Hi Matt, The thickness of the jaw should be .227"-.230".
The clearance between the jaw and the groove in the selector dog should be .002"-.009".
Groove width of the selector dog should be .232"-.236".
These are the dimensions with everything in tolerance.
However,lots of gearboxes are running beyond these limits. If you are slightly over these limits the effect is to lose some of the movement of the selector dogs but this is not critical as long as the wear is minimal.
Remove any burrs from the faces of the forks with a fine oilstone. Next examine the selector dogs...if the grooves are badly worn or show signs of heat discolouration and/or the sides of the grooves have a rough finish they should be replaced.
Wear on these parts is usually caused by incorrect shimming of the shafts or incorrect setting of the selector mechanism on assembly, generally they last well.
You will need to check the end float of all three shafts...after shimming (if required) there should be .005" end float. Really it is worth buying a dial gauge to do the job properly...these days you can pick up a Chinese dial gauge for very little money which will be quite adequate...try Machine Mart or a similar supplier.
Note when you tighten the two nuts that retain the kickstart ratchet assembly the first one on the shaft should only be done up finger tight. After tightening the second nut, and before bending over the tab washer check that you still have the correct amount of end float on the mainshaft.....Ian
email (option): julie@wright52.plus.com
That's it Matt....though there is no reason why the bush on the outer end of the layshaft couldn't have a hole drilled through the end to give access for a (plunger) clock gauge or depth micrometer....Ian
email (option): julie@wright52.plus.com
Ooh. Clever.
email (option): spacemonkeym@gmail.com
Are the pegs in the selector shaft a press fit or are they screwed in?
In trying to track down some replacement selector forks I've come across two types, this odd fork has a wider slot to take a larger diameter peg about 3/8" presumably to reduce the wear on the peg.
It would be a simple solution to change the pegs and the forks to suit but this fork is narrower where it runs in the selector dog by about 1.5mm so would need to change the dogs as well.
Would it be worth adding weld to the forks and remachining? I'm aware that they would need heat treating again. Hmmm sounds like too much work I think I'll keep looking.
email (option): spacemonkeym@gmail.com
Hi Matt, The pegs in the selector fork shafts are pressed in. They can be removed with a press or a hammer and punch. Selector assemblies with the larger diameter pins and tracks are from later gearboxes than the WD ones but can be 'retro fitted'.
Having wider tracks increased the radii at the 'corners' of the grooves and this gave a smoother and easier gear change action.
The dog tooth form was also altered in these later boxes on both the selector dogs and their matching gears. Here two slight angles were machined onto the top face of the dog tooth...again to help improve engagement. Earlier types have a completely flat top face. These parts can also be retro fitted to the earlier WD box and are worth looking out for.
All plunger M and B Series boxes,49 onwards rigid B Series boxes and rigid/tele M Series boxes have these parts as standard.
Selector dog grooves and selector fork arm widths are the same in both WD and later types. If you have narrower ones these may well be from the 46-48 (B1) B Series gearbox and they will not be suitable for fitment to your box. One possible exception to that statement is the very early (early 48)versions of the later box..they are distinguishable by the fact they have no oil seal fitted in the main gearbox casing. They MIGHT have narrower arms and dog grooves but I would have to look at one, or a parts list, to determine that fact.
Gears and selector dogs with the later dog tooth form are directly interchangeable with the WD types and can be 'mixed' without detriment if, for instance, you were to change just the selector dogs...Ian
email (option): julie@wright52.plus.com
Thanks for that info Ian. The Box is certainly of the later variety but it appears that someone in the past has put in the earlier selectors and dogs as the teeth on the dogs are flat while the gears have the angles on the dog teeth.
The forks with the narrow arms I found at a local bike shop but I didn't buy them.
Another question. Was it common for BSA to stamp the outside edge of the inner gearbox cover?
email (option): spacemonkeym@gmail.com
Hi Matt...BSA themselves didn't stamp the gearbox cases. On WD types stamps were sometimes applied by Army workshops...I imagine to aid identification of parts, but this often took the form of just asingle number or letter... On later boxes I haven't seen any markings. The 'M' in your picture has been done using an old 'seriphed' style stamp, whereas the '20' appears to have been done using a more modern font style, Also this is a later type box so I would imagine this is a more modern addition....Ian
email (option): julie@wright52.plus.com
Ian.
I'm about to assemble this spare gearbox, I have some new and good used selector components to go into it but on examining the exploded view of the gearbox, I am missing the oil flinger. p/n 24-4235, item 91, from the mainshaft of both this spare box and the one box that was previously in the bike. Will the absence of this part affect the end foat(I suspect it will) if so what is its thickness?
http://www.draganfly.co.uk/images/shop/b3/19/BM_gearbox.gif
Lastly, the 18T gearbox sprocket that I'm using from the old box has an oil groove machined into it as the gearbox shell had no provision for a seal, I assume that the sprockets for the shells with the rubber seal have no oil groove.
Is this correct?
email (option): spacemonkeym@gmail.com
Hi Matt, You are correct...sprockets for the later boxes were plain on the back for the oil seal to run on. I am unsure if you can get that type with 18 teeth..the standard sprocket for most BSA boxes is 19T.
Either will run OK on the M20 though.
I'll have to measure the oil flinger plate...you will need to fit one, or at least a washer of the equivalent thickness, to maintain correct end float.
I'll be going to the workshop later...I'll post the dimension then....Ian
email (option): julie@wright52.plus.com
Hi Matt, The oil flinger is 1.820" OD and .031" thick...Ian
email (option): julie@wright52.plus.com
I installed this gearbox back in January and put about 200 miles on it through Feb and March. Unfortunately it would jump out of 4th gear sometimes when coasting and 4th gear selection never felt positive even after much adjustment of the control rod. On dismantling the box again I found my previouly in spec 4th gear selector fork and dog were badly worn. I have replaced these items again and I am in the process of bolting it up again, all shafts are within .005"-.010" end float however there seems to be a large gap between the 4th gear and it's selector dog.
A pic of 3rd gear engagement.
4th gear engagement, note gap.
when I assembled this box last time I placed a shim behind the 4th gear pinion to move it closer to the selector dog but I suspect that this is what caused the fork and dog to wear so quickly. Without the shim I have this gap and a greater possibility of it jumping out of gear.
Any thoughts?
email (option): spacemonkeym@gmail.com
Hi Matt..3rd and top gear are on the mainshaft and 1st and 2nd on the layshaft. When top gear is engaged by the selector dog (ie it's in top gear) the layshaft selector dog sits between 1st and 2nd gears, but is not engaged with either one.
The gap between the selector dogs face and 2nd gear and the gap between the selector dogs face and 1st gear should be equal. This can be adjusted when the box is assembled by using the adjustable tie rod that connects the footchange mechanism to the selector fork shaft operating quadrant.
Making an adjustment here (if required) will also alter the movement of the mainshaft dog so can have some effect on top gear engagement...Ian
Hi Matt..you are right, it is difficult to measure the gap between first gear and the selector dog.
The way I do it is to measure the gap between first and second gears with the layshaft out of the box.
Then measure the thickness of the selector dog.
Divide the difference between those two figures in half and you have the gap each side.
You then only need to measure the side you can get at...Ian
Thanks Ian.
I spent some time on the box again yesterday and think I've found the problem. When I previously shimmed the selector shaft I had close to 040" endfloat so shimmed both ends of the shaft to centralise it, this was my mistake. I have removed the drive side shim and placed one of the same thickness under the selector side bush, this leaves me with good engagement on all gears with near equal gaps between the dogs when the control rod is set. Hopefully put the box back in the bike tomorrow.
email (option): spacemonkeym@gmail.com
Hi Matt, Glad to hear you are making progress. Once you get your head around how everything operates it gets a bit clearer...These are difficult gearboxes to get right compared to many and I am still never 100% sure I will get the result I want first time.
However, if you get close but not quite there, you get a feel for what small adjustment may be required.
Let us know how it goes...Ian