Lucas NR1 Safety Gap

[Iron Head]

Hi All just about to rebuild my NR1 with a rewound armature and EasyCap and was scratching my head about the means by which a safety gap functions on an NR1 without the little screw that the N1 has. Perhaps there is a path via the points or something? No doubt Groilly can advise!

[Groily]

Hmm. Interesting question!

The workshop instructions for N series refer to 'safety gap screw, when fitted' so the necessary inference is that they don't all have one. I don't have an N1C or R to look at, but if there were a dedicated screw it would to be in the usual place. Sadly, the Lucas Parts Lists I have don't differentiate in enough detail between versions, and the safety screw is just one of many items in a 'sundry parts set' so not listed under the individual variants of the N series.

If there isn't one it doesn't really matter and certainly won't stop it working. The most similar Lucas offering in terms of its spec is probably the MO1 magdynamo, or the earlier MN type with ring cam points like the NC and NRs. Most (I'd have said 'all' but I have actually seen exceptions) of them have no safety spark gap screw at all, and are none the worse for it.
An option on some magnetos was the use of one long HT pick-up screw with a rounded nose, to double up as a safety gap screw. But on an N series it wouldn't be ideally placed if fitted, as it would be off-centre ref the armature spindle axis. Bosch (I think) and some others (for sure) used them with pick-ups so disposed on the magneto body that one screw was central to the mag axis.

Not having one - and many mags didn't - just means there's no secondary escape route for sparks which can't get out via the normal pick-up and HT cable for some reason. Not usually fatal to the coil, but could certainly stress it if it can't discharge (for this reason it isn't good practice to spin magnetos without safety gaps unless the HT leads are connected to spark plugs earthed to the mag body, or simply earth HT leads).

I suppose one could drill tap and counter-bore a special hole (3BA would be standard), do a bit of measuring and put a screw with a rounded nose in, with a gap of about 6.5mm between nose and slipring . . . But if Lucas were happy without . . . and a million mags whizz round without them . . .??

[Iron Head]

Thanks so much Groily for your comprehensive response. As you say it would appear that Lucas racing magnetos can obviously survive without safety gap screws!

[Joker_Bones]

Ohhhh...
I have a NR1 magneto, when it was playing up I stripped it down to investigate.
I was puzzled to find the threaded hole for safety gap screw and the nearby vent hole had been filled with weld and ground flush.
I could not think why this had been done so I re-drilled the holes and fitted a screw.
Perhaps the comp magnetos never had the safety gap screw... I would be curious to know if yours does not have the holes at all or if they once present but welded up?

[Iron Head]

Hi Russell my magneto is definitely all original and has no drillings through the case apart from the carbon brush sitting under the name plate. Have seen lots of N1’s converted over so it may be you have an N1 body(?)

[Joker_Bones]

Makes sense... Or perhaps the case was swapped out, loss of magnetism maybe, and the holes filled as they were not found on the NR1.

Thinking about it now, I can see why the vent hole near the safety screw would be omitted as the points cover is waterproofed and has the fitting for a riser vent tube... I don't know why the safety screw is omitted.

[Groily]

It does (make sense, that is JB). I don't know 'why no safety screw' either. C and R versions of ?all? the Lucas range would not have had a vent hole, relying on the breather as you say. A standard N1 or MO1 will also have a small hole in the clip-on face cam cb cover.

If face-cam contact breakers were good enough for pretty much every Gold Star ever made, a standard N1 with the same bits ought to be good enough for AMC singles too. Unlike manufacturers beginning with V, AMC never used BTH magnetos when they were in their heyday (AFAIK): they'd have been a plus in many people's view.

A lot of magnetos decorated with snazzy ID tags aren't what they pretend to be, but at the end of the day it's a question of getting things as close to how the factory intended them to look as possible using what's available - and more to the point often - affordable.
The premium on C mags, let alone Rs, which are basically no better in practice than standard ones for 99% of us, is a scarcity thing pure and simple.

Personally, on twins, I actually prefer standard K2Fs over C versions for reasons to do with the HT pick-ups: - accessibility; avoidance of regular problems with stripped threads on the C type bodies requiring helicoiling; and above all the difficulty in getting good quality leak-proof pick-ups for them.
I have a C on my Mod 31 because I had it sitting around without a home, but for no other reason. My 650SS Dommie, which you'd think would be a candidate for a C, has a standard one and is none the worse for it.

[Iron Head]

Couldn’t agree more with your comments Groily, as long as it is strong and timed correctly, a spark is a spark in my book!

The other problem for the rivet counters in trying to build an NR1 would seem to be the extreme scarcity of L/H pull, slack to advance, CB housings. I’ve never seen one for sale.

Iron Head

[Taid]

May I ask, what purpose does a safety gap, in a magneto, serve?

[Groily]

It just provides an escape route to earth for the HT charge in a coil in circs where for some reason the spark can't get to the plug by the intended route of brush, pick-up and HT cable. For the safeguarding of the winding's insulation really.
The safety gap must be quite large - or the magneto may choose to spark across that gap rather than go to the bother of firing the plug under compression. The safety gap is 'in plain air' and sparks are easy to make in plain air, but much harder under compression. The higher the compression ratio, the higher the voltage needed to get a spark and therefore the larger the safety gap theoretically needs to be to avoid the spark's taking the short route. Typically somewhere a bit over 1/4inch is about right. Lucas specified 6.5mm - 7.5mm. The former is good normally but in the case of 'racing' engines with very high compression ratios, a bit more is wise. A washer under the screw head, or a grind of the pointy end . . . .

It is probably ? because of the risk of the spark jumping the safety gap on high performance engines that the screw wasn't fitted to some models, as in the case under discussion.
That said, I have run engines at around 10:1 cr without altering the typical safety gaps provided by the use of the original screws.