Ignition Timing

By Hans Muller
WDM20 58764
May 2007

Setting the Ignition on your BSA WDM20 Magdyno

  1. First loosen the Magdyno driving pinion with the special extractor (fig. 2); do not remove!

  2. Figure 2 Using the pinion extractor tool. Can be made yourself, but unless you are an ace with the lathe, I strongly recommend buying one! The hole in the timing cover is the one used on later models, but I made this myself; the cap is the same as the chain case inspection cap.

  3. Remove the spark plug and the small cylinder head timing bolt. Put the gearbox in 4th gear and let the piston move upwards on the compression stroke by slowly turning the rear wheel. You have to remove the valve spring cover to see if you are really on the compression stroke; both valves should be down, and both tappets should rotate freely.
  4. Now insert a thin wooden stick (I use a bamboo kebab skewer) and continue turning slowly the rear wheel until the stick does not go up anymore; you are now at TDC (Top Dead Centre). Put a mark on the stick, repeat a couple of times and average, to be sure you are really at TDC !
  5. Remove the stick and put another mark at 11.1 mm (7/16" for the metrically-challenged) above the first mark. Use a green (or any other colour at hand) felt pen to mark that 11.1 mm section. (Fig. 3)

  6. Figure 3. The skewer with the piston in TDC position.

  7. Put the stick in the cylinder and turn the rear wheel backwards, until the upper mark is flush with the cylinder head; the piston is now at the fully advanced position.
  8. In the mean time you have made a piston stop that can be threaded into the cylinder head timing bolt hole, using materials from the box in your workshop with things-that-really-should-have-been-thrown-away-a-long-time-ago-but- that-you-kept-in-case-of-need. Adjust the stop so that it touches the piston at the fully advanced position (11.1 mm before TDC). The thread in the timing bolt hole is 1/4"x25 CEI; mine was knackered beyond recognition so I re-threaded to metric. (Fig. 4)

  9. Figure 4. The piston stop tool in position; now the piston is in fully advanced position.

  10. My Magdyno has the advance cable on the left side (cable slack when fully advanced) which is not original WW2, but strongly recommended! (Conversion can be done by a good specialist) If the cable breaks, you are in the fully advanced (= riding) position, and you only have a slight problem when starting (risk of kickback). But try to drive in retarded mode, and your exhaust valve will soon burn away.
  11. Now, carefully pull the contact point with the little hook, and insert a strip of cigarette paper between the points. Slowly turn the contact breaker assembly in its direction of rotation until the paper comes loose. You will have to do this a couple of times to get the `feel' of it. (Fig. 5). When fully open, the contact points gap should not exceed 0.30 mm (0.012"); adjust if necessary.

  12. Figure 5. Inserting the fag paper between the points.

  13. Give a tap on the magneto driving pinion to secure it on the taper; this can best be done using the extractor, screwed in with the bolt fully retracted, giving it a smart blow, and tighten the nut. . The internal and external tapers should be reasonably grease-and oil free, otherwise your ignition will get out of pace rapidly; remember, there is no key and keyway to prevent the pinion turning!
  14. To check, remove the piston stop, put in that kebab skewer (no green should show) and set the advance lever in fully retarded position; slowly turn the rear wheel until the paper strip comes loose. The green mark should now be fully visible.

That's it!

Figure 1. A handy set of tools I alway carry with me. You can see the extractor tool, the hex key wrench for my modified cylinder head timing bolt, the kebab skewer with the green marking (note that I put it upside down on the photo; the shortest part should go down in the cylinder, the other end had been cut askew to prevent putting that part down in the cylinder!), the home-made piston stop, the small hook to pull the contact points slightly apart for the paper, the Magdyno spanner, additional magneto spanner set, a package of fag paper (will last you several lifetimes) and a reserve contact breaker set.