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The MR7F is: 10mm diameter, 3/4" reach, 5/8" hex, projected tip, gasket seal, solid terminal top

The CR8EIX is: 10mm, 3/4", 5/8", gasket with a loose nut on a stud terminal and one heat range hotter

Both plugs are designed for and should be gapped at .032"

While there is no 'upgrade' plug listed for the MR7F, the CR8EIX can be replaced with the CR8E, CR8EB, CR8EK, CR8EKB and the CR8EIB-10

Of course it will create spark and the engine will run, but it would take a discussion with the engineers who designed the engine and the ignition system to know why they selected the spark plug they did.

The question is; Why do you want to run a plug other than what the manufacturer specified? What are you trying to achieve? Are you trying to overcome a specific problem? As long as the engine builder (you) accept responsibility for your choice of the spark plug's affect on (among other things) the ignition system (strength and duration of spark and life of system components), engine (performance, combustion pressure, combustion temperature and valve life) and choice of fuel necessary for acceptable combustion characteristics, you can run any damn plug you want to!

I have experimented with spark plugs, fuel, cylinder compression, valve timing, spark timing, ignition systems and pretty much everything else in search of performance and fuel economy. Rewarded by some and punished for others. One of my biggest mistakes was using 100 octane fuel in my CRF250 which was designed for 93 octane. It caused a loss of power, the exhaust ran exceedingly hot, fuel consumption increased and the fuel was $20 a gallon with a 5 gallon minimum. Through trial and error I determined the highest octane I could use without a loss of power was about 94.5. Since then I have settled on 91 octane non ethanol pump gas which is available at one of our local gas stations currently for about $3.30 (changes daily) a gallon.

If you are satisfied with your choice of spark plug, then it was the right one, but what works for you may not be the right choice for someone else. It's a personal choice.
 

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Discussion Starter #3
Thanks Latebird, I got that number off a couple of threads in this site. Was just looking for info. I just got off phone with NGK and tech said CR7EIX is better match. It does not have the fixed stud though. You ever hear of stud coming loose? Haven’t made decision yet, just researching.
 

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The CR7 is the same heat range which makes it a better match

I have not seen a stud come loose as it is the other end of the center electrode, but I have seen the center electrode break and the stud piece come out (see pic) - I was checking out a high speed miss on a Honda CRF70 - one of the first things I do is put in a new spark plug - pulled on the cap and it came off awfully easy - looked at the plug and there was nothing but a hole in the end of the porcelain - looked in the plug cap and there was the threaded end of electrode. I took a pic of it as it was kinda odd. So, I guess anything can happen.
 

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Just my $.02..I always put anti-seize on spark plug threads. Just a smear..And usually some dialectric grease in the boot.

Was a mechanic at a Saab dealer in early 90's..I think a new head was about 3K...in 1991.
 

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Just my $.02..I always put anti-seize on spark plug threads. Just a smear..And usually some dialectric grease in the boot.

Was a mechanic at a Saab dealer in early 90's..I think a new head was about 3K...in 1991.
I agree anti-sieze on the threads and die-electric on the terminal.

Side not about Saabs. Growing up there was a Saab/Volvo dealer in town. Well long story short my mother bought a Volvo and that became my first vehicle. Having some years and miles on it some tlc was needed. Coincidentally my buddy owned a Dodge Omni same year and approximate mileage. Both our cars needed new alternators at the same time. So we decided to help each other out. So we both went to the NAPA store and ordered or parts. His alternator was on the counter for the price of $30 bucks. Mine was going to take a couple days and cost $180 bucks. That was a lot of money for a teenager!! Well that was last foreign vehicle I ever owned!!!
 

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The MR7F is: 10mm diameter, 3/4" reach, 5/8" hex, projected tip, gasket seal, solid terminal top

The CR8EIX is: 10mm, 3/4", 5/8", gasket with a loose nut on a stud terminal and one heat range hotter

Both plugs are designed for and should be gapped at .032"

While there is no 'upgrade' plug listed for the MR7F, the CR8EIX can be replaced with the CR8E, CR8EB, CR8EK, CR8EKB and the CR8EIB-10

Of course it will create spark and the engine will run, but it would take a discussion with the engineers who designed the engine and the ignition system to know why they selected the spark plug they did.

The question is; Why do you want to run a plug other than what the manufacturer specified? What are you trying to achieve? Are you trying to overcome a specific problem? As long as the engine builder (you) accept responsibility for your choice of the spark plug's affect on (among other things) the ignition system (strength and duration of spark and life of system components), engine (performance, combustion pressure, combustion temperature and valve life) and choice of fuel necessary for acceptable combustion characteristics, you can run any damn plug you want to!

I have experimented with spark plugs, fuel, cylinder compression, valve timing, spark timing, ignition systems and pretty much everything else in search of performance and fuel economy. Rewarded by some and punished for others. One of my biggest mistakes was using 100 octane fuel in my CRF250 which was designed for 93 octane. It caused a loss of power, the exhaust ran exceedingly hot, fuel consumption increased and the fuel was $20 a gallon with a 5 gallon minimum. Through trial and error I determined the highest octane I could use without a loss of power was about 94.5. Since then I have settled on 91 octane non ethanol pump gas which is available at one of our local gas stations currently for about $3.30 (changes daily) a gallon.

If you are satisfied with your choice of spark plug, then it was the right one, but what works for you may not be the right choice for someone else. It's a personal choice.
How often should these plugs be changed on these units? Is it by the hours or mileage?
 

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It can be done by routine or performance; routine, once each year, every oil change, every air filter change, after each outing - by performance, when it gets hard to start, starts misfiring, becomes fouled, fuel mileage decreases dramatically.

I ran the original spark plug in my CRF race bike for 8 years before I had any problems - the plug is $45 so I only change it when it is needed - after 8 years, the only problem I experienced is the engine would stall when chopping the throttle going into a high speed - it would restart as soon as the clutch was engaged, but I couldn't put up with the stalling - a new plug solved the issue - the second plug has been in use for 7 years, so I may expect to need a new plug soon, but I will wait till it starts acting up - as soon as it gets hard to start, misfires under acceleration or starts stalling on throttle closing, I will put a new plug in it.
 

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It can be done by routine or performance; routine, once each year, every oil change, every air filter change, after each outing - by performance, when it gets hard to start, starts misfiring, becomes fouled, fuel mileage decreases dramatically.

I ran the original spark plug in my CRF race bike for 8 years before I had any problems - the plug is $45 so I only change it when it is needed - after 8 years, the only problem I experienced is the engine would stall when chopping the throttle going into a high speed - it would restart as soon as the clutch was engaged, but I couldn't put up with the stalling - a new plug solved the issue - the second plug has been in use for 7 years, so I may expect to need a new plug soon, but I will wait till it starts acting up - as soon as it gets hard to start, misfires under acceleration or starts stalling on throttle closing, I will put a new plug in it.
Good to know, thanks.
 

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I decided for the 450 HO to go with NGK BKR7EIX(fixed stud) and the 570’s getting NGK CR7EIX(threaded stud?). Both Iridium and were recommended by NGK tech. Do you recommend using anti-seize?
so your 570 likes the new CR7EIX ?
 

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There is a hell of a difference between a BKR7EIX and a CR7EIX! The "B" plug is 14 mm diameter threads and the "C" 1s 10 mm diameter - they are not interchangeable and cannot be installed in other than the correct size hole.

The BKR7EIX is interchangeable with a BR7EIX, but the BK has a 5/8 hex and a solid terminal and the BR has a 13/16 hex and a threaded terminal with a removable terminal nut or a terminal nut can be installed if needed - you need a special thin wall plug socket to install and remove the STD 13/16 hex plug in the plug well of the engine. The "C" series of plugs also have the 5/8 hex.

Anti seize is not recommended - the plug's threads are anodized (unlike Champion's plain steel and others with black oxide coating) - the anodized threads do not corrode and do not gall to the aluminum of the head like plain and oxide coated threads can.
 

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Latebird, i've never seen a plug get this hot on threads, you ? in all my sleds, dirtbikes i have never seen discoloration on threads, these 570 sure run hot !
 

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Discoloration on the threads? Black on the first two or three threads? A pic would be helpful.
 

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Discoloration on the threads? Black on the first two or three threads? A pic would be helpful.
shit sorry pic did not load, my bad .
140705
 

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Looks like good color to me. The color on the threads would be oil gummy gunk. Shoot it with starting fluid, it should wash off for the most part.
 

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Looks like good color to me. The color on the threads would be oil gummy gunk. Shoot it with starting fluid, it should wash off for the most part.
yeah its running good fuel mixture wise but pic not really showing that its heat discoloration on threads, its not gunk . Not overly worried just wanted input. 👍
 

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Yeah - it pretty much looks like most every old NGK I've pulled out of every engine, but as close as I can examine the plug, it looks like it was under torqued - the crush gasket does not look like it has been crushed, but I can only see so much in a pic.

NOTE: NGK plugs are cadmium plated and do not require anti-seize like the plain steel Champion plug

140710
 

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Yeah - it pretty much looks like most every old NGK I've pulled out of every engine, but as close as I can examine the plug, it looks like it was under torqued - the crush gasket does not look like it has been crushed, but I can only see so much in a pic.

NOTE: NGK plugs are cadmium plated and do not require anti-seize like the plain steel Champion plug

View attachment 140710
Tks, it was the Original plug, and you are right it was not very tight, but not loose either.
 
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