hi everyone, back in my teens i had a b40 wd frame with a c15 distrib model engine, i put a 30mm concentric and port and polished the head,i went every where on this and loved it, then the rear wheel hub chain sprocket was worn bad, at the time of no internet i could not find a new wd hub, so i had the sprocket machined of and a blank machined out and welded on to the hub, i think it was 44 tooth, well the ole girl would cruse at around 70, now 42 years later i have an original b40 wd , reading the posts on tuning a b40 and gear ratios is what i want to do,so to get to a standard road b40 spec, the rear wheel sprocket would need to come down to a 47 tooth from the original 52 wd tooth rear wheel sprocket , and a standard c15 gearbox cluster fitted to get somewhere like my last bike, would also like to put a higher cam in but they seem to be hard to find for a points model, wondered if anyone had ever done the hub sprocket change, any info on the above questions would be great, its time for me to get out on the roads again, thanks mandy.
I never really understood why the military fitted the gearbox they did to the B40WD - the first 3 gears are so low that they are almost unusable on the road and its not as though they were needed off-road because the bike has very little off-road capability. All the later bikes used by the military had normal gear boxes.
Swapping to a C15 or B40 gear cluster is the normal way to go. Not sure what the issue is with the rear sprocket because both the B40 and the B40WD had a 52T sprocket fitted? In terms of increasing the engine performance, I would have thought the first step would be fitting a higher compression piston. The sportier camshaft (40-0706) can be fitted and seem to be available at a price?
i have seen some sprockets for sale for a b40 at 47 teeth so i thought that was the standard road going, would you know why some sellers say a distributer hi cam will not fit a points version engine, i cant seem to understand why, i will try and find a hi comp piston and rings on that size,or is there any jap substitute that will fit. also does it mater if the gear cluster comes from a disributor model to a points. thanks.
The B40 and B40WD both used a 52T rear sprocket as standard - the B40 used a 19T engine sprocket as standard while the B40WD used an 18T engine sprocket as standard, although they can be changed.
The obvious way to go for a higher compression engine is to use a 8:75 CR piston from an SS90. The B40 used the same 79 mm bore as a B44 and so a high compression piston from a B44 might be a possibility.
The reason the earlier distributor model camshaft won't fit is that you can't bolt the later side points auto-advance mechanism to it! It also had a big nut on the timing end which, of course, the later camshaft did not!
The mainshaft and layshaft for the distributor model C15/B40 were completely different to the later side point models although some of the gear pinions were common. You will probably therefore need to start off with a later gearbox. As far as I can see, the mainshaft was the same as the B40WD but the layshaft was not the same part number and it may be a question of swapping over the layshaft pinions.