# 4 Sroke Sparkplug Hole Threaded ?



## markuk25 (Apr 27, 2007)

Hi , 

I was was wondering if anyone could help with this .

I have a Makita EM4251 Petrol Brushcutter / Strimmer .

The sparkplug blew out after a repair job didnt work to rethread the sparkplug hole .

The sparkplug used in the model is a Spark plug CMR6A (C type), NGK .

I was wondering if i could bore out a larger threaded hole and use a sparkplug with a slightly larger circumference , as i cant see anyway to fix this now because theres basically nothing left of the old threads on the existing sparkplug hole .


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## Changeling (Nov 7, 2005)

There is an engineering procedure called "Hele-coiling" (unsure of spelling) The procedure involves oversize rethreading of the hole and then something which resembles a spring is screwed into the hole and the old spark plug should then fit.


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## crazijoe (Oct 19, 2004)

Changeling is correct. There is a company called Heli-coil that makes spark plug repair inserts that actually installs new threads in the hole. Part number 5334-14. Here is a link to one sold in the US. You should be able to purchase this locally at a hardware store.
http://www.sjdiscounttools.com/hel5334-14.html


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## markuk25 (Apr 27, 2007)

Thanks for the information .

I am going to try the HeliCoil method .

So it's basically just threading a bigger bore , then screwing in the threaded HeliCoil and then seating the Spark Plug inside the HeliCoil .

That sounds pretty simple and longlasting .

I think the only problem will be finding a HeliCoil Kit in the UK for the small CMR6A (C type), NGK Spark Plug .

Most of the Kits i've seen online are for Cars and Bikes , I'm going to look at some UK Lawnmower Parts shops .

P.S I was looking at the ad at the bottom of this post http://www.timesert.com/
They look pretty much the same

Thanks , Bye :wink:


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## Midnight Tech (Dec 31, 2001)

Make sure you put some red threadlocker on the outside of that Helicoil to help secure it in place...you may be able to use one for a metric bolt instead of one for a plug.


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## kendallt (Feb 21, 2007)

First choice ould be to try the helicoil, but if that fails or can't be done for whatever reason, I -have- done as you proposed and tyhreaded the head for a larger plug.

Critical points are the reach of the plug, and the heat range, make sure that you have enough room outside the engine also.

If that's a flathead type engine, you have very little to worry about, if it's valve-in-head, make sure you won't be cutting into a valve seat, and that the electrode of the plug won't interfere with a valve.

My old lawnmower ran with a pipe reducer in it as a plug 'adapter' for 5 or 6 years after my son was nice and tuned it up for me.

Ken.


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## newnewbie (Apr 7, 2007)

GOD SAVE THE HELI-COIL!ray: :grin:
Personally i cant praise the heli-coil enough. the clever little thing has saved me and my past employers several tens of thousands of dollars. heres how it works;

(1)you tap the striped hole with heli-coils special sized tap ( if you use a powerfule vacume or compressed air to pull or push the chips out while you run the tap. you need not even remove the cylinder head.)
(2) screw the coil onto the heli-coil mandrell and then screw the works into
the hole.
(3) remove the mandrell and snap the "drive tang" off the coil.
thats it ,your done.

the coil kind of shrinks as you screw it in and then springs out as you stop.
they never leak and i've never heard of one failing,and if need be they are easily removed. they can be had in almost all sizes metric and yankee alike (my local retailer only carries the most popular ones) thier special stainless steel alloy makes them compatible with virtually all materials. I/we have used them on head bolts, water pumps,refrigeration compressors, manifolds, boilers, forklifts, boat hulls and motor cycles.

another good brand of threaded insert is "keen-sert"

good luck mate ,let us know how you fare.
mike


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## trailrider894 (Jan 2, 2008)

and next time you are pulling spark plugs in and out don't force them just when you screw them in just let them go until the are hard to turn then stop. and also put some anti seize compound and put it on the inside on the threads of the engine. and also put some engine oil on the threads of the spark plug and that should at least make you thread's last longer.


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## kerpal (Jul 4, 2008)

markuk25 said:


> Hi ,
> 
> I was was wondering if anyone could help with this .
> 
> ...


I don't think you have to put a larger thread hole. Just have it as it is, and the spark plug will just fit in it. Why do you want to have a bigger thread hole anyway? What difference will it make?


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## Fix (Sep 23, 2008)

If by chance you are not able to find the right Heli-Coil, find someone with a Lathe ('er actually, you COULD get by with just a hand drill I suppose). Yrs ago, before Heli-Coils, we'd find the right size Iron Pipe Bushing, Drill & tap the I.D. for the correct Spark Plug Thread, then drill & tap the Head for the right size Pipe Thread (I.E.: 1/8"npt, 1/4"npt, 3/8"npt, etc.)
It's a tad more involved than I mentioned, but I think you get the idea. IF you can't find the Heli-Coil you need & DO require more info........... contact me.


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