Noise issues on 350 Scrambler engine

[Rocla]

Hello,

Thanks for your replies. i've moved the oil pump cover and the findings are :
-A the nut on the crankshaft was free because the screw thread is dented, only 2mm deep are usefull to tie the bevel pinion on the crankshaft

Dsc04898A.jpg

-B as a result, the bevel pinion was flying, few mm away its normal position, and is dented because it was clinking on its counterpart (the source of clinking sound)

Dsc04900.jpg

-C its vertical counterpart is dented too (but less), of course, as you can see

Dsc04897B.jpg

-D in addition, the bevel shaft has 1 mm slack when you turn it. That means 1 mm slack at the head cylinder level

Dsc04896B.jpg

Given you know very well the single (reason why such interesting forum!), here are my questions:

Q1: how could you fix the screw thread on the crankshaft ? Insteed of removing a brand new cranckshaft, for me there are 2 solutions: do do a new thread on the smallest diameter in order to add a second nut to stuck the genuine one; to stick the genuine nut and to fix it with a little wedding bead
Q2 : how to replace the lowest vertical bevel pinion? Does it requires to remove the cylindre I guess, but not the crank case?
Q3 : how to replace the upper vertical bevel pinion (in the head cylinder) ? Does it requires to remove the cylinder?
Q4 : do you think that 1mm clearence when you rotate the bevel shaft is normal or particularly huge? For me it is huge and probably the origin of my very noisy engine, but I have no experience of Ducati singles...
Many thanks indeed!

[Bevel bob]

HI, The bad thread would normally mean another crank, if the nut does tighten then some nut lock may do the job as this nut is not heavily loaded. leaving out the tab washer may free up an extra thread or so.I would not do this untill you have investigated the 1mm slack which may be on the shaft joint or the top bevel shimming or a missing shim under the housing at the bottom of the shaft.The smaller diameter of the crank is used to seal against the cover oilway bush which feeds the big end with oil.New bevel gears would be good ,but i have seen damaged ones still running .

[Rocla]

Hi,

Exact, I can cancel the washer to gain half a milimeter, and cancel one or two washers behind the bevel gear too, in order to minimize the shim between the 2 bottom bevel gear and to gain additionnal space on the safe part of the thread. I think it will be possible to tighten the nut, otherwise, I can add a little welding point to secure its fixing.

About the upper bevel gear, I've check, there is a big shim between them and I need to rise the bevel shaft. So, is it necessary to dismount the head cylinder or not, to add washers where it is necessary?

[Rocla]

In addition, following the advice from StewartD, if I understood well, the mean to minimize the shim on the upper bevel gears is the thanks to the part 89 in your first picture, that means the shim can be reduced on the camshaft axis, or on the vertical bevel shaft as well, at its bottom or top. Is it right ? And how to do so? Is it necessary to dismount the head cylinder or not? At what place it is easiest to add a washer?

Thinks are getting more and more clear for me thanks to your posts!

[graeme]

The bevels have to be meshed correctly.
There is a grind mark on both pairs of bevel gears that should be found and shim them to this grind.
This is a good start point.
Not too tight and not loose. But they must be shimmed for proper contact meshing.
Do you have a manual?

The thread on your crank shaft may be ok with a new nut?
Look to see if the damage to the thread is left from the nut thread and may come off the crank shaft thread?
The shaft thread is stronger than the nut thread and usually the nut thread will strip before the shaft thread.
When you have the lower bevels meshed correctly you could lock tight the nut in place without a washer as Bob says.

The top bevels also need to be shimmed correctly.
You may have to remove the head to shim the vertical bevels.

[StewartD]

Hi Rocla,

The screw thread on the crank looks bad. I agree with Bevel Bob that it normally means you need a new crank. As to avoiding this by leaving out the lock washer: I agree with Bob in leaving out the lock tab washer, but check how much good thread is engaged without the lock tab washer. As the nut is about 5mm thick, I don't think the 2.0 - 2.5mm that you are talking about will be enough.

I think lock tab washers are a pox idea. They must be made of a soft steel, otherwise you wouldn’t be able to bend them. The softness is like the weak link of a chain. It will give way slightly and even without the nut rotating, the mutual compressive force between bevel gear face, tab washer and nut is slightly reduced.

Bob says this nut is not heavily loaded. I disagree with this. The torque required to drive the cam fluctuates to a large degree for every rotation of the cam. The torque being transmitted between a pair of bevel gears results in separation forces that will fluctuate to a corresponding degree. This separation force is trying to force the bevel gear of the crankshaft against the tab washer and nut, every second turn of the crank. I think the torque and the resulting separation force, would reduce to zero, or near zero, at the time of the inlet valve closing.

This fluctuating force in addition to the force resulting from tightening the nut is acting on a soft steel washer. If this load were evenly distributed across the washer area then there would probably be no problem. In real life however, the nut, washer and bevel gear faces are not perfectly flat and mutually square. If the bevel gear bore is slightly larger than the shaft then the gear will deflect slightly. All these possible imperfections will mean the fluctuating loads may impact on a small area of the soft steel washer, any yielding of the washer will result in the components moving slightly. Movement between components will lead to wear and therefore more movement and ever increasing wear.

If you decide to try reassembly without the tab washer, then I would clean up the crankshaft thread with a metric thread file. The thread is M14 x 1.0mm pitch (left handed) Use a new nut and torque it up. My Clymer manual does not give a torque figure. I think a recent post by Nigel of Lacey Ducati gave some torque figures. If the thread strips, then do not think you have lost anything. I am sure it would have failed in service anyway, and possibly damaging other components.

I agree with the rest of Bevel Bob and Graeme's comments. I don't like the idea of welding the nut in place.

The 1mm backlash in the vertical shaft is very bad. A badly worn or collapsed top bearing (item 99 in previously posted drawing), or cam bearing (item 206) could be the cause of this.

Cheers,

Stewart D

[Bevel bob]

Taking out shims to move the bottom crank bevel inwards in order to gain/expose an unused thread is definately going to cause more problems with bad gear meshing . I would remove the valve rockers (catch and note where the rocker side shims go) to allow you to turn the motor with the bevels unloaded ,this will make it clearer where the problems are.In theory shimming is a simple process but in practise its complicated by the need to have the ball race steel bushings held down and the gaskets at both bevel sets under just enough pressure to seal.You will need a micrometer and spare shims to get anywhere ,also circlip pliers and new clips.Working on the head bevels with the head off will require some means of centralising the shaft in its tube(i used a snug fitting socket from my Elora set).Its possible that the indentations on the gears will make it impossible to get the gear clearances right at all points.I agree with Stewart that the lockwashers are suspect and without the proper means to hold things still and tighten to the correct torque they will come loose.The bevel side crank nut is the only one that i have never found loose!!, perhaps i've been lucky.New gaskets will also cause problems due to thickness issues. I have heard of motors from new having shimming problems ,so engage full brain power!!.If everything was set up correctly first ,i can't see that anything would be lost by spot welding the nut( the parts are scrap anyway!), however i doubt that you will get the shimming/sealing right first time, It took me a few goes. It may be possible to skim off a couple of mm off the outer face of the crank bevel gear to bring the good threads more into action ,but this is a hardened part and would not be easy to work.All bodges tend to be self defeating in the long run,but if you get the bike back into action it may enthuse you to do the job properly next time.

[StewartD]

Hi again,

Bevel Bob's statement that, "The bevel side crank nut is the only one that I have never found loose!!" surprises me. The crank nut and the cam nut are both left hand thread M14 x 1.00 pitch. The orientation of the bevel gears means that the crankshaft nut is the only one that takes the separating forces of the bevel gears. The separation forces between the camshaft bevel gear pair push the cam shaft gear directly onto the camshaft bearing inner ring. The force is taken through the bearing balls to the outer ring and into the head casting.

I must admit that I have never seen a crankshaft nut come off, but I have seen the result of a camshaft nut that come off. It lodged between the bevel gears as there is not a lot of room for it to fall elsewhere. The part of the head casting outside the vertical shaft bearing was sheared right off when the nut jammed the gears!

Cheers,

Stewart D

[Bevel bob]

Hi Stewart, The most common damage seems to be from the drive side crank nut comming loose, usually destroying the keyway and flywheel tapers, This seems to be a poor design to me,with just a key to carry the drive load ,other designs use splines or a taper and key for the drive. This stems from the design being for a 100/125 with little power.My 250 was thrown together by a modern bike shop and came to me with a multitude of horrid bodges ,but i'm gratefull I got the chance to buy it and at least its now on the road and not rusting away in a Norfolk barn.

[ducwiz]

Hi,

in my opinion this engine was under blowsy service, which means it may contain more of such surprises. All parts and components must undergo a comprehensive inspection after total dissection. There is only one way to go now - not the cheap but the proper one.
If you have no experience with DUCATI engines, you must have a workshop manual and a parts catalogue on the bench for reference. Especially Ducati singles do not forgive most of the service mechanic's mistakes.
Download information from http://ducwiz.minus.com/uploads or elsewhere on the internet.
Be sure to have all special tools necessary at hand.

The crankshaft thread can be welded up and reworked on a lathe, a new crank isn't essential. A smaller thread is not recommended, and to do this job, the crank has to be removed from the engine, anyway.
The nuts on crankshaft and camshaft have to be torqued accurately and secured by lockwashers. I additionally apply some Loctite 242 onto the threads.
The bevel gears cannot be moved into an arbitrary position by adding or taking out shims, their meshing has to be set perfectly for minimum backlash and working geometry. Read this http://i.minus.com/1378033040/IxGMgJ0xgVLgE9a1o7niJQ/d8gmyql9spk4P/mesh_job.pdf.
If the nut on the left crankshaft side is in same condition, it will loosen soon and the alternator rotor will come off his conical seat. Immediately the tapers an crank and rotor will wear out, and this joint will be damaged forever. Again, expensive repair measures will strain your budget.
The connection between lower and upper bevel shaft must not show backlash either. Check the faces of the Oldham coupling, and the condition of the connecting sleeve and fixing circlips.
The end play of the bearings for the lower bevel shaft and gear is set by shims inside the steel pot that surroundss the shaft. This can be pulled out by a special tool. The shaft/gear will remain loosely inside the right engine carter. It can only be taken off after separating the carters and removing the crankshaft.

good luck Hans