Hi Orabanda (plus anyone else with a BTH mag).
I think I've found the explaination to the problem I observed, mainly because I found an "Ignition Timing in mm Before TDC Calculator" on the net. The link is below if anyone is interested.
www.lambretta-images.com/archive/porttiming.php Eyeballing the mag pinion there is about 2mm movement at the pinion perimeter when the BTH mag is locked, I calcualte this equates to about 3 deg, which in turn equates to 6 deg at the crank pinion.
Using a dial guage, and after locking the mag and setting the timimg to 8.73mm (11/32") I observed the variation in piston height between magneto stops was 6.15mm to 9mm (2.85mm variation). In a perfect world the crank would of course be locked by the mag and not move at all.
According to that calculator using rod length 165mm, stroke 84 mm
34 deg btdc is 8.86mm
28 deg btdc is 6.10mm
======
2.76mm
So this more or less proves that the (observed) play in my new BTH mag locking/timing mechanism can result in the timing being up 3mm out.
The 3mm that really surprised me, that such a big piston height difference results from just 6 degrees crank rotation.
Anyway, I guess this is just something to be aware of if your (new style) BTH mag has any play in the stops like mine has. I'd still buy the BTH mag knowing this.
I'll just experiment with the timing once the bike is going, I'll just use the mags slotted mounting holes to make "fine" adjustments to the timing should the engine pink etc.