# Exact Spark plug necessary?



## drumsonly2002

Installed a new plug on the Yamaha 8-28. The stock plug was a NGK BRP 4HS. The store had only had an NGK 6HS. The new plug seems to work fine. The machine now starts a lot better. The old plug was toast.

Does the 4 vs 6 make any difference? Any reason for concern? Thanks for any advice.


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## drmerdp

In short yes it does.

The long of it is, the number designates the heat range. For Ngk 6 is colder then 4. Compression ratio and operating conditions dictate an optimal heat range for a spark plug. A colder plug will dissipate heat faster between combustion cycles. 

Too hot of a plug can add to the risk of pre ignition or knock. Too cold can lead to misfires or fouling. In the automotive performance world I sometimes drop to a colder plug when power adders are introduced.

For your snowblower, it’s best to keep what’s specified.


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## Yanmar Ronin

Here's the NGK plug chart: NGK Plug Chart

Going from a 4 to a 6 might not be the end of the world... a picture of the old plug as removed would be helpful in determining if a cooler plug would be detrimental, or perhaps beneficial.

Couple of my machines I've found that the Mfg.'s recommendation isn't quite right for my use anyhow... fine tuning ftw.

Anyhow it's not gonna blow up overnight, no fear.


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## JLawrence08648

Drumsonly2002 - Both drmerdp and Yanmar Robin have it right. So much so, and complete, no one can dispute what they say. My three cents.


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## drumsonly2002

That is great information. Ordered the proper plug and got some spares to boot. For the cost of plugs, having spares on hand, no more searching around when a plug is needed. I did not maintain my Honda and it packed it in. Going to take care of the Yamaha as it is a hard blower to find in my area, and will work when needed. Thanks for the help and good advice.


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## stromr

Fortunately the 6 is colder than the 4 so as was stated you might eventually foul the plug. If it was too hot you could have done engine damage. The reason it starts easier now is just because the new plug is better than the old one, gap correct, electrode not eroded, etc. I've had plugs last for years and other times be wasted in less than a year. Sparkplugs, oil, etc. are cheap, change them often!


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## sscotsman

Here is my recent experience with the wrong plug:

Engine stalls when warm. - Engines - RedSquare Wheel Horse Forum

It took me 2 weeks, and $28 for a coil I didnt need, just to find out it was the spark plug. 
So yes, its important to have the correct one, and it will now be the *first* thing I check! 
Lesson learned..

Scot


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## Yanmar Ronin

You guys think my o-ring addiction is bad, you should see my spark plug rack... :blink: :icon_smile_big:

Protip: Avoid poorly packaged plugs. Some places throw 'em nekkid into a baggie and then loosely into a box for shipping. They can be damaged this way, without visible evidence. Or often the ground electrode gets bent out of shape, requiring much fiddling to gap properly. The fiddling can weaken the attachment point compromising the integrity of the plug.

Always go for boxed plugs, second choice blister pack.


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