Burman 4th Gear Selector Meltdown

[tatty500]

Matty,
I’m glad there is at least one box out there with correct meshing. Perhaps there were only ever two that are incorrect. That would be just my luck.

While you have your new box in bits, please can you measure the depths from the joint face:
A) to the output bearing inner race without the output gear in place,
B) to the face of the output gear when this is in place in the bearing.
On my box, these are 135.8mm and 110mm respectively.

Still haven’t got mine back together yet…..I’m having the kick start stop boss welded........(Kick start stop sounds very strange)
Merry Christmas
Tatty

 

[Matty]

Hi Tatty

Measured with my new box and gears from my bike. A - is 135.8 same as yours and B - is 109.65.

These are measured as well as I can with a cheapish caliper gague and look very much like yours.

Have compared the "damaged" gears with mine and the ones on the mainshaft look dimensionally identical - including the position of the selector grooves.

Could you see from the photo when enlarged how far the dogs on mine were engeged, it looks OK doesn't it?

So it is still all a bit of a mystery.

I could have another problem which will have to be checked out.

My mainshaft was roughly straightened in 1957 when I first had problems and I am a bit worried that the slight bend remaining in it may be causing the output bearing to be "worked" in the casing.This may possibly be the cause of the slackness there on my old casing, which was in any case cracked in the 1957 incident - will check this when I reassemble the gearbox onto the mainshaft.

I have been a bit lazy and have avoided stripping the primary chaincase and clutch by putting a Pad behind the clutch inspection plate to stop the mainshaft moving back and dislodging the chaincase seal plate, pulling the gears out of the gearbox and then pulling the gearbox off the mainshaft, leaving it in place on the clutch.


To check that the mainshaft output bearing is not being pulled around by any shaft bends I shall have to slip the box with output bearing onto the shaft with the kickstart end bearing in place and check for tight spots by turning the drive sprocket.

If it is tight, I shall have to take it all apart and either find another mainshaft or have a go at straightening it a bit more.

Waiting for a new tab-washer for the drive sprocket nut, so will proceed when it arrives.

Happy Christmas

Matty

 

[Matty]

PS

I have checked the amount of engagement with just the output gear, 4th gear sliding dog gear and selector assembly in the box. I put the selector into 4th, pushed the selector shaft fully in to its bearing and then pulled the 4th gear dog assembly as far back as it would go to make a worst case scenario. Then I used a course felt tipped pen to mark the 4th gear dog on a few teeth where it went into the output gear so that I could see the depth of engagement. It looked like nearly 4mm - and I am sure the pen went right into the corners.

Trying the same thing with Vics damaged parts produced a simular result, but of course the dogs on these gears are very worn and tapered so the contact area is much less.

The 4th gear dogs on my old box gears look virtually unworn, with just a slight taper for a lead in - perhaps I am just a very sympathetic gear changer - but where is the missing 2mm or so ? !!!!


Matty

 

[tatty500]

Matty,
Thanks for the information.
As Hercule Poirot suggests, when you have eliminated the probable, there remains only the improbable.
So, looking back at the evidence, perhaps the actual cause of meltdown is that of an earlier event the evidence for which is obliterated by the melt.
I have looked again at the pictures I took of the 5 gear pairs.
My set shows only a tiny engagement wear patch.
Vic’s set shows a two-stage, stepped wear pattern.
The other two and the CP box show one, longer wear patch.
So, is the actual cause an event long ago that wore the side of the selector without destroying it?
In the case of my box this must have been early in its life, as there is little wear at full engagement. The box then continued to function with the tiny engagement until finally melting and destroying the evidence of old selector wear.
In the case of Vic’s box, the event must have occurred at about half it’s life. The wear shows these two phases and the selector was finally totally melted, once again obliterating the earlier problem.
The other three sets have worn at their original engagement positions.
If I manage to send the pictures, they are as follows:
0CP A very warn lightweight CP box.
1my My metdown box
2new My new box
3vic Vic’s meldown bits
4prewar Chris’s pre-war BAP.
Who knows?………“the old man in the mountain, that’s who.”
Tatty

 

[clevtrev]

One I measured this morning. 135.6 and 108.9

Measured with a digital very expensive comparator.

 

[tatty500]

Clevtrev,
Thanks for doing those measurements.

That means your output gear is deeper/longer from the bearing shoulder to the face. On my gear this distance is 26.2mm, so is yours 26.7?

My gear was the smallest by 0.25mm and my box the deepest by nearly 0.5mm……..but these biggest discrepancies were relative to the pre-war box.
Relative to the other post war box, my gear was down by 0.1mm and case deeper by 0.3mm.

Unfortunately, the face that fits against the o/p bearing isn’t dead square. The radius on the bearing inner bore is quite large and the contact patch on the gear has become more round shouldered than me. In fact , doing the sums, it must be that the gear is only 25.8mm deep to the actual contact of the shoulder on the bearing. We must also take account of my possible measuring errors. I found measuring to the small bit of visible bearing quite tricky with the depth prong on a vernier calliper.
When doing the comparisons I only measured differences for this very reason.

But, the face of your gear is a full 1mm further into the box, so how much further can it be pushed through the bearing before it bumps into the big diameter part of the selector shaft? Is this only 1mm as opposed to the 2mm on mine?

Your box presumably shows a desirable 3mm or more of dog engagement which I am providing with my offset selector pin and big shim washer…….

Thanks,
Tatty
PS don’t know why the pics ended up in that order.

 

[Vic Youel]

The selector fork in my original one was replaced by somebody before I purchased my Comet. It was an original part. The inners were all replaced when Trevor kindly sorted my gear box.

Vic

 

[Vic Youel]

Just a thought; Bruce Harrison who runs the Martston Sunbeam spares scheme used to work at Burman's..... maybe he has an opinion on all of this?

Contact details at http://www.marston-sunbeam.org/spares.html

Vic

 

[tatty500]

Vic,
Bang goes another theory!........unless the the deeper engaged wear happened after the new selector was fitted.

Rgards,
Tatty

 

[Matty]

Hi All

I rebuilt my gearbox on Christmas Eve with 1mm of shims behind the output gear and it did not quite touch either the gear selector shaft or the gear on the layshaft - roughly 1mm clear I guess.

I have replaced my cracked casing thanks to Clev trev and both mainshaft bearings with sealed ones from Peter Barker.

Finished the job while lunch was being prepared on Christmas Day and refilled with 800gm of thixotropic grease, via a grease nipple I put onto the casing.

Did a 20 mile run today (Boxing Day) and it is all fine, with very positive changes and pretty quiet in 3rd gear.

Hope all your boxes are good as well - though I worked out I have spent about £380 on my box in the last 2 years, with several new gears, some bearings, layshaft, the case, grease etc. etc.

Suppose it's not too bad over the 55+ years I have been using it !!!

Happy riding next year.

Matty