Hi, I have a Scout 1928 37 "with an engine or transmission problem. If I drive at the highest gear and over 60-70 km / h, a vibration feels in the footplates and feel on the gear lever. If I'm on the throttle Quickly, I perceive it as it disappears during the actual accelration.

The engine and transmission including the clutch have been recently refurbished but not the gearbox itself.

Does anyone have a tip where the problem is?

Regards

Jorgen Fritz

Visby/Sweden

The rear Most mount point  if loose  can give vibration issues ,That is the 7/16 Bolt at the rear of transmission .It needs to be tight .

Both mainshaft ball bearings should be changed, especially the thin bearing is exposed to tremendous forces from chain whip and jank. When pulling/pressing or knocking in/out those ball bearings, heat up the case with something ( I use a hot air gun) to when spit sizzles on the case. 

When mounting a new bearing, some do, but I don't bother to freeze the bearing as I have the opinion that steel contracts little with temperature compared to aluminium and the cold steel does chill the aluminium quickly. I do lube a little with a light oil like WD40. See to that the bearing has seated right at bottom of the seat. Be careful not to press or knock on the bearing too hard when the case is heated because the aluminium is soft, best is to use some kind of anvil to support the backside. Abort and pull out if it looks like the bearing isn't sliding in with constant force.

Same shims or distance washers at both ends between the bearings and the shaft splines should be used for the mainshaft, the thicker one at the clutch end. Perhaps thicker if you must reduce side play, but do not reduce the thickness. The reason I mention that is...

I made the mistake once to think the sideplay was reduced with new bearings, so I reduced the shim thickness to get proper sideplay. What I didn't check was if the clutch side bearing had seated fully. Assembled and out on my first test ride, the gearbox warmed up and when I pressed the clutch pedal, the bearing sunk in. The result of that was that the clutch disengaged suddenly, pedal went to the floor and the bike lurched forward. I had no chance to engage the clutch or stop the bike so it was a heart stopping moment there for a good while...  

And check play in the cluster gear bushings, both in the gear and in the case, large play makes gears whining. Loose or oval worn gear case bushing on the right hand side is particular bad as the bushing seat is fairly weak, and gear vibration that will result can pound the bushing loose in the case with more elaborate restoration in the waiting..

The cluster gear shaft is constructed a bit daft, it really should have a threaded hole at the clutch side end, in order to fit an extractor. Because the far end bushing should be so tight that you have to lightly knock in the shaft.

There is a tell-tale hole below the chain sprocket. The reason for the hole there is two-fold. One is so you can insert a thin mandrel and knock out the cluster gear shaft. The other is that it tells when the bushing is getting worn and the shaft starts to rattle, then oil seeps past the bushing and the leak is visible on the outside. That is the reason why I advocate don't seal up that hole.

"I hope I mounted it in the right direction. The axial load comes from the transmission side." You are right if I get what you mean, the load direction comes from the transmission side. Sort of...The axial load is from when you depress the clutch pedal and it is pulling on the clutch. But the main load is severe from the gear drive on the clutch basket as the clutch basket is positioned off-side of the bearing. 

The gears inside the gearbox is not pushing in either direction as they are straight cut. The problem is a combination of issues that decide the wear and axial play of the bearings. Excess play in the bushings on either side will allow clutch basket and/or sprocket gear to tilt and that accelerate bearing wear, together with the weak thin ball bearing's rapid wear from chain whip, that all will result in more side play at mainshaft.

With a bearing that is set up correctly there should be very little tilting movement at the clutch basket. A plain ball bearing has a very small play when new but since the basket is large diameter and the bearing is offside to the basket, the small play in a new bearing can be felt, but only just! 

A tapered roller bearing should be set to zero or pre-loaded so there is no tilting or up-down play at all. The bearing rolls are at risk of shudder or vibration if a bearing of that kind is set with play. Since the mainshaft has to have a certain side play in order not to pre-load the thin ball bearing at the other end, a tapered roller bearing is not at all ideal to use! 

Original in both types (small and large clutch) gearboxes there was a angular contact ball bearing that has to be axially limited by the main shaft thrust washers, but could take a certain sideplay without going sloppy. That is why side play is very important to set correct, in conjunction with that kind of bearing. 

The contact angle of a Indian original ball bearing (Fafnir or Timken) was 15 degrees so it could take some wear or larger axial play in the mainshaft, without getting the radial play out of hand, but such shallow bearings is not made anymore in the size needed for our gearboxes. Modern angular contact ball bearings is made with 40 degrees so a little axial play makes a huge difference in radial play (ten times the difference, measured one time ago by George Yarocki) so clearly they are not suitable to use.

The bearing situation is a problem in the older gearboxes where the bearing is mounted from the outside. Those bearings are of a non standard dimension. I have seen for sale some specially ground bearings that I assume are ball bearings. Or machining has to be done on the case to fit a standard size bearing. There are several ways to fit a standard size bearing there... 

Newer boxes where the bearing is mounted from the inside has a bearing size of standard dimension and a common, preferable 6207 2RS ball bearing fit right in. Of course the gearbox may work with either the 40 deg. angular contact ball or roller bearing when tight, but only for a short while. 

With plain ball bearings on both sides, the side play in the mainshaft is good to set to specifications, but not essential, as the clutch bearing is not dependent on support.