Brightspark Magnetos

The 'Complete Works' condensectomy - splitting the armature

Special tools/materials required

• For all Lucas magnetos, and for some other makes too, you will need a puller for removing the inner race of the bearing next to the slip ring. You may also need a puller for the slip ring if it is stuck. Drawings are available for download here.

Procedure

Please note that the pictures below show the armature of a Lucas K1F, K2F or KVF magneto, which has the slip ring at the CB end and the condenser at the drive end. For MO1, MN2 and N1 instruments, the slip ring and condenser are at the opposite ends.

You need to remove the inner race of the bearing next to the slip ring, and the slip ring itself, from the armature in order to access the heads of the two long screws that hold the brass end pieces to the bobbin of the armature, so that the brass end-piece containing the old condenser can be separated from the bobbin.

You may find techniques described on the internet for removing the inner race without any special tools. However, with them, and on a K-series magneto especially, there is a great risk of damaging the CB-end brass piece and rendering it useless. Universal 2- or 3-legged pullers are unsuitable for the job, because there is no space between the inner race and the oil flinger into which the feet of such a puller can engage. A special bearing puller is required; you can make the required pullers shown in the photos from the drawings which you can download by clicking here.

It is sometimes possible to remove the slip ring by hand without damaging it, but it is far safer to use a special puller as is also shown in the downloadable drawings.

The clamp halves are secured to the inner race. The bridge is secured to the studs on the clamp halves, with the end of the extractor screw bearing against the cone which is placed in the internally-tapered end of the armature. Turning the extractor screw draws off the inner race and the small collar outboard of it.

Possibly one or more shim washers can then be removed (note and keep safe), as too can the oil flinger which prevents any oil getting onto the slip ring.

The slip ring can then be removed in a similar fashion using a larger clamp. In the picture, note the use of the cone to protect the armature shaft, as for the bearing puller. Often there is no need for a puller, but in the case of original Lucas armatures which have never been stripped, shellac can make extraction extremely delicate.

With the slip ring pulled, note the spike of the HT coil, which fits into a very snug hole in the boss on the rear of the slip ring. Take extreme care not to bend the HT spike unintentionally.

The particular slip ring in the picture is in generally good condition, being neither broken on its flanges (through being removed without the safety screws having been fully withdrawn) nor heavily 'tracked' where the HT pick-up brushes have run against it. It should be serviceable for a good while yet. A clean and polish will be enough. In some cases, the lightest possible skim in a lathe is recommended, or replacement in cases of deep scoring or any breakage. Chips or other damage to the outer edges of the flange of a slip ring can cause surprising amounts of spark-tracking when the magneto is working hard, and should not be accepted.

Unscrew the two through-screws which hold the brass ends of the armature to the central 'bobbin' - the laminated core around which the low and high tension windings and their insulation are laid. Once you have removed the screws, it is a good idea to put the slip ring temporarily back in place so as to protect the HT spike.

Before going any further, scribe or punch corresponding marks on the bobbin and on the drive-end brass end-piece (but not on the track for the earth brush) so that your can put them back together, later on, the same way round.

Also, if you have an ohmmeter or multimeter, measure and make a note of (a) the LT resistance between the earth brush track and the nut for the CB centre screw (hidden between the bobbin and the CB-end brass end-piece) and (b) the HT resistance between the earth brush track and the HT spike. You should get readings of about 0.6 Ohms and 5000 Ohms. Anything vastly different from these figures probably indicates that the bobbin needs rewinding, although a lower figure for the LT resistance could be a result of the old capacitor providing a short circuit.

Then, the brass end-piece containing the condenser needs to be gently pulled from the bobbin. At this stage you do not need to separate them completely, just by about 5/16" (8mm).

Don't try to separate the other end-piece from the bobbin - all being well, it can be left undisturbed.

For K1F, K2F and KVF magnetos:

The drive-end brass end-piece and the bobbin are also connected by two dowel pins, which are often quite tight.

Resist the urge to hold the drive-end brass end-piece in the vice. You can, however, place a spare 3/8" BSF nut (or an interlocked pair of nuts) on the drive shaft, clamp in the vice, and then try wiggling and pulling the bobbin by hand.

Also, resist the urge to use any significant crowbarring action against the large diameter edge of the drive-end brass end-piece - it is all too easy to distort or damage it and the track for the earth brush.

It is sometimes possible to start the separating movement by screwing the long 2BA through-screws the wrong way round into the drive-end brass end-piece (i.e. from the drive end) and inserting something into the gap between the end piece and the bobbin to prevent the screws passing into their holes in the bobbin.

For MO1, MN2 and N1 instruments:

Dowel pins are not usually provided between the bobbin and the CB-end end-piece containing the condenser, and so things are usually a lot easier.