Brightspark Magnetos

Removing the magneto armature

Remove the HT pickup(s) and contact breaker end cover.

Be careful not to lose the pick-up brush(es).

With the K2F magneto in the picture, the pick-ups are retained by pivoting spring blades. Other types of pick-up may be held by a pair of screws.

The end cover may be retained by a pair of nuts (as in the example), a pivoting spring blade, or by a large ring screw-threaded onto the magneto body.

Remove the CB assembly.

For ring-cam instruments:

Release the contact breaker centre screw using a 4BA or 1/4"AF nut driver or socket. This will allow removal of the contact breaker assembly as a unit. With screw undone, wiggling it will always free the taper with its integral key. The assembly in the picture is an original Lucas brass one (to be contrasted with later Lucas steel ones and more recent steel pattern ones).

Some brass backplates have an auxiliary earth brush on their rear face.

For face-cam instruments:

Undo the screw holding the moving point and remove the point. This gives access to the centre screw, which needs to be removed after straightening the locking tab washer. The CB assembly can then be withdrawn.

Be careful that the points tappet does not fall out.

Remove the safety screws.

The safety screw(s), if fitted, lie in the same transverse plane as the pick-ups.

Many twin magnetos have two safety screws opposite each other. Many single magnetos have a single safety screw. Some don't have any safety screws.

The safety screws are there to provide a safe route to earth for the magneto's spark energy in the event that the normal path (via the pick-up, HT cable and sparking plug) is interrupted.

ANY SAFETY SCREWS MUST BE REMOVED BEFORE PROCEEDING FURTHER, otherwise they will interfere with, and probably break, the slip ring when withdrawing the armature from the magneto's body.

Remove the earth brush.

The earth brush connects the armature body (and thus the earth end of the HT coil) to the magneto housing (and thus the engine) to provide a return path for the sparking plug current. The earth brush assembly comprises a carbon brush, spring, special hollow screw and usually a fibre washer. Simply unscrew it.

On some magnetos, the earth brush is sometimes hidden beneath the name plate.

Remove the end housing.

The housing for the cam and CB-end bearing is removed by undoing the two, three or four screws holding it in place. One of the screws may also double-up as the mounting post for the spring blade that held the CB end cover in place.

Note, and retain, any large brass shims (two of which are shown in the picture).

The outer race of the CB-end bearing should be firmly fixed in the end housing. However, the insulator between the outer race and the housing may have disintegrated, leaving the race loose. In that case, the insulator will need replacing.

Apply a keeper

The magnet strength of Lucas magnetos (other than early versions with a horseshoe magnet) isn't significantly affected by removal of the armature, so you can ignore the rest of this section.

However, with most other magnetos including BTH, if the magnet is fully magnetised, then it will lose some of its strength the first time that the armature is removed (even for a split second) unless some form of keeper is applied before the armature is removed, and stays in place until after the armature has been replaced.

A large steel or cast-iron vice makes a good keeper. If the magneto has an alloy cover over the magnet and pole pieces, remove it first. Place the magnet pole-pieces between the jaws of the vice and close the vice just enough to hold the magneto. Don't squeeze it hard. As an alternative, you can use one of more steel or cast-iron G-cramps. Again, don't do them up too tightly; with some magnetos you might loosen the pole-pieces in the casting of the magneto body.

Remove the armature from the main housing.

The armature can now simply be pulled out of the housing. The inner race, cage and balls of the drive-end bearing should come with it, but the outer race of the drive-end bearing should remain in the housing.

Again, the insulator between the outer race and the housing may have disintegrated, leaving the race loose, so that it, and the oil seal, may fall out. In that case, the insulator will need replacing. In any case, having stripped the magneto this far, it is prudent to replace the insulator for the drive-end bearing and the oil seal (if provided).

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