Brightspark Magnetos

FAQ sections:

FAQs about the EasyCap
FAQs about magneto condensers
FAQs about magnetism and remagnetising
FAQs about ATDs and timing
FAQs specific to Lucas magnetos
FAQs about magnetos generally

In this section:

What do safety screws do?
Are suppressors and resistor sparking plugs OK to use with magnetos?
In an emergency, can the earth brush be used as an HT brush?
Why are some magnetos driven clockwise and others anti-clockwise?

FAQs

What do safety screws do?

If your ignition system is properly set up, the safety screw(s) shouldn't do anything. However, they are there to provide a non-destructive path for the HT current in the event that something does go wrong with the system, such as an HT cable or plug cap coming loose. The safety screw is earthed and has a tip pointing towards the slip ring, with a gap between the two. The size of the gap is such that a spark shouldn't jump between the slip ring and the screw during normal operation, but with a HT voltage slightly higher than encountered in normal operation, a spark can jump the gap.

Without a safety screw, the magneto will do its best to get rid of the energy stored in the LT winding when the CB points open, and it does that by increasing the HT voltage well above its normal values in an attempt to find an escape route. Without a safety screw there are many potential escape routes, such as:

• breaking down the insulation of the HT winding and jumping to the steel of the armature bobbin;

• breaking down the insulation of the HT spike and jumping to the magneto housing (see photo top right);

• tracking across the boss of the slip ring from the HT spike to the armature end piece (see photo mid right);

• tracking up the sides of the slip ring from the brass segment/ring to the magneto housing (see photo bottom right and video clip);

• finding a crack in the HT pick-up.

Once an escape route has been found, especially if it continues for any length of time, the damage done is usually irreparable without replacement, i.e. a bobbin rewind or a new slip ring or pick-up.

In the Brightspark lab, we have a modified K2F magneto with a transparent acrylic housing for the slip ring. The video clip below shows the lightning effects that can occur at the slip ring if the HT current can't take its normal route, in this case because we had disconnected the HT lead and there were no safety screws.

For information on the safety screw gap for Lucas K series magnetos, please see our FAQ What is the recommended safety screw gap for a Lucas K series magneto?

Breakdown of the insulation on the HT spike. (In this case, we managed to do a repair, but only for lab. use)

The effect of continued tracking across the boss of the slip ring to the armature end piece.

Our 'lightning chamber' K1/2F on the  Brightspark rig