coupling riding up the shaft and the Subsequence damage.

[Bevel bob]

The upper and lower bevel circlips will be badly weakened by the incident ,anyone who could pull down the head against the obvious resistance of the bevel drive has to be a mechanical clot, beware , what else have they done?.And yes I have done stuff like this,before I knew better.Its much easier to work on the top bevels with the head off,you just need a socket or something similar to stop the vertical shaft from wobbling about in the tube.Also remove the rockers to allow you to feel free running.You may be lucky and not have to remove the cam.Watch out for debris in the lower bevel bearings.