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Burman 4th Gear Selector Meltdown

tatty500

Well Known and Active Forum Website User
Non-VOC Member
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
 

tatty500

Well Known and Active Forum Website User
Non-VOC Member
Clevtrev
No, thats my point, it isn't just wear/age. The total wear only seems to account to a maximum decrease of engagement of 0.6mm or 24thou. It is the fact that this makes up half the original engagement that is the downfall. The engagement should have been more in the first place.
Tatty
 

tatty500

Well Known and Active Forum Website User
Non-VOC Member
Blogger,
Thanks.
This of course is the easy bit, for unless a new double gear is about 3mm longer from the selector groove to the 26 tooth end then the whole thing will happen again.
The selector cam moves the selector 6mm in both directions. When moving to 3rd this 6mm prodoces 4mm dog engagement so there is 2mm dog clearance when not in 3rd.
The 6mm movement to give 4th only produces the 1.2mm engagement, so there is 4.8mm dog clearance when in the other gears.
This must be wrong in my box.
Is it wrong in all?
So if another sliding gear is the same dimension I will make a spacer to move the output gear inboard by 2.5mm or so... or whatever I can achieve whilst maintaining some safety margin between the output gear and the fat shoulder of the selector shaft.

Regards,
Tatty
 

vince998

Well Known and Active Forum Website User
VOC Member
Blogger,
Thanks.
This of course is the easy bit, for unless a new double gear is about 3mm longer from the selector groove to the 26 tooth end then the whole thing will happen again.
The selector cam moves the selector 6mm in both directions. When moving to 3rd this 6mm prodoces 4mm dog engagement so there is 2mm dog clearance when not in 3rd.
The 6mm movement to give 4th only produces the 1.2mm engagement, so there is 4.8mm dog clearance when in the other gears.
This must be wrong in my box.
Is it wrong in all?
So if another sliding gear is the same dimension I will make a spacer to move the output gear inboard by 2.5mm or so... or whatever I can achieve whilst maintaining some safety margin between the output gear and the fat shoulder of the selector shaft.

Regards,
Tatty

Hi Tatty,
Sounds like a plan.
On assembly, do a dry run first with plasticene on the contact patches to make sure clearance is still present.
I did the same on my twin gearbox but the problem was with first gear which jumped out at the slightest whiff of acceleration
The witness marks on the engagement dogs showed that first had never been properly engaged :)
 

Black Flash

Well Known and Active Forum Website User
VOC Member
if I understand you right, why not try a shim between the sleeve output gear and the inner face of the output bearing. this would seem a logical solution to me. we use shims all over or bikes due to the manufacturing tolerances that were possible 60 years ago. if you are sure that is the problem, I would certainly go that way, but beware the last thing you want is your gearbox locking solid while doing 70 mph. double check twice and when you are sure about dimensions give it another 0.25 mm extra clearance ( width reduction of your spacer) Just to be on the safe side.

Bernd
 

Matty

Well Known and Active Forum Website User
VOC Member
Oh Dear you do cause me worry - I have just done a rebuild on my Burman because it was noisy in 3rd. I have not spotted any evidence of the problem you have had because I have never had a problem with it wanting to jump out of top, so did not check how far the 4th gear engaged. Have replaced recently the layshaft and its bronze bearings, both 3rd gears, and the small gear on the layshaft which mates with the output gear. This was a rather greasy job and I do not want to strip the box again to check the dog engagement if I can help it.
I did not remove the box from the bike to do the job and just pulled the gear assembly out.
WARNING when pushing the gears back in, the mainshaft and clutch moved back and the metal seal behind the clutch which keeps the oil in the chaincase dropped down behind the flange it sits on on the mainshaft and I had to then strip the primary side and clutch down to replace the disc on the mainshaft - a real pain. The trick would be to take off the access disc over the clutch and put it back on with a pad of cloth behind it before taking the gearbox apart to ensure that the mainshaft can not move back when the gear assembly is worked on.
I have refilled my box with Thixotropic grease, which a friend who worked as a lubricant expert at Ford Research said he has used with great sucess in Burman Boxes and claims it helps with the leaks as well.
I think I will take a chance and if mine ain't broke I won't try to fix it - I have only had the Comet 55years and it has only done 80,000 miles so I hope it has some life left in it yet (that goes for me as well!!)

Matty
 

Black Flash

Well Known and Active Forum Website User
VOC Member
hello Matty,
from what Tatty 500 wrote in his first post this problem seems to occur on very few boxes made at a certain time. If you never experienced the problem on your bike, then it is quite likely you do not have this problem.
so enjoy your quiet gearbox now and do not worry.

Bernd
 

john998

Well Known and Active Forum Website User
VOC Member
Hello,
Not any help in this case but there is a close parallel with twin box here. If you are unlucky and the tolerances go against you, the depth of engagement between
the constant mesh and double gear in top is marginal.
To over come this on my Rapide I pressed of G6 constant mesh pinion and machined the inner face of the gear so it would press on further, this made my little
Myford work for it's living. You then have to make up a spacer to go between the G6 and the bearing.
This has worked well for the last 20 years so you could consider it a success.
Regards John.
 
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