Spark Plug 101

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spencer13cat

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How many guys go to their equipment dealer, ask for a plug for their "XYZ" and get something other than the listed plug in the owners manual????? How many never check??????? I hope this thread helps somebody. Sorry to bore you experts. I ran into this for years while restoring cars too.

I think very few outdoor equipment or car dealers for that matter really understand spark plug differences. I kept having problems with my Stihl KM110 (4-mix trimmer/saw). I looked in the manual the other day and noticed it was the wrong manual. Guess what, a KM85 takes a plug with a different thread size than a KM110. They gave me the wrong manual 4 years ago. I had no way of knowing the "correct" plug. I finally get that straightened out and realize They gave me the wrong plug again!!!! I spent a few evenings trying to figure all this out. Including plugs for my three Stihl saws.

According to Stihl(I talked to their tech support three times this week, they confirmed all this with the Stihl Product Manager), the only approved plug for their chain saws is Bosch WSR6F
I know guys have used all kinds of other plugs with success as have I, but for the average guy this is the plug. The reason is The "6" in the Bosch number represents the heat range of the plug. And regardless of what "Billie-Bob" at the local parts store tells you, 6 in Bosch is different than a 6 in Champion, or ngk, or Denso, etc. Most foreign plugs the lower the number the hotter the plug. Domestic brands are the opposite.

All this makes sense after my Stihl Dealer gave me an NGK CMR7H instead of a Bosch USR7AC for my KM110. Guess what????? Stihl says the correct plug is NGK CMR5H or Bosch USR7AC. The early models (like mine) came out with CMR6H originally...... So I ended up two heat ranges too cold........I wonder why I kept fowling plugs????????? I think this is an easy thing to over-look because spark plugs seem so simple. They can cause a lot of problems.
 
There are other approved spark plugs for Stihl saws... example - the NGK BPMR7A. Use this or the Bosch WSR6F and you'll have no problems. Stihl's saws ship with either. They also used the Champion equivalent a few years ago.

In addition to the heat range and physical size, the non-screw top is specified (and needed).

If you're fouling plugs in your 110, look into your fuel/mix as well as your plugs.
 
Last edited:
Wrong Plugs? not surprised...

Lakeside53 said:
There are other approved spark plugs for Stihl saws... example - the NGK BPMR7A. Use this or the Bosch WSR6F and you'll have no problems. Stihl's saws ship with either. They also used the Champion equivalent a few years ago.

In addition to the heat range and physical size, the non-screw top is specified (and needed).

If you're fouling plugs in your 110, look into your fuel/mix as well as your plugs.
Click to expand...

Yes there are other approved spark plugs for Stihl but for the 4mix trimmers specificly my FS-90R Stihl does not designate one; not in the manual and not when i called. But of course my local service dealer used the NGK CMR7H and yes this is two heat ranges too cold; result, I had allot of carbon on the exaust valve and combustion chamber. I contacted NGK to ask what their equivalent is for the Stihl designated USR7AC and low and behold it is the CMR5H, yes the CMR7H is two heat ranges too cold. I called another service shop and they were using an NGK plug that was too hot. This problem seems to be pervasive amongst service/dealers and i would suggest double checking them or doing the service yourself. I use a very well measured mix of 50:1 all the time, every time and only use Motul 800 2t as my oil. This is an Ester based true synthetic that can be considered a "designer oil" for very high rpm air cooled 2 stroke engines. Stihl Germany considers it to be superior to their full synthetic offering I would suggest it be used in 4mix engines as I have had no carbon buildup with the correct plug and this mix.
 
Paging GAS... Anyway, I wonder what Lake meant by the non screw types are needed. I bought a bunch of BPMR7A plugs and they all unscrew at the top. Seem to run fine. I do wonder why they unscrew though. Any guesses, smart folks on here? They make those without the screws on the top?
 
WildnCrazyGuy said:
Paging GAS... Anyway, I wonder what Lake meant by the non screw types are needed. I bought a bunch of BPMR7A plugs and they all unscrew at the top. Seem to run fine. I do wonder why they unscrew though. Any guesses, smart folks on here? They make those without the screws on the top?
Click to expand...


GEEZ ALERT!

In the old days....lots of small engines had a fork/loop terminal on top...you unscrewed the top, put the terminal on...screwed the top back on
 
WildnCrazyGuy said:
Paging GAS... Anyway, I wonder what Lake meant by the non screw types are needed. I bought a bunch of BPMR7A plugs and they all unscrew at the top. Seem to run fine. I do wonder why they unscrew though. Any guesses, smart folks on here? They make those without the screws on the top?
Click to expand...

I have 3 Yamaha 4wheelers that you need to unscrew the tip. Not sure why they are like that. Maybe just a different way to grab onto the plug.:confused:
 
mictrik said:
Yes there are other approved spark plugs for Stihl but for the 4mix trimmers specificly my FS-90R Stihl does not designate one; not in the manual and not when i called. But of course my local service dealer used the NGK CMR7H and yes this is two heat ranges too cold; result, I had allot of carbon on the exaust valve and combustion chamber. I contacted NGK to ask what their equivalent is for the Stihl designated USR7AC and low and behold it is the CMR5H, yes the CMR7H is two heat ranges too cold. I called another service shop and they were using an NGK plug that was too hot. This problem seems to be pervasive amongst service/dealers and i would suggest double checking them or doing the service yourself. I use a very well measured mix of 50:1 all the time, every time and only use Motul 800 2t as my oil. This is an Ester based true synthetic that can be considered a "designer oil" for very high rpm air cooled 2 stroke engines. Stihl Germany considers it to be superior to their full synthetic offering I would suggest it be used in 4mix engines as I have had no carbon buildup with the correct plug and this mix.
Click to expand...

Mate motul 800 has always been designed for 2 stroke motorcross bikes which are water cooled not that it makes any difference. But your just wasting your money running it in a saw, I run the stihl 2 stroke oil in all my saws it's costs me like $25 aus for 4 litres, the way I look at it if stihl makes the engines don't you think they would know what oil to run in them? Off topic but I know about oil due to working in a Honda motorcycle store and Honda did a study on oils and they found they had more engine failures In there cbr1000 with people running motul oil as opposed to people running the geniune oil Honda hp4.
 
Rookie1 said:
I have 3 Yamaha 4wheelers that you need to unscrew the tip. Not sure why they are like that. Maybe just a different way to grab onto the plug.:confused:
Click to expand...

I've had sleds like that, on one I swapped out the boot (it needed one anyway) for one that took regular top plugs. I still run the same plug, just don't screw the top off.
 
Thanks guys. This place is a wealth of info... I don't unscrew my tops, just wondered why they did that. Who woulda thunk that they made different size boots for the same spark plug. I woulda thought the opposite. They woulda made different size spark plugs for different applications. Wait, they do. :dizzy:
 
I always try and run the same plug that came from the factory, my Polaris snowmobile came new with champion plugs, if I run ngk I will have fouling trouble, even though the cross reference chart says its the correct plug.

I'm thinking sometimes when you cross reference to a different brand, the heat range may not be exactly the same. I don't know how else to explain why else this would happen (assuming the catalog is correct)

Maybe someone here is a spark plug expert and could shed some light
 
All of my 4-mix equipment has come with CMR6H plugs and my saws have come with both the Bosch WSR6F or the NGK BPMR7A plugs.

I keep a box of both the Bosch and the NGK saw plugs and 2 boxes of the 4-Mix plugs because if it even seems like the thing is having a hard time starting, I change the plug and the will normally solve the issue.

Filters and plugs solve a lot of issues and keep me in the field v. in the shop.

I need to look into this deal with the CMR5H being the new standard for the 4mix equipment.
 
I have run both the Bosch WSR6F and the NGK BPMR7A in my 028 AV Super - they are both listed in the service manual. Both seem to run great and I don't notice a difference in performance, ease of starting, etc.

However, the screw-top terminal does seem loose sometimes.

Using sparkplugs.com - the BPMR7A comes in two varieties (otherwise identical it seems):

# 4626 - screw top terminal
# 6703 - solid terminal

The Bosch WSR6F shows two varieties as well (both have the screw-top terminal):

# 7813 - "Super Start Spark Plug" - nickel plated rolled threads
# 7547 (this is the one stocked at my local Stihl dealer, and "the only one they run in all their Stihl 2-smoke saws") - for some reason this one is listed as "For Low Power Engines"

The Bosch plug lists a nickel-chromium ground electrode, and an interference suppression resistor. Do either of those features make squat of a difference? I wouldn't think there'd be freq interference on a chain saw, from what and to affect what I don't know (the only radio a chain saw should have is the sound of it's screaming 2-smoke engine!!).
:greenchainsaw:

Anyone have any experiences between any of the above-listed plugs, for durability or performance?

Anyways, I think next time I'll try some of the solid terminal NGK's, just for giggles.
 

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