Spark Plug Gaps "IMPORTANT"

I've just been reading a piece by a magneto specialist in a club magazine (It's called "Fishtail", it's the Vellocette owners club magazine), he states that:

Anything with magneto ignition should have a maximum plug gap of .018" (that's 18 thou)

He suggests setting plug gaps to .015" (15 thou) to take any future wear into account. I've set mine to a loose .015" but tight enough that you can't get a .018" feeler in the gap.

Here's the bit I didn't realise:

Use of too big a gap will damage a magneto!

Now, we all know what a pain in the backside magnetos can be, & just how expensive it is to get them re-wound, but I never realised opening up the plug gap could actually damage a mag.

Thought I should pass it on, as I reckon it's a useful bit of info. He also reckons it's a good idea to check continuity, with a multimeter, between the centre electrode & the top terminal, as apparently many cheap plugs have no continuity here, but they will still work. They'll give trouble though, & could also damage the mag, as there will be 2 sparks going on, one useless one inside the plug!
He also suggests checking the earth electrode, to make sure it's properly fixed to the plug body. Another good idea, as if an electrode drops into the combustion chamber it could cause all sorts of damage.

We should all know not to use resistor plugs & not to use resistor or suppressor caps. We should also "only" be using copper cored HT leads, but I guess we all knew that.

Whilst I'm on the subject, I don't suppose anyone's got a coil for a Lucas SR1 mag out there? I think they were used on Lister D's at some stage. The one on my brother's Stuart Turner has had it. I've tried testing it & there's nothing on the HT side & almost a dead short from the LT side to earth.
Typical! if it was a Wico it would probably be working fine!

Interesting info thanks for sharing, think I need to alter my plug gaps as most are set at 25thou.

Stu.

My manuals for mag-equipped stuff state between .018" and .025", depending on which one it is... That's from the magneto as well as the engine manufacturers. I usually use my thumbnail to set the plug gap - that's where everything seems to run best whatever the 'book' says. But then I normally use a fingernail to set the points - nothing like a bit of accuracy! (I just measured my thumbnail and it's .022")

Now I agree with what I've seen about firing a mag to nothing - i.e. completely open circuit or pulling the cap off to stop the engine - that can cause proper damage to any coil, but I really fail to see how less than a .007" difference in the plug gap can cause a mag coil to break down and fail unless it's almost dead or badly rebuilt anyway.

I remember always being told by old-school mechanics that too tight a plug gap results in a less intense spark, incomplete burn, more smoke and less fuel efficiency - all of which have been borne out by my subsequent experience.

All I've said there is a little immaterial anyway - I'm willing to bet at least 17p that if you get a group of 5 people and give them a plug and some feeler gauges not more than two of them will give you the same measurement.

Oops, forgot to say the bit about checking plugs is spot on. Even expensive plugs can have the same issues so it's not just cheapies that can catch you.

Also forgot - if I use any plug caps I only use suppressed ones, some of them even have 'wico' or 'wipac' moulded into them...

I'm odd though, I like magnetos

I think for the amount of time most of us run these engines it probably doesn't matter that much, but the piece I read was written by one of the country's biggest magneto experts & I'm inclined to believe him. It takes voltage to jump that gap (can't remember now, but I've got a feeling it's something like 350v per millimetre) so it's obviously going to put some extra strain on a mag to make it jump a bigger gap.

Funny thing though, I changed the gap on the Tarpen & it took me 3 goes to get it started! It starts fine now, & seems to run well, but I thought it would be sod's law for the thing to not start with a smaller plug gap.

The Homelite generating set tells you to use aircraft screened plugs, which have a .025" gap, & re-gap them to .018"

I personally don't think plug gaps effect combustion that much, I think that's more down to timing, mixture & combustion chamber shape, but that's just my thoughts, I know the gap does effect it, but I've always thought it's a small part of the whole story.

Well, 350v/mm is about right, but a voltage that low is unlikely to arc, unless the electrodes are touching and then separated. In dry air, at standard pressure an approximate figure is 33kv/cm - I'm quite sure the change in volts/mm distance isn't linear though. Different temperatures, pressures and electrode shape and size affect this figure as well. The high pressure and temperature as well as the different dielectric strength of the fuel mixture in a combustion chamber mean it needs a greater voltage, somewhat reduced by the fact that the spark plug electrodes are fairly 'pointy'.

0.018" is about 0.46mm, and 0.025" is 0.63mm - a properly operating and good condition magneto should be producing somewhere in the region of 15-25 kV.

I'm not saying a leading expert is wrong, but he may just be a little 'finicky'. How many mags that have been working for the last 50+ years have always had a correctly gapped plug, let alone one gapped to his spec? I'd say the vast majority of people take a plug out of the box and put it in the engine, if you're lucky a few of them might look at it to make sure it's a spark plug first...

As far as I have seen, the gap can affect the combustion fairly significantly, especially if something else isn't quite right. You're right it is a small part of the whole story, but then every part is quite small. Admittedly, the difference between .018" and .022" is probably not enough to affect it all that much.

Now, I have no intention of starting any sort of argument, I'm just relaying my experiences and opinions.

No, I quite agree, & this specialist was writing his piece for a motorcycle club monthly journal, stationary engines are pretty different, especially with show engines, they're not put under anywhere near the strain of a regularly used classic motorbike, so it's probably not quite so important.

I'm glad I found it out though, as many stationary engines these days have magnetos which are pretty much on their last legs, this bit of info could help to get a few more years out of them, especially as the cost of having a magneto re-wound or reconditioned these days is usually more than the engine is worth!

I will have to have another read of the article though, as I can't remember what he said about winding mags over with a complete open circuit (IE no plug connected).
Personally I don't see that it would be that bad for it, or at least doing it the odd time (like clearing the barrel & crank case out on a 2 stroke with a good pull) should be OK. There's a couple of magneto specialists in my dad's old bike club, I'd like to have a chat to them some time, see what they make of all this stuff & how important they reckon plug gaps are. (Also it give me an excuse to go down the pub where they all meet up & have a couple of pints )

One thing you did mention was a manual saying to use aircraft plugs which are gapped as standard to 0.025" but knock them down to 0.018". Most aircraft use magneto ignition...

Once or twice winding over an open circuit mag shouldn't hurt it, but personally I'd connect the plug and lay it on the block to complete the circuit (or stick the lead to the block to short it - same as an old kill strap does).

Strangely (or maybe not) the issue you described after regapping the tarpen was exactly one of the problems I have with a too small gap - an absolute bugger to start first time, but then will start easily and run all day. Next cold start (i.e. more than 48hrs since last one) and it'd take a load of pulling to go again.

If I was you I'd certainly take the opportunity to go down the pub though!

I find magnetos quite interesting, maybe I'm a bit odd, but they couple my interests in electrics/electronics & old engines. I also think working on magnetos (not just gapping points etc, I mean actually rebuilding them professionally) is almost a kind of "black-art", or so I'm lead to believe. I would like to know more about them & understand them better.

I've built a few different coils for other hobbies, one of them was a 4000 turn coil for high voltage experiments, which I wound with a home made holder thingy & a cordless drill, they are interesting things (to people like me at least) I would love to be able to build my own magneto coils, although I doubt I could afford the expensive motorised winding equipment, & something tells me to hand wind one would take a lot of time & probably a lot more patience than I'll ever have.