Hi, l just wanted to know if the fibre gear on a BTH magneto on a twin should have much backlash when cold, as mine has hardly any. ( I note that the Vincent instruction sheets say to have no less than 8 thou backlash for the original ATD) My problem is the breather pinion with which it meshes overheated and welded itself on to it's spindle with the alloy from the steady plate. Consequently the whole lot was turning in the case with the tab washer tearing a big hole in the steady plate . I'm trying to find if the cause is the expansion of the fibre gear.
E: Engine BTH fibre pinion.
- Thread starter gmcculloch
- Start date
The first thing I'd sort is the reason for the breather pinion getting hot. This should not happen. You can selectively size the breather pinion by pressing a smaller sized half-time pinion, ET49, on to a new sleeve. While you're at it, give the slot knife edges and widen it to 3/8". Are you sure you're talking about an original "Tufnol" pinion? They rarely grow, just strip teeth if used for too long. There were some nasty white nylon ones about some years back which were given to dynamic diametrical divergence; the new "Acetal" ones are O.K. I believe. If unsure, post a photograph.
Hi, thank you for your input. The breather pinion has around 7 thou backlash against the cam wheel. ( 5 to 10 thou is permitted) so cannot fit a smaller one. What I'm asking is how much does the BTH fibre pinion expand when hot. Because l have next to no backlash between it and the breather pinion when cold, if it expands as the engine heats up it will become too tight putting pressure on the breather causing friction, and I was thinking this might be the cause of my problem. The pinion is a newish BTH supplied one.
Most likely the breather pinion had little in and out end float.....if the breather is an alloy tube type, then this happens readily. As the crank case heats up the gears should move apart due to expansion.......None of the gears should be tight or have tight spots.
Thanks Greg, I've only just taken the bike over. I'll have a look at that when I put it back together.