This is not my work, I believe it was the work of Dave Kath
some years ago. GStar Ron
may have contributed as well. I keep a copy of this in my book with all of my pre-unit printed information. Lots of good information here. I will make it a sticky thread since questions about pre-unit gearboxes come up from time to time.
Through the magic of cut & paste, here are the wise words from a number of years ago:
"I frequently come across problems of the incorrect assembly of the layshaft thrust washers in the swing arm type gearbox, of the type with the needle roller layshaft.
There are four large hardened washers used in the gearbox, three on the layshaft and one behind the ratchet pinion bush and it's possible to assemble the gearbox with some of these four in the wrong position. Unfortunately the BSA spares list illustration does not show clearly the correct assembly.
Working from the clutch end of the gearbox, the washers should be assembled on the layshaft in the following positions.
42-3078 ... 1.156" OD, 0.687" ID, o.108" thick
fits between main shell and layshaft 'B' gear.
67-3203 ... 1.094" OD, 0.750" ID, 0.093" thick
fits between the 'F'
layshaft 3rd and 'H' 1st gears.
42-3079 ... 1.250" OD, 0.750" ID, 0.113" thick
fitted between the 'H' layshaft 1st and the middle 'sandwich' case.
67-3161 ... 1.162" OD, 0.750" ID, 0.075" or 0.068" thick
fitted behind the kickstart ratchet pinion bushing.
Pay particular attention to the washer p/n 67-3203, which is also used on the gearboxes fitted with bronze layshaft bushes. The bore of the washer often only has a chamfer on one side only. If this is the case, the washer must go onto the layshaft with the chamfer against the spline to give clearance for the small radius on the shaft at the start of the spline.
Be very very careful of the improperly sold repop 'T' layshafts. There are a couple of subtle dimensional differences between the STD bushing layshafts and the STD T needle roller shafts but some suppliers want to sell a 'one part fits all'.
'Bushing' Box info
The 42-3019 'bushing' lay shaft has oil grooves on the end adjacent to the clutch.
bearing surface diameters:
clutch end ... ~ 0683"
speedo drive end ... 0.7475"
'B' gear, aka 'layshaft 4th' tooth width is 0.660"
'A' gear, aka mainshaft sleeve gear, tooth width is 0.560"
note: both are narrower than the 'T' box gears.
'T' Box info
The 42-3094 Timken needle bearing lay shaft used in 'T' Gold Star boxes is almost identical to the shaft in a bushing box, but there is Never a oil groove in the bearing ends. The bearing ends are also 2.5 thou larger than the bushing shaft. Measure.
bearing surface diameters:
clutch end ... 0.686"
speedo drive end ... 0.750"
'B' gear, aka 'lay shaft 4th gear' is 0.740" wide across the teeth.
'A' gear, aka 'mainshaft sleeve gear, is 0.603" across the teeth.
note both gears are wider than the 'bushing' box gears.
'T' box thrust washer info:
42-3078 - Fitted between main shell and the layshaft fixed 'B' gear.
1.156" OD, 0'687" ID, 0.108" thick
67-3203 - Fitted between the layshaft 3rd 'F' and layshaft 1st 'H' gears.
1.094" OD, 0.750" ID, 0.093" thick
42-3079 - Fitted between the layshaft 1st 'H' gear and the middle case.
1.250" OD, 0.750" ID, 0.113" thick
67-3161 - Fitted behind the kickstart ratchet pinion bushing.
1.1625" OD, 0.750" ID, 0.075" (or 0.068") thick
67-3203 is also used in bushing boxes. If this washer has only one chamfer on the ID, fit the chamfer against the layshaft splines to give clearance for the radius.
There were 4 different mainshafts used in these gearboxes.
RRT2 shaft 42-3131 for use with needle bearings in the 'A' sleeve gear
STD(T),RR(T),DAY(T) 67-330 shaft used in both the bushing and 'T' boxes
SC(T) shaft 67-3315 is slightly different than the STD shaft. This shaft can be used in a STD box, but not visa versa.
TRI(T), STB(T) shaft 67-3313 identical to the SC shaft except the 'G' 1st gear is made as part of the shaft vs pressed on.
There are 17 gear sets listed to make up the 7 different ratio transmissions both with and without the Timken bearings. Some gears are used in multiple applications, but some are unique to a particular box. And you can create your own ratios with a bit of 'mix and match' of the gear sets."