Burman 4th Gear Selector Meltdown

[Diogenes]

My Comet let me down a few days ago. Just like Vic Youle's did a few years ago. The fault was exactly the same, a melted selector on the 4th gear side.
I was beating myself up for using the wrong lubricant which could be oil or grease depending on what you read. However the real reason is far more serious.
So what I'd like to determine for all Burman users is if this is a manufacturing problem that applies to boxes made in March 1951 or if all are the same.
For those who don't know, the model number and date are stamped on the outsde of the inner cover. G97 is the Comet BAP model and in my case C51 is the date using letters A to M for the months from Jan to Dec with I not used.
The meltdown occured because when the mainshaft sliding gear is in the direct drive 4th gear position its 26 teeth only overlap the dogs inside the 33 tooth output gear by 1.2mm and some of this was on the lead-in chamfer! (When in third gear, the overlap at the other 20 tooth end of the sliding gear is 4mm.)
Having spent most of its life in top, this little contact patch slowly wears the corners away so that now the selector has to provide a force to hold these teeth in position.
This force produces wear in several places......the gear groove, the selector face, the selector pin, the groove in the camshaft and last but not least the inboard end of the camshaft bush where there is steel to soft steel contact.
As the wear increases the force increases and as the force increases the wear increases until the lubricant breaks down and the selector melts from one side.
My question is this. Is this a design fault that can affect all BAP boxes, OR is it a manufacturing fault that occured for a few days or months? So if your Comet has suffered like this what is your gearbox production date?
Also, will everyone with lots of Burman bits go and look at the shiny contact patch caused by the dogs on the tooth ends of the sliding gear and reassure us all by finding lots with a good 4mm at both ends.
Now where can I buy a new selector from?
Good hunting,
Tatty

At the risk of sounding silly, have you considered the option of fitting a non-Burman gearbox and clutch?
Provided you can tolerate the non-standard appearance, there seem to be many advantages which make the Comet a more pleasant bike to ride, and more reliable.
Plenty of advice written by others about which items to use and how to do it.
Can't be more expensive than sticking with Burman, which was only chosen by Vincents as original equipment because it was cheap in those days.

 

[tatty500]

Diogenese and others,
Yes, I have considered fitting a different box, but surely there must have been lots of BAP boxes with no problem. How many Panther 100/120's were there coupled to triple adult sidecars?
On the other hand, it does seem strange that the spares company can supply output gear, sliding gear and selector. There must have been demand.
I am still hoping anyone out there with lots of BAP bits will examine the wear patterns on the ends of the sliding gear teeth.
If there is a problem perhaps new sliding geras can be modified to suit. After all it would be easy to reduce the length if it isn't needed.
I still hope everyone with lots of bits will clean off the grease and examine the wear pattern on the ends of the mainshaft sliding gear.
Also I still wonder if there is a part in some other Burman box type that can be mistaken for the BAP part and so create this problem. Perhaps they know at Daganfly. I'll call them tomorrow.
Regards, Tatty

 

[tatty500]

Ooops, sorry, I seem to have repeated myself there.....
Tatty

 

[Bill Thomas]

Hello 500, Is the bearing and the circlip in the back of the box OK, Some years ago I lost 4th on the road, Came home in 3rd and found the circlip half out, The casting had broken that holds the circlip, Had it welded and has been OK, Cheers Bill.

 

[Diogenes]

"Yes, I have considered fitting a different box, but surely there must have been lots of BAP boxes with no problem. How many Panther 100/120's were there coupled to triple adult sidecars?"

Quite right.
So the next port of call could be the Panther Club?
They have a good technical section, for example they sorted out the stripped splines clutch centre problem long ago.
Maybe they have spotted the gearbox faults you mentioned?
Worth asking them as they are usually very helpful.
http://www.pantherownersclub.com/

 

[tatty500]

Well, I think I have measured everything there is to measure. Most of the critical dimensions are hard to find without cutting the side off the casing. But here's what I think I've found.
The bits on my layshaft show hardly any signs of wear on the axial contacts. Here I think that owing to the lack of side-thrust the graese holds eveything in mid-slop position when running. In this position Mr Burman seems to allow 1mm separation between bits that could touch and cause problems.
If I use these rules on the mainshaft then the output gear and the third gear should be 2 gaps of 1mm further apart than the 84mm length of the sliding gear, i.e. 86mm. ( a metric design)
In my box the separation is 87.6mm.
My plan to simply move the output gear inboard is limited by the selector shaft which is only 2mm away from it. So to stick with the 1mm safety gap rule the o/p gear can only be spaced in by 1mm.
After hours of pondering I think I have worked out the failure mechanism:
In all the other gears (and 4th when new) there is no side force from the dogs on the selectors and the grease being forced into the gaps by the motion of the parts keeps all the bits in mid position.
But, 4th gear is different from all the others. The pull of the chains creates a small sliding action on the dogs.......the ball race is only a simple one, and the final drive sprocket is pretty overhung so it must rock as it rotates. Slowly this polishes away the contact patches and little by little the angle to create side thrust appears. Once there is side load the grease films become asymetric and the dog engagement is reduced. All the bits then wear faster and so on.
I think I'm getting a look at another box this week so there'll be more data.....probably conflicting........and my old Panther handbook says the level of the engine grade oil added to the box is correct when it is "just below the mainshaft". You'd need to fit a nappy on it.
Oh, and please stop worrying Matty
And yes Bill Thomas, all the circlips and spacers were present and correct.
Thanks
Tatty

 

[Matty]

Am interested in your findings Tatty 500, because I may have to have my box apart again some time soon.
Way back in 1957 my box "blew up" because a tooth came off one of the gears and jammed between two mating gears, bending the mainshaft, cracking the casing and breaking most of the "lip" off behind the output shaft bearing. Being very young and poor at the time I straightened out the mainshaft on a fly press, replaced the damaged gears and got the welder at work to mend the aluminium casing as best he could.

This is basically how it has remained for 60,000 miles until a couple of years ago when it seemed to become more noisy and also leak a lot of grease from behind the output bearing onto the chain. I stripped the box and replaced the layshaft (which I found had also been bent) layshaft bearings, third gears etc.

I also found that the output bearing was moving a little in the gearbox housing which was the cause of the grease leak, though what lip was left on the inside was OK to stop the bearing going in too far. The only way I could fix it was to put it in with a new oil seal assembly from Dragonfly and some bearing Locktight on the bearing, the shims and circlip.

This has worked fine for about 6,000 miles but I notice now that there is a leak of grease which has not been cured as I hoped by using Thixatropic grease which I put in last week when I changed the gear which mates with the output gear, in a fairly successful attempt to cure a whine in 3rd!!

I did not remove the gearbox to do this, though may do so in the Winter - but I really need another gearbox case or some serious welding and machining to do the job properly.

Will measure all my clearances, meshes etc then, because the box seems to work fine as it is, except for the small grease leak which is for some reason not getting on to the chain but is running down the outside of the breather pipe!!

Anybody got a Burman gearbox case to spare please or can advise how to "shim up "the output shaft bearing?

Matty

 

[Vic Youel]

I have placed the remains of the damaged parts from my gearbox for sale on this forum with the express condition that they are loaned to Tatty500 for analysis and therefore diagnosis and feedback to this forum in the interests of Burman users.

See http://www.vincentownersclub.co.uk/classifieds/showproduct.php?product=27 for advert..... if you win this you are under a public obligation.

Vic

 

[Gordon North]

Burman BAP gear selector meltdown

Post war Panther heavyweights (500 & 600cc) used the BAP box too. I managed to destroy the selector on my Panther twice. On each occasion the selector fork seized (brazed?) itself into the groove, but this was operator error. The gears in my box were pretty worn and occasionally it would slip out of fourth gear; to prevent this I kept my foot pressed down on the gear lever until I needed to change down. Twice I destroyed a selector which tore into two major pieces (and a few other minor bits) without locking anything up and enabling a short ride back to base. Fitting a different make of box in a Panther is not an easy proposition but if the gears aren't too worn the box seems bullet proof.

Certainly new selectors are available from the POC (members only) but Draganfly can also supply, I believe but if the VOC can supply, please disregard the last remarks!

Cheers.

Gordon North

At the risk of sounding silly, have you considered the option of fitting a non-Burman gearbox and clutch?
Provided you can tolerate the non-standard appearance, there seem to be many advantages which make the Comet a more pleasant bike to ride, and more reliable.
Plenty of advice written by others about which items to use and how to do it.
Can't be more expensive than sticking with Burman, which was only chosen by Vincents as original equipment because it was cheap in those days.

 

[tatty500]

Vic and everyone,
I'd love to see some more bits.
Anyone going to the AGM could pass tham on to Big Ed Grew or George Spence to get them to me.
Iv'e just been given a selector, output gear and sliding gear from another box. The output gear is different, being 0.25mm longer to the bearing shoulder.
Now,with these pices fitted there is 2.75mm engagement. For comparison the other gears have 2.95, 3.97 and 3.79mm dog engagement(1st to 3rd).
I still, possibly stupidly, expect more in top, perhaps 3.5mm which is the average of the layshaft engagements.
Attached are some pictures of the dogs to show the tiny engagement wear patterns. It is more difficult to spot the longer 3rd gear pattern, but it can be seen.

Tatty