Spark plug -what's the difference

[Tom RS]

Here's possibly the dumbest question of the day. If a plug is exactly the same size and fits in the hole just fine can I use it no matter what manufacturer it is or the number on it? (I know I can't use a resister plug when the engine doesn't use those).

 

[old]

Here's possibly the dumbest question of the day. If a plug is exactly the same size and fits in the hole just fine can I use it no matter what manufacturer it is or the number on it? (I know I can't use a resister plug when the engine doesn't use those).

That is a Yes No or Maybe thing. It could be to cold so it would foul easy or it could be to hot which can cause burned valves or a hole to be burned in a piston etc.

 

[timcasbolt]

As to the manufacturer part of your question, the brand doesn't matter if the application is correct. Everybody has their favorite brand, but they all work.

 

[MarkB_MI]

It's a bit more complicated than just fitting the spark plug hole. I remember back in my teenage days one of my buddies substituted a random spark plug in his six cylinder Ford pickup. It went about 15 miles before it threw a rod out the side. I remember it well because his truck rolled to a stop in my yard. I'm not sure, but I think the substitute plug was a wee bit too long.

There are obvious physical differences between spark plugs, particularly how far they reach into the cylinder, whether they seal with a taper or a washer, and of course the hole size. But some differences aren't so obvious: The heat range of the plug, which is determined by the length of the insulator, is critical. Too cold and the plug will foul quickly. Too hot and you can have pre-ignition which can quickly destroy your engine. There are heat range charts that will tell you how similar plugs compare, in case you want to run a slightly hotter than stock plug. (Nobody ever goes colder than stock.)

As for resistor versus non-resistor versions of the same plug, they're pretty much interchangeable.

 

[Bob Bancroft]

Here's possibly the dumbest question of the day. If a plug is exactly the same size and fits in the hole just fine can I use it no matter what manufacturer it is or the number on it? (I know I can't use a resister plug when the engine doesn't use those).

I learned the most about spark plugs, believe it or not, working on chainsaws in the 70's. The spark plug is often the first part to be assumed/replaced as a problem, or even on a regular basis in a small engine. I found very few worn out, or bad spark plugs. Once in a great while I would find a worn out spark plug. Likewise once in a great while I would find a perfectly good looking, bad spark plug. Frequently I would dig/clean/wash/ burn carbon out of a spark plug and it would work fine. Of course trying a new spark plug in a problem engine is a quick, easy diagnostic measure. But if that didn't solve the problem, I didn't sell a new spark plug. It was easy to see the different heat ranges by the thickness of the insulator.
Now in my antique tractors, which don't see any hard work, I run the hottest spark plugs I can find. They never look like they have been too hot.

 

[Geo-TH,In]

It's a bit more complicated than just fitting the spark plug hole. I remember back in my teenage days one of my buddies substituted a random spark plug in his six cylinder Ford pickup. It went about 15 miles before it threw a rod out the side. I remember it well because his truck rolled to a stop in my yard. I'm not sure, but I think the substitute plug was a wee bit too long.

There are obvious physical differences between spark plugs, particularly how far they reach into the cylinder, whether they seal with a taper or a washer, and of course the hole size. But some differences aren't so obvious: The heat range of the plug, which is determined by the length of the insulator, is critical. Too cold and the plug will foul quickly. Too hot and you can have pre-ignition which can quickly destroy your engine. There are heat range charts that will tell you how similar plugs compare, in case you want to run a slightly hotter than stock plug. (Nobody ever goes colder than stock.)

As for resistor versus non-resistor versions of the same plug, they're pretty much interchangeable.

Mark,
When I was in HS we had the Japanese invasion of motorcycles.
A kid had a 175 CC Kawasaki. At the time it was the fastest thing on 2 wheels for its size.
He put too hot of a spark plug in it to go faster.
He blew a hole in the piston directly below the spark plug.
That was a lesson. I use OEM recommended spark plugs.

 

[John T]

Here's possibly the dumbest question of the day. If a plug is exactly the same size and fits in the hole just fine can I use it no matter what manufacturer it is or the number on it? (I know I can't use a resister plug when the engine doesn't use those).

The hole size and brand and threads are only one part of the equation. The heat range is critical if too cold they can build up deposits snd foul too soon.. mfgrs list the correct plugs and there are cross reference charts for different brands.

Hope this helps there are other good gents here see if they agree with me or not

John T. Not a spark plug expert

 

[marloweg]

that Ford did not pitch that rod out the side because of the spark plug no way in hel-

 

[Blackhole49]

It's a bit more complicated than just fitting the spark plug hole. I remember back in my teenage days one of my buddies substituted a random spark plug in his six cylinder Ford pickup. It went about 15 miles before it threw a rod out the side. I remember it well because his truck rolled to a stop in my yard. I'm not sure, but I think the substitute plug was a wee bit too long.

There are obvious physical differences between spark plugs, particularly how far they reach into the cylinder, whether they seal with a taper or a washer, and of course the hole size. But some differences aren't so obvious: The heat range of the plug, which is determined by the length of the insulator, is critical. Too cold and the plug will foul quickly. Too hot and you can have pre-ignition which can quickly destroy your engine. There are heat range charts that will tell you how similar plugs compare, in case you want to run a slightly hotter than stock plug. (Nobody ever goes colder than stock.)

As for resistor versus non-resistor versions of the same plug, they're pretty much interchangeable.

He did say EXACTLY the same size. And I agree with marloweg.

 

[MarkB_MI]

that Ford did not pitch that rod out the side because of the spark plug no way in hel-

that Ford did not pitch that rod out the side because of the spark plug no way in hel-

Well then it was a heck of a coincidence. Same cylinder, a short time after the substitute plug was installed.

You're free to play mix-n-match spark plugs in your own vehicles. Let us know how it goes. YMMV.

 

[Mule Meat]

Plug heat ranges are determined by the length of the ceramic insulator inside of the metal jacket. The shorter the insulator the hotter the plug due to faster heat transfer.

So no you can not go on looks, thread size and length when installing plugs. Most engines will allow two or three plugs of different heat ranges to be used. Going above or below the recommended heat range can either make the engine run poorly or damage it. This is especially true when it comes to two stroke engines.

 

[da.bees]

It's a bit more complicated than just fitting the spark plug hole. I remember back in my teenage days one of my buddies substituted a random spark plug in his six cylinder Ford pickup. It went about 15 miles before it threw a rod out the side. I remember it well because his truck rolled to a stop in my yard. I'm not sure, but I think the substitute plug was a wee bit too long.

There are obvious physical differences between spark plugs, particularly how far they reach into the cylinder, whether they seal with a taper or a washer, and of course the hole size.

that Ford did not pitch that rod out the side because of the spark plug no way in hel-

I couldn't swear that rod exited block in Mark's front yard or in hell but I can attest to some pistons that were cracked by plugs protruding too far into cylinder. I suspect the rod was thrown as result of piston cracking when it struck spark plug. We could argue that plug didn't cause it but it could have helped the teen's lead foot cause it.

 

[kencombs]

Don't forget the odd ones 40s chevy 10mm and fords 18mm tapered seat.

 

[cumminstinkerer]

how about newer ford 10mm tapered seat

 

[Texasmark]

Mark,
When I was in HS we had the Japanese invasion of motorcycles.
A kid had a 175 CC Kawasaki. At the time it was the fastest thing on 2 wheels for its size.
He put too hot of a spark plug in it to go faster.
He blew a hole in the piston directly below the spark plug.
That was a lesson. I use OEM recommended spark plugs.

I just bought a 2004 Kawasaki Prarie 360 ATV 4 wheeler with 500 or so hours, at my age.....but the reason was running around the hood, I needed something better than a repurposed riding mower. That little sucker is nothing but guts and glory. Even in the low range of 2 forward ranges, that sucker will pull the skin away from your eyeballs. It does have a peuliar spark plug and I replaced it with the OEM. Some times I embarass my self to my self for goosing it and shutting it down immediately due to the acceleration.

Moving on:
I bought a new boat with a 125 Johnson V4 outboard and kept it for 7 years......absolutely no problem rig. Don't remember the reason but sold it to a worker buddy. He had a cabin on the Intercoastal water way in S. Tx. so he said he loaded it up with gear and attempted to drive the 40 miles at full throttle with an overloaded boat to the hide out as he told me at work the next week. Well that was his plan...he didn't get to the end of his journey because he burned a hole in one of the pistons just under one of the OEM spec'd spark plugs. He didn't realize that if he was going to go that distance at full throttle with an overloaded boat he should have dropped a couple or so inches from the prop so that the engine could operate in it's normal RPM band. That was in the 1979 time line and instruments on regular folks boats were non existent......at least in my boating circles.

So, life is full of stories...some we win some we loose.