There is a pretty decent article on spark plugs in Wikipedia, heat range is discussed...

including -

'By examining "hotter" and "cooler" spark plugs of the same manufacturer side by side, the principle involved can be very clearly seen; the cooler plugs have more substantial ceramic insulators filling the gap between the center electrode and the shell, effectively carrying off the heat, while the hotter plugs have less ceramic material, so that the tip is more isolated from the body of the plug and retains heat better.'

You might see similarities or differences between your two plugs.

and -

'There is no direct relationship between the actual operating temperature of the spark plug and spark voltage. However, the level of torque currently being produced by the engine will strongly influence spark plug operating temperature because the maximum temperature and pressure occurs when the engine is operating near peak torque output (torque and RPM directly determine the power output)'

How important heat range depends somewhat upon how hard you work the engine.

and under reading plugs -

'For example, a sandblasted look to the tip of the spark plug means persistent, light detonation is occurring, often unheard. The damage that is occurring to the tip of the spark plug is also occurring on the inside of the cylinder.'

or -

'As another example, if the plug is too cold, there will be deposits on the nose of the plug. Conversely if the plug is too hot, the porcelain will be porous looking, almost like sugar

Good luck, I appreciate a person who is inquisitive and willing to experiment!
--
'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 435,000 miles put on 9 bricks

Good thought James,

I reverted back to my Champion plug.

I suppose that part of the problem is that the manufacturer's don't readily provide a decent comparative heat range sheet per plug.

From what I could tell from an internet search, the heat range of a plug is related to the length of the electrode and amount and size of the insulator.
Since the Champion and Volvo plug were similar, of course, with the exception of the electrode, I made the assumption that the heat ranges are similar. This may have been an erroneous assumption.

However last night at about 85 degrees F, I ran the mower for an hour with no adverse effects. I'll pull the plug today and look at it to see.

It's kind of irritating that despite all my research I cannot find a way to compare the two plugs.

Thanks for the response,

Marty

Hello James.

If you are saying that it is good practice to follow manufactures plug recomendations I would be the first to agree.

Generaly the car plugs that I have on the back of the bench are long reach so dont fit much garden machinery, excepet for some four stroke lawn mowers, perhaps.

These lawn mower engines are not high perfrmance types,and will probably be quite happy with a plug of average heat range. The plugs in my turbo Volvo will be 'harder' (or 'cooler') than that, so--- may foul IF the engine burns oil. If that happened there would be no harm done, except for frustration.

I regard small high performance two stroke engines as found on strimmers to be special cases, and in any case would never accept a Volvo plug.

Smilarly I would never 'econimize' with two stroke outboards.

If it gets the grass cut, and we can get back to brickbord, and talk about sparking plugs, then I think its OK to use an average to hard type of plug in a four stroke lawn mower, especially if The 'BOSS' wants it done now. BUT! it might not be right for you. Very best regards as ever .

Howard. Isle of Skye.

hello,your volvo uses nkg bcpr6es plugs and rc12yc plugs, they are the same plug your mower uses,