Check the sparkplug application chart for compatibility to your engine. This should be available at the store where you bought these plugs, or any store selling these plugs.

The two vehicles in question - details please.
It pays to have this detailed information on spark plugs - they may or may not be interchangeable.. The heat range is the same.
Check your owner's manual. But you may end up going to the dealer for this info.

You cannot use the CR9EK plugs. The different placement of the electrode is important for an engine that requires that extended reach because of swirl patterns of the air/fuel ratio.

This is pretty complicated as far as how it actually works, but when a spark plug fires in the cylinder, the spark is blown into a horseshoe shaped ball by the turbulance. It's important to picture that this arc will be 1/4 to 1/2 an inch in diameter. Changing the length of the electrodes changes the location of this arc ball and will affect the burn in the cylinder.

OK gents.
The CR9EK was used for '03 Kawasaki 1200STXF (personal watercraft) - it has a 4 stroke 4 cylinder 1200 cc DOHC 4-valve per cylinder fuel injected engine. It's sold.
The CR9E is used for '05 Kawasaki Brute Force 750 (All Terrain Vehicle) - it has a 4 stroke V-twin 750 cc SOHC 4-valve per cylinder carbureted (Keihin 34 mm) engine. It's my new WORK machine (hope my wife reads this).
I'm trying to find out if CR9EK can be used on the ATV.
The NGK spark plug chart does not show them as interchaneble. The dealer parts guy says the same thing. While the CR9E looks like an ordinary plug (copper and single electrode), the CR9EK looks quite fancy (laser cut platinum tip and double electrode, etc). To my untrained eyes, the CR9EK may give more complete combustion and last longer w/o fouling as well. I just hate to waste the CR9EK which I bought 2 years ago for spare.
Hey John,
You seem to know A LOT about how plugs work. Could that altered flame front cause pre-ignition, hole on piston or any other harmful consequence?

Get off your wallet and buy the right plugs. Better safe than sorry.

Mister H said,,"You seem to know A LOT about how plugs work. Could that altered flame front cause pre-ignition, hole on piston or any other harmful consequence?"

I do know a lot about how plugs work, thats part of being able to look at an engine analyzer and watch secondary ignition waveforms and understand what is happening and whether or not what I am seeing on the scope relates to a customers description of a vehicles symptoms.

Pre-Ignition, definately not. By description pre ignition is a burn of the fuel that occurs from some source other than the spark occurring in the gap of the plug. But understand, an overheated plug, or a plug of incorrect heat range could initiate the burn. Thats one of the reasons for being very carefull to use the correct plugs all of the time.

As far as creating holes in pistons, or some other problem the only way your going to find that out would be to find someone that used the incorrect plug and suffered a failure, or attempt to find out on your own. If you can "afford" to risk it and try, go right ahead otherwise you cannot afford to do anything but purchase and install the correct plugs.

BTW there are a lot of fancy spark plug designs. The substitution of a fancy spark plug IMO has never proven to substantially improve an engines performance over it's original design. In fact its quite likely to impair performance because the design engineers have normally already tested extensively to find the best design for a particular engine. Thats how the recommended plug gains that status.

John,

I understand not using the right application plug. In newer cars with electronics, I've heard you shouldn't even use a different BRAND plug even if they are supposed to be interchangeable.

What about platinum and double platinum plugs? Are they worth the extra cost or are they more of a gimmick? Do you really get more life from them? I have known people who have used the standard $1-2 plugs for over 100,000 miles and are still working well. I have also known people whose platinum plugs were shot in the same period of time. Some of those platinum plugs can go for ten times what a standard plug costs. Are they worth it?

There are some brands of plugs that I wont use in a customers car.

I will not use Champions. I try to never use Bosch, especially Bosch Supers. As far as platinum go, there is clearly some out there that last many times longer than conventional plugs. I use Autolites in almost anything that they directly list. AC's for any GM, and NGK's for Asians.

I have seen people run conventional plugs for way too long on a number of occasions. By the time they encounter a failure with todays ignition systems they end up needing wires, coil packs and occasionally even a module simply because they didnt service the plugs when they should have and allowed the ignition system to be stressed to the point of failure.

I avoid Champions as well these days, except on my lawn mower. I've been replacing plugs on my cars and toys for last 30 years and had two occasions of breaking the ceramic body of the plugs - both were Champion brand (I used them because it was OEM part). Ever since, I've always cross-referenced and tried to use other brand plugs.

Interesting. I thought Champions were good plugs. Has anybody ever used this website??

Here is a link that might be useful: Spark Plugs

I will buy 1 or 2 of those plugs they are the stock plug for my Yam WR250