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G: Gearbox Burman Box Again


brian gains

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VOC Member
Reassembled box and on selection of 1st it can be graunchy once engaged and not all the time, which is rather frustrating as at least a consistent issue can be dealt with.
What does happen is that once 1st is engaged the drive can be a little snatchy and only in first,

Can the internal gear on the layshaft be installed accidentally the wrong way around?

How crucial is the spring compression on the selector layshaftshaft pawl?

Does the issue sound consistent with not having the 'O' indicator marks on the selector layshaft and toothed selector quadrant incorrectly aligned?
 

chankly bore

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Non-VOC Member
If you are getting all gears then the selector shaft is probably correctly aligned, (9 o'clock with gearbox vertical). All meshing gears add up to 54 teeth, so layshaft from kickstart end is 24 teeth, 34, sliding dog, 28 and 21 teeth. Note that the splines for the last mentioned 21 tooth gear are wider than the other end of the shaft. The sliding dog cannot mesh any other way because the fit of everything is determined by the 33 tooth output gear which is fitted first. Snatchy drive can also be caused by the outer clutch plate lifting beyond the drum slots. You may be able to cure this by fitting a thicker plain plate outermost, or fitting one less pair of plates until they bed in a bit. The spring pressure on the selector pawl is not critical.
 
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brian gains

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MartynG ; good to highlight those vid's, tremendously useful in orienting yourself around the box but needless to say on the assembly / dissembly vid' to take note teeth numbers are different on Comet application.
Chankly B'; so if all the gears are there all be it with an issue for 1st and allowing that nothing is bent internally , missing teeth etc it would be reasonable to turn to the clutch and I'm thinking that the amount of torque when pulling away in 1st is significant?.
I;ve run the clutch with no issues previously so am thinking have I assembled it incorrectly. There are two washers either side of the clutch rollers a thick and a thin with the thick one at the rear so there may be a possibility these have been misplaced. Al;so the clutch plates, I'm sure the last one in against the pressure plate was a plain one could this be incorrect? I think I'll review Richardson's then go back into the clutch to check things out.
 

chankly bore

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The first and last plates must be plain ones, but if the outermost is a thick one then there's less chance of the outermost friction plate going too far out and clearing the slots. It is a matter of transposing plates to achieve this state. Thicker washer innermost and rollers must be 6mm. wide not 1/4" of course. Check the plain plates for flatness and get the glaze off all plates. Some owners report that A.T.F. causes less "stiction" than engine grade oil in the primary. Lack of use is also a problem nowadays!
 

MartynG

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Interesting information about the effect of ATF on clutch plates


Summary - DON'T!
 

MartynG

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When assembling a burman BAP box as fitted to a Comet should the 3rd gear 24BA (which is free to revolve on the mainshaft) be pulled up hard and tight against the inner race of the small bearing by the kick start mech and the nut 70X on the other side of the inner cover?

I ask because I suspect that with my recent rebuild that the face of 24BA is JUST making contact with the inside of the inner cover when all is done up tight.

I have fitted a slightly thicker gasket (0.030") between the gearbox case and the inner cover but logic tells me that I should be looking at a shim twixt 24BA and the small bearing as this seems to be the only way to increase the clearance between 24BA and the inside of the inner cover??

Surprise is that the std METRIC wheel bearing shims are a great fit if needed.

Ta Martyn
Burman Pic.jpg
 

greg brillus

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Third gear should not be pulled hard up against the bearing, the input mianshaft should have some end float when all assembled, regardless of the thickness of the inner cover gasket used. Yes the wheel axle shims work ok in the area behind the starter ratchet.
 

chankly bore

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Non-VOC Member
Metric wheel bearing shims are 20 mm. i.d. as is 6304 bearing on mainshaft. I've not tried standard wheel bearing shims, (nominally 3/4" i.d.), but they may well be a little tight. Suck it and see!
 

brian gains

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I have end float in the main shaft so 3rd 24T must be free to revolve freely. However on the last rebuild the gear cam shaft did not have the 0.002" end float and did not revolve so I fitted a 0.030" for this reason where as when I had stripped down initiallt there was no inner case gasket at all.

However it should be noted that inner cam steel bush had worn considerably and the PO had shimmed out with hardened? washers and no doubt also omitted the gasket to take up end play. This all being negated by the newly machined inner steel bush, it is worth remarking that there is notable hydraulic damping when the gear cam shaft is pushed into the steel bush and this has to be compressed to bleed out trapped air.

Reading around the issue and taking advice here I shall check the plain plates for flatness, all measure at approx' 0.06" thickness. The fibre plates are all approx 0.17" thick and I shall check for flatness, scrub up on coarse sandpaper and clean with petrol or acetone. If this doesn't cure the issue I see a big spend and m/c off the road for foreseeable future 8-(
 

MartynG

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VOC Member
I have end float in the main shaft so 3rd 24T must be free to revolve freely. However on the last rebuild the gear cam shaft did not have the 0.002" end float and did not revolve so I fitted a 0.030" for this reason where as when I had stripped down initiallt there was no inner case gasket at all.

However it should be noted that inner cam steel bush had worn considerably and the PO had shimmed out with hardened? washers and no doubt also omitted the gasket to take up end play. This all being negated by the newly machined inner steel bush, it is worth remarking that there is notable hydraulic damping when the gear cam shaft is pushed into the steel bush and this has to be compressed to bleed out trapped air.

Reading around the issue and taking advice here I shall check the plain plates for flatness, all measure at approx' 0.06" thickness. The fibre plates are all approx 0.17" thick and I shall check for flatness, scrub up on coarse sandpaper and clean with petrol or acetone. If this doesn't cure the issue I see a big spend and m/c off the road for foreseeable future 8-(
If perchance the there has ever been ATF in the primary case then you will most likely need to fit new friction plates as ATF contaminates the friction plates causing stiction that can never be fully resolved.

When I assembled my cases I thought I had it all wrong, but after a few minutes the camshaft overcame the hydraulic lock you mentioned and all was then well.

When I look at the design around the mainshaft/third gear/ small bearing and k/s ratchet it seems to me that if you pull the k/s ratchet nut up tight there is no way there can be endfloat on the mainshaft. There is a shoulder on the mainshaft that the 3rd gear abuts then the rest of the parts I just listed are a slide fit on the mainshaft - ergo tighten the k/s ratchet nut and they all get clamped together (unless I have missed something).

So to get main shaft endfloat and thus a free revolving 3rd gear that nut needs to be NOT tight. To have it not tight, but secure I assume one needs to use either (or both) loctite or punch locks.

Could this explain why during the teardown I found that nut punch locked?
 

MartynG

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I just checked the end float on my main shaft and found it to be 0.007" which should be OK. So time to put all back together me thinks.

20191225_113654.jpg
 

vibrac

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So to get main shaft endfloat and thus a free revolving 3rd gear that nut needs to be NOT tight. To have it not tight, but secure I assume one needs to use either (or both) loctite or punch locks.

Could this explain why during the teardown I found that nut punch locked?
I understand the logic but I can't be!ive it is true, this thread has the word 'again' in the title for good reason and I don't think I have ever seen the loose nut theory before. Certainly I have done a few boxes and never found the need to leave it locked and loose, However it's MartinG speaking and he is not given to wild theories so experts after Turkey please consider.....
 

chankly bore

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Non-VOC Member
I could be wrong, but the nut should be tight. The sleeve PR50-39BA that the kickstart pinion runs on is critical here,too long and it will sandwich the bearing and mainshaft third gear together. A local owner has had to rig up a holder and cut .020" off a recently supplied item to cure this very drama.
 

MartynG

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Could it be that the assembly relies on the nut bottoming on the thread? In all this gear box caper I am a very new boy and am struggling to understand the correct assy procedure. Now if we had access to the works drawings or works assembly instructions (Burman) only then could we be certain.

The attachment from 1941 in the second last parra of the item suggests that the nut (70X) must be done up tight. Take that in concert with Mr Bore's comments (above) and may be it is all resolved by having the nut bottom out on the thread - thus making the length of the PR50-39BA the critical dimension and one that 'may' be varied in order to achieve the claimed desired end float of ~0.010" on the main shaft.

I wonder if there are Burman documents in the hands of the Spares Co that may provide some clarity here?

I hate leaving mysteries like this unresolved. It may work - but why?

Martyn
 

Attachments

greg brillus

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I don't think it is a big deal, but it can be if the inner cover gasket is left out......... It is quite typical of an assembly that is worn in many areas. The third gear mentioned must be able to spin freely on the main shaft on account of the different speeds it rotates at pending what gear you are in, not including top gear of course. If the gear was nipped up tight against the intermediate cover bearing, that is the bearing inner race, the gear and the shaft were all locked up together, the gearbox would fail rather quickly I would imagine.
 

MartynG

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VOC Member
I will confess to be a slow but persistent learner - I think I have finally understood how to control main shaft end float in a Burman BAP gearbox.

Up front be clear that the ONLY time that I think you can get a true measure of main shaft end float is with the mainshaft with the 3rd gear on it, fitted into the inner cover and the k/s assembly in place and tight on the other side of the inner cover.

On the mainshaft at the k/s end there is a threaded portion that nut 70X fits onto and there is a shoulder there - when tightening 70X it is (should) be tightened up against that shoulder. Starting at the outer k/s end of the mainshaft between the tightened 70X nut face and the outer face of the small bearing firstly there is the Driven Ratchet PR50-41BA and then the Ratchet Pinion Bush PR50-39BA (this bush is installed with its top hat flange against the small bearing inner race). Slip fit over the Pinion Bush is Ratchet Pinion PR50-38BA and the spring PR50-40BA. Essentially this assembly makes a spacer between the nut 70X and the top hat part of the Pinion Bush. Let's call this assembly the Float Control (ok I made that name up!)

Now on the other (inner) side of the small bearing is the 3rd gear that is a slip fit on the mainshaft, with (hopefully) end float between the inner race of the small bearing and the shoulder on the mainshaft. It is the stepped face of the 3rd gear that abuts the small bearing and the mainshaft shoulder on the other side of it that becomes the partner to the Float Control. So we are looking at an assembly that can slide on the mainshaft between the mainshaft shoulder that abuts the 3rd gear and the nut 70X.

I have not been able to locate any reliable Burman works documentation that tells what the main shaft end float should be however the suggestions received suggest that around 0.010" end float should be adequate. Richardson states that main shaft end float should not exceed 0.030".

So if we are to control the end float of the mainshaft then it can only be done in one of two ways.

If there is excessive end float to reduce it we can fit a shim between the Ratchet Pinion Bush and the Small Bearing. Standard Vincent metric wheel bearing shims available from the Spares Company are ideal for this use - Order H17M/set. Thanks to Greg Brillus for this insight.

If there is insufficient end float then as Mr Bore correctly suggested we can reduce the length of the Ratchet Pinion Bush.

M
 
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greg brillus

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VOC Member
Sounds like you are onto it M......One issue with these boxes and even the pre war version on the series "A" machines either twins or singles........that bearing that Martyn mentions is a problem in that it often comes loose in the housing, sometimes the outer part of the cir-clip groove is actually broken away, seen this several times and the usual fix is to drill and tap 3 or 4 holes, taped 3/16 Whit or similar, and short machine screws with a washer under each to "clamp" the bearing in place, this on the outer side of the bearing housing........Now this can only happen if someone has literally bashed the clutch end of the main shaft with a hammer, for whatever reason........So the shaft is held in place by that bearing and its fit in the housing only. Perhaps what is felt to be end float on the shaft is actually the bearing loose in its housing and moving left to right. In any case, the assembly is quite fragile, and needs to be treated with a little care. I have to say, these are not one of my favorite boxes, the pre war variety can be very expensive to overhaul on account of the ware in particular to the outer kick start cover and its workings.
 

brian gains

Well Known and Active Website User
VOC Member
for my 2 cents' the kick start pawl nut is tightened up fully and end float checked, in my instance the end float is OK. Have read elsewhere, here or Ariel forum that if the end float is too great the retaining nut can be
counter bored on the mating face so it pulls on to the main shaft further\ make sense?
 

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