Which spark plug does my lawn mower take?

The number is printed on the plug that's already in there, so here's how to read it and match it.

The spark plug your lawn mower takes is the one that's already in it, so pull the old plug out and read the number off it. On a Champion plug the number is printed along the white ceramic or stamped around the metal hex, and you buy that same number again. If you've still got the engine manual or there's a sticker on the shroud, it'll usually be listed there as well.

The common mower plugs below are all a 14mm thread, and they differ in reach, seat and whether they carry a resistor.

Where is the number on a spark plug?

Take the lead off first, then undo the plug with a plug socket that fits the hex on it, which is 13/16" on the J19s and 3/4" on the CJ8s. Have a look at the ceramic once it's out, and if it's gone sooty just wipe it with a rag and the printing will come up. Old plugs do get to the point where the number has worn off completely, and that's when you go by the shape of it instead.

Four things to check against the one you've taken out:

• Reach is how far the threaded part goes down into the head. Most of these are 9.5mm. Get this wrong and the plug either sits too deep or doesn't reach into the chamber properly.
• Seat is how it seals. A gasket seat has a small flat washer around the base of the thread. A tapered seat has no washer and the plug body itself is cone shaped where it sits down.
• Hex is the spanner size, 13/16" on the J19s, 3/4" on the CJ8s and 5/8" on the DJ8JMP.
• Resistor or not. On these Champion numbers the R at the front is the resistor version, so RCJ8YMP is a resistor plug and CJ8MP is the same shape without it. Match whatever came out, because a mower that wants a resistor plug and doesn't get one can throw interference into anything nearby.

What do the common mower plugs measure?

Champion number	Thread and reach	Hex	Seat	Resistor	Heat range
RJ19LMMP	14mm, 9.5mm reach	13/16"	Gasket	Yes	19
J19LMMP	14mm, 9.5mm reach	13/16"	Gasket	No	19
RCJ8YMP	14mm, 9.5mm reach	3/4"	Gasket, projected tip	Yes	8
CJ8MP	14mm, 9.5mm reach	3/4"	Gasket, projected tip	No	8
DJ8JMP	14mm, 7.8mm reach	5/8"	Tapered, cut back ground with modified gap	No	8

If the number you've read out is one of the others, they're all here.

There's also the RN9YCMP, which is listed for lawn and garden equipment and small engines of that sort. It's a 14mm thread with a long 19mm reach, a 13/16" hex and a gasket seat, so it's only right if the one that came out is that long.

What if a new plug doesn't fix it?

More often than not when a mower won't fire it's the fuel rather than the plug, especially if it's been sitting in the shed since autumn with half a tank in it. Drain the old fuel out and put fresh in before you go chasing anything else. The other one that catches people is the lead not clicked back down properly on the plug, so give it a firm push until you feel it seat.

Either way, a fresh plug once a year before the grass takes off is worth doing, because it's the cheapest part on the whole machine and it's the one that decides whether you get to mow today.

If in doubt, bring the old plug in and we'll Hardware CSI it. We'll read the numbers off it, check the thread and the reach, and match it on the spot. Too far to pop in? Give us a ring on (08) 8536 4333 or send us a message with the make and model of the mower and the numbers off the engine, and we'll work it out from there.

Cheers,
Candeece