At last, here’s what I have discovered.
Since I think the lightweight CP box was the design that was beefed up to create the BAP, this was my starting point. In a CP the design rule seems to be that any moving parts that could touch are kept 1mm apart. Seems reasonable for a metric design.
Moving to the BAP box this same rule holds true everywhere except 4th gear. Here there are two discrepancies, both of which reduce the engagement to its pitiful amount.
1. The inboard face of the output gear is 2mm away from the shoulder on the camshaft. This gear could therefore be 1mm longer with the simple flat face, or maybe even 2mm if the conical face was used here as it is in the CP.
OR
2. The sliding gear is 3mm away from the face of the output gear. This gear could be 2mm longer from the selector groove to the 28 tooth end.
Sadly my particular shell is deeper than two others by 13 and 19 thou, and my output gear is shorter by 2 and 10 thou. Thus 4th gear engagement is reduced by 15 thou and 29 thou in total. These figures are small but the 29 thou = 0.73mm would have increase engagement from 1.3mm to 2mm, or a 50% improvement.
In all of the four output gears I have examined the worn surface of the internal dogs is at best half the 4.5mm dog length. At worst it is less that 1.5mm. (see earlier pictures)
And now I’m stuck, for, even if I buy new gears and selector, I will still be assembling a box with pathetic engagement. Is there any way I could get the sliding gear made 2mm longer?
If I make a 1mm spacer to fit between the output gear and bearing, it will need a wall thickness of 3mm so will it break up?
Recommendations.
1. It is the wear of outer bush in the driving gear and the surface of the mainshaft that creates the rubbing action on the 4th gear dogs. So lubricate the point where the shaft emerges and check for play whenever possible. If the mainshaft is ever removed pack the space between the bushes with fresh grease.
2. If any wear of the 4th gear selector face is noticed this should be the early warning. Investigate further.
3. It was evident that the wear products do not escape from the groove behind the dogs. This must mean the there is no movement of lubricant in this area. So, keep the box as full as you can without too much leakage. Try to use the left hand prop stand after a run.
4. Keep the camshaft as far to the drive side as possible by eliminating the wear at the steel bush at the k/s end.
5. If the revs rise in 4th and you trust your clutch not to slip, then maybe the selector is on the way out.
Stop using 4th and get home in 3rd.
Finally many thanks to Chris, Vic and Matty for the loan of parts
Tatty500