Hitchcocks Forum

I've been meaning to make something like this for ages.

Most of us test for a spark by earthing the plug on the cylinder head and turning the engine over.
This doesn't tell you much as the HT voltage needs to be about twice as much to produce a spark under compression so a spark in the open may not mean a spark in the engine.
It also doesn't tell you if it's the plug or the ignition system at fault if there's no spark (or you just didn't earth it properly)
With this spark tester I can attach the plug lead to one end and earth the croc clip. If all is well there should be a spark, even when the gap is adjusted to it's widest. A good spark shows that the HT system is probably OK.
If the croc clip is then clipped to the plug and the gap on the tester adjusted to around 25 thou there should be a spark across the gap if the plug is OK.
The HT system should easily be able to jump two gaps.
It also can be used with the engine running to test for situations when the engine stops just after starting - just watch to see if the spark stops.
It's not foolproof I know but will tell me more than the usual method.

Body is turned from a 18mm piece of delrin, contacts are 8mm brass rod threaded into the delrin. Plug cap connection has a removable cap to cater for both cap types. A bit of shrink sleeving covers the rod to help prevent HT shocks.

An excellent tester! Surely there's a market for such an item. Thanks for showing this on the web site.

Presto, someone got there first - there are plenty on the market for not a lot of money. Ask our hosts, I'm sure Mr H would be able to point you in the right direction.

I'm not saying it's my idea, Ebay is stiff with similar testers.
I just like making things.

If you hold the spark plug against the head and you accidentally lift it away, it it it it it it it it it hurts.

A spark plug discharge is governed by Paschen's Law which says the breakdown voltage is approximately proportional to the product of pressure and gap distance.



Basically when a 8.5:1 piston comes up there's 8.5 times as many molecules to push the spark through than there is at 1-atm. In dry air at 1-atm electrical discharge is 10kV/cm, it's not linear but at least that's the one to remember when you're holding a live spark plug, 25kV per inch. Don't hold it, use a long stick, something insulated for HT, not the little electrician's screwdriver

PeteF - I like it, it's very neat.



My bodge used on the lawnmower is a spark plug and a pair of battery croc clips with a short length of stout wire betwixt them. Fortunately the new mower doesn't need this dance every spring.

The lawnmower is going to be the first subject for testing jefrs. I struggled to start it the last couple of times so I'll give a proper seeing to before winter storage.

That is really neat Pete,

Is the gap in your spark displayer fixed, or adjustable? (Screw it in, screw it out?)

Adjustable Scaly, up to about 8mm