if the expense and effort was spent on repairing the engine back to original you'd be just as well off.
I won't defend BSA's design of the magneto mounting system, but it could be worse for a system that allows for interchanging Magdynos and various magnetos. That mounting system was used for the tens of thousands of singles that rolled off the assembly line at a fairly rapid rate, each with the timing having been set and the pinions reasonably meshed. The system that was good enough for BSA back then is good enough for me now.
The fore-aft position of the slots machined in the engine case ensures the pinion is set so it properly meshes with the mating gear and at the same time is centered in the oil seal, while the use of slots rather than holes allows the magneto to be moved left-right to compensate for later wear in the tapered shaft that results in the pinion moving closer to the magneto body. A strong strap then holds the magneto firmly in place so it can't move from its set position. All of this is great "in theory" (in the popular use of that term), but not quite perfect in practice. But, its defects can be dealt with.
At the risk of jinxing myself by writing the following, I haven't had the problems with this that others have had. I'm not saying others who have owned Gold Stars much longer than me are doing it wrong. I'm just saying I haven't (yet?) experienced problems despite quite a few thousand miles on the various Gold Stars. That's not to say I love fiddling with the mounting system each time I need to install a magneto, because I don't, but the system gets the job done.
Addressing the strap first, the way it comes around the back of the Magdyno on my BB does not allow the Magdyno to move backward even if the strap hadn't been properly tightened. Its metal-against-metal from where the strap is bolted to the case to where it bends around the base of the Magdyo so only if the strap somehow stretched could the Magdyno move to the back. However, there is
a ~1/8" gap at the base of the Magdyno at the front that certainly would allow for fore-aft movement to let the pegs "worry" themselves into the metal at the edges of the slots if loosely clamped and left as-is. So, I inserted a ~1" high piece of 1/8" Al the width of the strap that completely fills the space between the base of the Magdyno and the strap. With this "shim" in place, even if the strap were only somewhat tightened it would be physically impossible for there to be any fore-aft movement of the magneto. Due to tolerances and differences in the body shapes of Magdynos and magnetos I have similar "shims" in gaps that are in both the front and back of the magneto bases of the DBDs that won't allow them to move fore or aft, either, irrespective of the 40 h.p. worth of vibration hammering on everything.
Turning to the fore-aft location of the magneto or Magdyno, unless the slots were machined in the wrong position at the factory they will place the shaft pretty close to the fore-after centerline of the hole in the timing case. Wear in the base or the platform area (or production tolerances) that cause the shaft to be below the up-down centerline can be easily addressed by a shim of the necessary thickness under the unit. Of course, it's much faster to write this paragraph than it is to do the work required to center the magneto, but this should be a once in a blue moon task.
Unfortunately, due to production tolerances, when you slip the pinion on you might find too little or too much backlash. The teeth mesh at a steep angle of 20o
from the vertical so every 0.010" the pinions move with respect to each other changes backlash by just 0.0036". Vincent addressed an issue of the meshing of gears in their timing cases by supplying one of the pinions in over- and under-sizes. In their case the gears were on shafts inserted in machined holes so there was no other possibility for adjustment. However, BSA didn't supply magneto pinions in a range of sizes at the time, and aftermarket suppliers haven't done it for us now, so a compromise will have to be made between perfect centering of the shaft in the oil seal, and perfect backlash of 0.007".
If the pegs were an interference fit in the slots the use of shims under the body would be the only means of adjustment. If the shaft were perfectly centered in the oil seal but the backlash was, say, 0.000" it would require moving the pinions apart by 0.020" to get the backlash correct. However, the gears are at ~45o
with respect to the magneto base so it actually would require raising the magneto by 1.4x this amount (1/cos45o
), or 0.028". Thus the shaft would be off center in the oil seal by 0.028".(*)
However, if there is some fore-aft movement possible the magneto could be raised by half this amount, 0.014", and also moved horizontally the same 0.014" to get the required 0.007" backlash, resulting in the center of the shaft being off center by only 0.020" (square root of 2 x 0.014").
So, in summary, "if the expense and effort was spent on repairing the engine back to original you'd be just as well off."
Unlike a "solution" that damages the cases and that might or might not work in the end, the original BSA system will
A Timken Seals Catalog says their Model 87 spring-loaded lip seal of the type sold for our timing chests can tolerate a shaft misalignment of as much as 0.100" at over 3500 rpm (7000 rpm engine). The catalog says it has "Aggressive shaft-to-bore misalignment capability."