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Can I shim a spark plug for fit?

Use this area to post problems that may arise, if you could use some help in figuring out what is wrong with your engine and what needs to be done to achieve top performance.


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Old 08-15-2013, 02:54 PM
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Default Can I shim a spark plug for fit?

I just purchased a NGK Iridium BPR7HIX spark plug for my first build. It's a two stroke engine. I have the cheapo chinese oem plug with the washer. Can I use another washer to shim for fit? By adding the extra washer, it appears the plug will be the right size. Thoughts?
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Old 08-15-2013, 03:18 PM
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Default Re: Can I shim a spark plug for fit?

Not following you. When you say "will the plug be the right size" are you talking about length of the electrode, or 'reach' as it is called. Just install your new plug as usual, slowly rotate the crankshaft through one revolution and check that you have no piston to spark plug interference. You shouldn't have any with that plug.

Tom
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Old 08-15-2013, 03:21 PM
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Default Re: Can I shim a spark plug for fit?

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Originally Posted by 2door View Post
Not following you. When you say "will the plug be the right size" are you talking about length of the electrode, or 'reach' as it is called. Just install your new plug as usual, slowly rotate the crankshaft through one revolution and check that you have no piston to spark plug interference. You shouldn't have any with that plug.

Tom

^^^ This.^^^

Two washers on the plug will cause it to leak compression. Even though it's got a longer nose, the plug should work just fine --- many of us have 'upgraded' to this plug.

Good luck!!
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Old 08-15-2013, 03:36 PM
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Default Re: Can I shim a spark plug for fit?

slowly rotate the crankshaft through one revolution and check that you have no piston to spark plug interference


I tried this and the piston hit the plug.
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Old 08-15-2013, 03:45 PM
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Default Re: Can I shim a spark plug for fit?

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Originally Posted by runkmaster View Post
slowly rotate the crankshaft through one revolution and check that you have no piston to spark plug interference


I tried this and the piston hit the plug.
Now you have a problem. Not a common one but one you can fix.

Take that high priced plug back to the store and ask to compare it with a standard B6HS NGK plug. I'm not well versed on the iridium plugs or their sizes and heat range except what I've read about them and there really is no advantage to them over regular plugs except they will last a long time. That doesn't equate to better performance or a "hotter spark", it just means deposits don't build or stick to them as readily as other plugs.

Check the thread length between the two plugs and the reach of the electrode below the threads. My guess is you'll find the standard B6-HS plug will look different, shorter. If so, buy it and your interference problem will be solved, me thinks.

Tom
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Old 08-15-2013, 04:24 PM
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Default Re: Can I shim a spark plug for fit?

Thanks boss this really helps!
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Old 08-15-2013, 07:00 PM
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Default Re: Can I shim a spark plug for fit?

Spark plugs with an "R" in the prefix before the number are resistor plugs.
With these weak sparked ignitions I personally would'nt use a resistor type plug, or a resistor type plug wire. Others may not agree.

TiM
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Old 08-15-2013, 08:08 PM
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Default Re: Can I shim a spark plug for fit?

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Originally Posted by timboellner View Post
Spark plugs with an "R" in the prefix before the number are resistor plugs.
With these weak sparked ignitions I personally would'nt use a resistor type plug, or a resistor type plug wire. Others may not agree.

TiM
I agree and the reason why I always say that the stock (kit supplied) spark plug wire is better than what you'll get at most auto parts stores today which will be resistance wire. The common myth says to replace the wire and the plug boot. I maintain that the boot should be replaced but the wire, being metallic core (conductor) is better than the resistance wire. Do what you feel is best but if you doubt me check the wires with a good ohm meter and make your own decision based on the resistance you'll read.

Tom
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Old 08-15-2013, 09:12 PM
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Default Re: Can I shim a spark plug for fit?

FWIW, I run a BPR7HIX and there was a huge difference in power delivery. Just my experience.

If it's hitting, you've got some pretty good compression, so like 2door said, get a B6HS.
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