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Gearbox shim confusion???

Hello from Southern California,

I am working on a 1950 M21 gearbox and am running in to problems with regards to shims. This gearbox has never been apart since it left the BSA factory and bike was last on the road in 1961 (have the log book). Bike has only 3,000 miles from new but very rusty. Upon taking it apart the 3rd/4th gear shifting fork and dog clutch it engaged with were in very bad shape. Blue in color and lots of metal worn away. All items as shown in the parts list were installed correctly. The main shaft had the oil slinger, bearing spacer and other washer between spacer and bearing but no main shaft shims. The high gear also had the correct washers on either side of bearing. Shifting fork shaft had no shims behind bushes. And no shims on lay shaft either. I read through the forum to get as much info as possible and still am confused. I replaced the entire shifting fork - shaft assembly with a new one and both main shaft and lay shaft dog clutches. Main shaft ball bearings also replaced. Lay shaft bushings are original as they were not worn at all. It appears the correct end float is supposed to be .005" on all shafts. With a .015" dry gasket I measure: .027" on main shaft, .015" on lay shaft and .030" on shifting shaft. Upon inspection it appears in 3rd gear that the clutch dog was not able to move far enough to the timing side to fully engage the main shaft gear (fork would not move any further) and was putting a big side load on shifting fork. So added .020" of shims and that now seems to be OK but just barely. I would like another .010" if possible but there is not enough room for another .010" shim. Now for the lay shaft, the parts manual shows no shims at all for it? So it appears this shaft was never shimmed by BSA? Next is the shifting shaft, if I shim it at all problems show up. Without the .030" of end float in 4th gear the shifting fork does not move over enough to allow the dog clutch to engage with 4th gear fully. Shimming it also does not allow it to move over enough to fully engage the dog clutch for 3rd gear either. So it appears that it needs lots of end float to work properly or I will burn up another shifting fork.

So the condensed version of this is, I want to move the main shaft another .010" to the drive side as the dog clutch just barely engages all the way in 3rd but I don't have enough room left. The parts book shows no shims for the lay shaft at all so it appears BSA did not care about it's end float? I can't shim the shifting shaft or I will destroy the shifting fork. The gear box seems to shift fine on the bench and am wondering why it won't work if all shafts are shimmed to specification? I don't want to have to take the gearbox apart again. I want to ride the bike without problems!

Also, one thing I noticed. If you have a gearbox that has never been touched by a bodger who beat the ball bearing out cold. I was told the bearings in the gearbox were standard clearance. So I installed new bearings this summer and they felt perfect. Now that it is winter and have time to resume the project at temperatures below freezing the bearings were locked up tight (work shop has no heat). I replaced them with C3 clearance and tested them down to -10 degrees F (-23 degrees C)and all was still OK. Had I finished the gearbox this summer I would not have know what bearing problems lay ahead!

Kind Regards,
Bruce

Re: Gearbox shim confusion???

Bruce
I want to move the main shaft another .010" to the drive side as the dog clutch just barely engages all the way in 3rd but I don't have enough room left.


Hello Bruce, Why doesn't it have room? , does the mainshaft hit the sleevegear? That would be strange as in my experience there is usually lots a endplay between these 2 parts. Are you sure the large bearing is fully in the housing?

Adjusting endplay on the mainshaft is not needed in my opinion as the mainshaft is positioned against the small bearing, the play between the mainshaft and sleevegear can be used to adjust the gears on the shaft to the gears on the layshaft by added shims on the timingside.

Strange that a gearbox with so few miles needs so much shimming. Sometimes in unmolested boxes, leaving the gasket out and using silicon instead takes up some wear and no shimming is needed at all.

By the way, the layshaft can be shimmed behind the bronze bushes so the bush has to be pushed out for this.

Re: Gearbox shim confusion???

['Adjusting endplay on the mainshaft is not needed in my opinion as the mainshaft is positioned against the small bearing, the play between the mainshaft and sleeve gear can be used to adjust the gears on the shaft to the gears on the layshaft by added shims on the timing side..']

If the two nuts on the kickstart ratchet are used to position the mainshaft (and set the end float), the end float is adjusted off the bearing on the kickstart end and the shaft as a consequence is necessarily pulled towards the kickstart end..

If the shaft is shimmed between the oil flinger and the thrust washer INSIDE the kickstart end bearing it is moved towards the main bearing on the drive side as it is being shimmed away from the bearing...

Using the first method would indeed negate the need for shims...

However, part number 66-3071 (.005") and part number 66-3072 (.010") are listed expressly for the purpose of shimming the mainshaft..

This indicates that the correct method is to shim the shaft inside the box thus moving it towards the main bearing to achieve the correct degree of end float and shaft positioning...Then carefully tighten the kickstart ratchet nut and locknut etc. so as not to compromise the end float setting previously achieved....Ian

email (option): ian@wright52.plus.com

Re: Gearbox shim confusion???

Yes, shims on the mainshaft may be needed, but in my opinion they are to be used for positioning the gears on the mainshaft to the gears on the layshaft and to the 3/4th gear dogclutch.

Adjusting minimal endplay on the mainshaft is not relevant as the mainshaft is pulled up to the small bearing by the nuts on the end.

At least that is how I see the construction, but I remember we don't share this opinion.

Re: Gearbox shim confusion???

OK, to make it clear what is happening,

The gearbox as found had the 3rd/4th gear shifting fork and dog clutch burned up. Put in all new parts as required and paper gasket. No shims have been added anywhere. Gear box is now as it left BSA in 1950.

Shift gearbox in to 1st gear, neutral, 2nd gear and all seems OK. Shift in to 3rd gear and you notice the gear dogs have only engaged about 90%. Either the shifting fork needs to move to the right or 3rd gear to the left. OK so lets shim the gear box so all is to specification. Have added .020" to main shaft to move 3rd gear this amount to the left and hope it's enough once nut is tightened bringing shaft snug against right side bearing. Shifting shaft is now also now shimmed. Now shift back in to 3rd and the problem is still there and now 4th has the same problem. Riding the bike like this, the shifting fork will burn up again as it will have a huge side load as the gear dogs try to fully engage. So take out the shifting shaft shims but leave the shims on main shaft and try again. Shift back in to 3rd and the gear dogs have just barely fully engaged. I would like to move main shaft a little more to the left to be safe but am out of room for more shims. But luckily 4th gear is now OK with shift shaft shims gone.

My conclusion is something was machined wrong at BSA and gearbox was bad from the day it was made. It was probably burning up the shift fork as the BSA test rider took it out for it's first ride. The fact it needed main shaft shims and had none shows the poor quality of work done at BSA. I am guessing the gearbox case is made wrong but not having another to take measurements from makes this only a guessing game. So for now I am going to leave the shifting shaft with lots of end float. I have installed a magnet in the drain plug and will ride it and see what shows up.

By the way the bike is from the London area. It lived from 1950 to 1958 in Crayford and 1958 to 1961 in Catford. It was taken in trade for a new BSA I am told then. Probably because the gearbox no longer worked.

Regards,
Bruce

Re: Gearbox shim confusion???

Hi Bruce,

There are also shims for the sleeve gear that haven't been mentioned yet, that go between the bearing's retaining washer and the sleeve gear to move the gear towards the third gear to improve forth gear engagement.
I'd be hesitant to use the box with so much end float on the selector shaft.

Re: Gearbox shim confusion???

Bruce
I would like to move main shaft a little more to the left to be safe but am out of room for more shims. But luckily 4th gear is now OK with shift shaft shims gone.


Difficult without seeing, but:

Why are you out of room for more shims on the mainshaft, is the mainshaft touching the sleevegear?

I understand from your description that the fork will "burn up" against the 4th (sleeve) gear, that makes me think that the sleevegear is not deep enough into the housing...

Btw. 90% engagement doesn't sound bad to me, the shape of the dogs will hold the gear I would say.

Re: Gearbox shim confusion???

Yes, both sleeve gear washers are installed and bearing is fully installed in case. Heated up case and it dropped right in and you can see there is no gap when looking in from outside of case from sprocket side. Were it not fully seated I would have the opposite problem of fork trying to push dog clutch over to the sleeve gear even though the teeth of dogs were already fully engaged. As for adding more shims to main shaft, with .020" of shims installed I only have .007" of end float left. I tried .030" of shims initially and it definitely does not have the room as main shaft was pushed up hard against sleeve gear. As for 90% dog clutch engagement being OK, I don't agree. As the dogs are undercut they will try to pull themselves together under load. If the fork is stopping this from happening it will burn up. I think the lack of main shaft shims is the reason the fork burned up initially when the bike was new. As for the shift shaft needing so much end float to work correctly, well?????

Other than the 3rd/4th gear fork and clutch being burned up, the gearbox looked new inside and no evidence of any abuse or running low on oil. I have been looking at this for a week and trying to make the gearbox as perfect as possible as this bike will ridden. Road side repairs are not something I want to face.

Other than the primary and clutch being touched, there is no evidence of the motor or gearbox ever being opened up from the time the bike left the factory.

Bruce

Re: Gearbox shim confusion???

Hi Bruce,

Just a thought, but are the fixed gears pressed fully home on the shafts?

email (option): glenn_mullan@postmaster.co.uk

Re: Gearbox shim confusion???

Hi Bruce, I assumed that burning up the forks came from pushing the dogclutches too hard against the gears, not realising that it can also occur by the pulling force of the undercut dogs, thanks for that!

Just looked inside two used boxes, they have lots of endplay on the layshaft and control shaft, yet they still go solidly through the gears.

Looks to me that this larger endplay has the advantage that the dogs have the opportunity to 'find' eachother without being obstructed by the limited forkaction.

So maybe minimal endplay is not that critical....

Re: Gearbox shim confusion???

As for the main shaft gear not being fully pressed all the way on, I have not checked that! Good idea. I will see what I find. Thanks.

Bruce

Re: Gearbox shim confusion???

Looks to me that this larger endplay has the advantage that the dogs have the opportunity to 'find' eachother without being obstructed by the limited forkaction.

So maybe minimal endplay is not that critical....

I'm investigating two gearboxes at the moment, each with burnt selector forks. One is a real problem, the other is fake due to the gearbox being assembled from someones rubbish :(

As with the gearboxes I manufacture I think the answer is to build them by calculation.
The random machining quality of the dog clutches (dog rings) enables you the change the throw buy swapping the faces.
Even the fork throw varies in position and offset.
Too little and endfloat will cause problems in the wrong places.

Mark

email (option): pes.sales@btconnect.com

Re: Gearbox shim confusion???

BSA M20 factory worksheet No. 608 states ['All shafts should have the minimum of end play']...The lightweight box that was fitted to the WB30 also operates in essentially the same way as the M20 box...

WB30 information states...['washers can be varied in thickness to adjust for correct end float of the shafts, which should be .005" for the mainshaft and layshaft and .003" for the selector shaft...']..

You can make of that what you will but it seems clear to me and I have always used these figures to build gearboxes, achieving correct operation in that condition...'Lost movement' caused by excessive clearances (either wear or setting) can have, in my experience, a detrimental effect on both the action of the box and the correct setting of the selector dogs on assembly and is therefore best avoided...

Another relevant point to check is that the washers either side of the main bearing, which have 'depressed' centers are fitted facing the right way....Ian

email (option): ian@wright52.plus.com

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