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Burman 4th Gear Selector Meltdown


BigEd

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VOC Member
VOC Forum Moderator
GL-4 Specification gear oil seems to be the safe one to use with bronze bushes as it does not have the sulph.ur additive
 

brian gains

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Non-VOC Member
terrifically useful thread having had the gearbox in / out and apart at least 1/2 dozen times this summer and having to go in again after 4th wouldn't engage again yesterday.
After all replacement parts there are only two options left, spacer behind main drive gear and on starter end of selector cam shaft/ At present the end play is 0.9mm which is excessive and by working in and out by hand seems to be the difference between gear engaging or not.
My question is what is the minimum end play that should be aimed for?. I thought I had seen this figure somewhere 5 thou'?, but can't find.
 

Bill Thomas

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VOC Member
Have you done the work, Without taking the Box out Brian, Only years ago, I lost top gear, On the road,
And found the case had split and the big bearing next to the drive sprocket had moved out a nats.
I stripped it out and had the end of the box alloy welded,
He welded it with the circlip in, So as not to lose the groove, So I had to replace that,
Been OK since. Cheers Bill.
 

tatty500

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Non-VOC Member
Brian,
The figure is in Richardson. Minimum camshaft end float 2 thou. But if you are using gaskets you need to check after it gets compressed....when its too late!
I presume you have checked the selector face and its peg that fits in the cam groove.
Happy hunting
Tatty
 

brian gains

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Non-VOC Member
Thanks, I was sure I had seen a figure somewhere. I'm not running an inner gasket although there is a part # specified I have read that there are those that do and those that don't, I shall be in the latter camp.
Yes, checked the selector and did replace the pegs despite the action still being smooth and minimal wear apparent.
I replaced the output gear just because the dog teeth looked to have minimal wear, however the teeth on the replacement seem marginally more substantial.
The inner selector cam bush was worn and had two 'split' shims in place by PO, had replacement bush made up in silver steel.
End play on mainshaft seems within tolerance at 0.0245" taken from Aeriel owners site.
Will take the inner cover out this arvo', if the outer cam selector shaft bush appears worn and a replacement will bring it back into tolerance I shall go that route otherwise I'll have some shims made up. I'll deceide on that when I'm satisfied that any fitted shims will not be torn up by snagging on the shaft.
 

tatty500

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Non-VOC Member
Brian,
Like Bill says, there must be something else to blame. The cam moves the double gear into the final drive gear by about 2.5mm. Even with 1mm (40thou) cam end float there should still be engagement with your new parts.

Check that the distance from the joint face of the shell to the face of the output gear down inside the box is not more than 110mm.

The KS end cam bush flange is 6mm. It is a difficult place to shim because of the undercut on the cam shaft. If the bush is easy to remove put the shim/spacer on the bush(think there's an undercut in this corner too). This can keep it away from 3rd gear's teeth better.

Tatty
 

brian gains

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Non-VOC Member
points taken and the suggestion that if a shim should be needed place it between the outer cam bush flange and the inner cover is a good one.
Having removed the inner cover it is apparent that the outer cam bush has had a step 'machined' into the flanged end by the action of the gear change cam shaft, this would account for the excessive end play so as you (tatty) say I will need to take into account the gasket thickness. Having run the box for thousands of miles with no gasket from the PO's rebuild does not mean that this was good practice.
Bill: I haven't pulled the box out this time but when I did at the start of the summer I found the drive sprocket retainining nut loose and the bearing / oil seal circlip not seated. Having replaced the circlip I am satisfied the bearing is in the correct position and did not notice any damage to the casing. What I did do was measure the depth of the casing from the sprocket end to the face of the mainshaft bearing in consideration of putting an up graded oil seal as per the Draganfly site suggestion but it appears that there is insufficient depth as they say can be the situation in some cases. Now considering the replacement gear change bush in the inner cover and whether to have one made up or buy standard replacement. I trust new parts aren't NOS as I am not impressed at all by the quality of the material of the existing part. Once again a consequence of Burman boxes being sourced on financial considerations and also quality materials being in short supply?
 

brian gains

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Non-VOC Member
"Check that the distance from the joint face of the shell to the face of the output gear down inside the box is not more than 110mm."
I make the measurement 111.30mm which makes me a consider that the shim behind the output gear is necessary. What I don't understand though is where the wear has suddenly occurred, the bearing does not appear to have moved and nothing is apparent on the gear despite this having been replaced on an earlier strip down during the summer.
 

Bill Thomas

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VOC Member
Brian, When I did my old box, 20+ years ago, It needed a big f/drive bearing,
And I was supplied with a " Sealed Bearing ", Loads of Money !!.
I have just rebuilt another box, That had been under the bench for 20 ish years, And it was locked up by old grease, I could not remember much how it worked, It got my old brain going !.
I did not find it easy to work out endfloat etc. Cheers Bill.
 

tatty500

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Non-VOC Member
Brian,
It sounds like the output bearing is not quite in the right place.

Sadly when I compared several boxes all those years ago a lot of the measurements I made were comparative, not absolute. However, I think you need to check that that the output bearing is properly home directly against the end of its hole.......as you have also said there isn't enough depth left for a Draganfly seal kit.

The gap between the KS and drive ball races is 138mm inside, 170mm outside....if you do remove the output gear to measure.
Only other thing I noticed is that the radius between the 37.1mm bore and face of the inner race of the output bearing is sometimes large, whilst the 44.2mm shouldered down face on the output gear that rests on the bearing can be only just big enough diameter to give decent contact on the bearing.
From this bearing face on the gear to its big inside face was between 26.2 and 26.7mm.

That 110 dimension I gave was the biggest I found . Probably "109and a bit" is correct.

The cam bush has always been made of rubbish, and made worse for wear by the tapered bell mouth.

Oh, and on the outside. The outside face of the final drive sprocket is 34.5mm away from the end face of the casing top mounting boss where the big bolt goes through.

Regards Tatty
 

Matty

Well Known and Active Website User
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
 

Matty

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VOC Member
Hi
In 2011/2012 there were many pages on the forum about this problem.
I think the thread number is php?4089.
My comet gearbox was mechanically OK but because I was about to fix some leaks and change bearings etc. I wanted to optimise the clearances etc for the rebuild and a few of us (tatty 500, Vic Youle, ClevTrev, self etc) embarked on a long series of correspondence on the subject of Comet Burman gearboxes.
I will try to find a summery which is in my computer –which may be on the forum anyway and will attach it to this email.
There are other bits and pieces of text on this subject which may answer other questions.
I have now done many thousands of miles with my rebuilt box and other than an exercise to fix a leak of grease from round the gearlever it has been fine and ridden from Essex to the Isle of Man at least seven times since the rebuild.
I hope the others involved in the investigation have also had similar success.
As for end play, I just tried to get around 4mm of overlap on the top gear dogs with all the tolerances pushed one way - that way (with a small spacer behind the output gear) it does not want to jump out of top gear.
The spacer in theory will put the sprocket out of line a bit but it does not seem to have caused any problems.
MAKE SURE THE GEAR INDICATOR IS NOT TOUCHING THE LEVER WHEN IN TOP.
Also I seem to remember I had to make sure the output gear cleared the bronze selector by around 1mm with the spacer in and is of small enough diameter to fit inside the casing – it turned out to be quite a thin ring at its circumference probably only a couple of mm I seem to remember.
Best of luck.
Matty
 

brian gains

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Non-VOC Member
Tatty thanks for the dim's which have been added to my 'library'. I ordered a replacement bush from the spares co' so shall see if it is NOS.
The output bearing appeared to be seated and I have checked it three times now. My comment about measuring for a replacement output oil seal was prompted by the Draganfly suggested oil seal replacement, and at the risk of complicating matters the statement was made with regard to Arial Sq 4's which I understand used the same case with changes to internals only ie mainshaft length, output bearing size etc as per end users spec'. "... measuring the depth of the housing, if it is 11/16” our seal conversion will not fit, the depth of housing on the later gearboxes is 13/16” and will accommodate the seal conversion". My understanding is that Vincent used a bearing with more depth and therefore the suggested supplied oil seal mod would not work.
Matty; "gear indicator" , not found on C series Comet.
I shall comment again once the replacement bush and gasket have been fitted. Thanks again for useful comments and knowledge.
 

vibrac

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VOC Member
Does anyone know what Panthers use in that position inside? that box is another identikit Burman
 

Matty

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VOC Member
Does anyone know what Panthers use in that position inside? that box is another identikit Burman
I used the spacer with oil seal from dragonfly but a member of the local VMCC club who line bored my leyshaft bush housing which was damaged and made an oversized oilite bush to fit, also turned a very narrow groove for an O ring in the outside of the Draganfly spacer/oilseal to prevent oil seeping past the outside of the bearing and past the spacer.
I also put some Loctite round the outside of the output bearing which as you probably know is a very odd size 72mm x 1.5 inches internal x 17mm deep - I used a sealed bearing and left the seals in.
I bought it in 2011 from Peter Barker 01454 228066 barkers@engrs.wanadoo.co.uk for £47 then but of course they may not still be in business now.
There should be a gear indicator on the inner shaft of the gearchange but this can not be causing any trouble if there is not one !!
All the best
Matty
 

brian gains

Well Known and Active Website User
Non-VOC Member
Q: Spares co' can't supply gear cam shaft outer bush. The original seems to be turned up from very cheap mild steel. If I have replacement made up in silver steel is this likely to make the cam shaft the sacrificia£ component?.
 

Matty

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VOC Member
Hi
If you look on the old se of information on the Burman box I contributed to some years ago, you will see that I took up the small amount of end float on the camshaft by moving the very tight bush in the casing.
I was going to put a shim behind the bearing flange but the steel bearing was so tight in the casing I just heated the case and banged it over a suitable amount.
I do not think the material of the flanged bearing is very important because it should not be subject to much wear.
Matty
 

brian gains

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Non-VOC Member
the outer bearing in my case looked decidedly rough with a chip in the outer flange which I guess was done when tightening up at some point with one of the rollers not being seated correctly.
Also, in the flange face there is a step which I'm guessing has been worn by the gear cam shaft shoulder bearing against it without sufficient clearance, as Tatty suggests another good reason to use a gasket and then check.
The bush was a tight press fit to remove so it is reasonable to think that it could have been pressed out slightly to obtain correct clearance but then I would be thinking will it 'walk' back at some point.
Now just waiting for machinist to do his magic.
 

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