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Discussion Starter #1
Good day all,

I have a 60 plate Alfa giulietta 1.4tb that has blown the spark plug out and stripped the threads on the cylinder head. Now I'm pretty mechanically skilled so I should be ok to use one of the many Heli coil kits on the market to repair it, the problem I'm having is trying to determine which size kit I will need for the size of spark plug I have. The diameter of the thread on the spark plug is 12mm so does that mean I would have to buy an M12 kit to make the hole bigger and insert the helicoil or would it be the M14 kit? I have tried googling and I can't get a definitive answer so if somebody knows I would be very grateful for your help
 

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I'm pretty sure you get the kit for the size you want the hole to end up.. but now you've made me doubt myself..

Do you have the head off? otherwise you need to break the driving tang off the thread insert, which will only end up in the bore if doing it in situ, not to mention the swarf from reaming the hole out to the correct size.

Otherwise, there are mobile companies who could do this for you.
 

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A 'timesert' would be better as there isn't a tang to break off. You need the one for a 12 mm thread (the tap is larger) but check the size, and the pitch and you could confirm with the seller. Also if you try without removing the head secure the tap just in case it falls into the chamber! Ive seen it but not done that mistake. Best to remove the head if you can.
This is the type even though the listing calls it a helicoil.
https://www.ebay.co.uk/itm/Spark-Plug-Rethread-Kit-Reamer-Tap-Thread-Repair-M12-Cylinder-Head-Helicoil-5874/292929488819?hash=item4433f547b3:g:olYAAOSw7cZcRwqu

This how a timesert is installed, if you have the chance of this being done professionally have it done.

 

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I had this happen on a twin spark, it is caused by either leaving a plug loose accidentally or overtightening.

I bought a kit from sparkplugs.co.uk to repair the thread. It consists of a single tool which cuts a slightly larger hole and then rethreads it. You then fit a collar to the spark plug and screw the lot in once you have cleaned all of the swarf from the hole. (I used a plastic tube attached to the vacuum cleaner) there is no need to remove the cylinder head.

This was about 12 years ago though, so they might not sell the kit but I am sure you could find similar.
 

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Discussion Starter #6
So armoore if the spark plug thread is 12 mm across the thread do I use M12 tap to make the hole bigger for the timesert to go in? I'm so confused😩😩
 

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Threads are specified by the width of the hole with the material left. So to put a 12 x 1.25mm thread in you would ideally drill a 10.75mm hole and the tap would remove the other 1.25mm. I would imagine the timesert outer thread will be at least 14mm to take a 12mm thread, if the threads are truly synchronised that would give a 2mm wall thickness. Not a lot but strong enough so probably need to drill around 12.75mm to get the 14mm thread insert to provide the 12mm thread for the plug. I am only guessing at the available thread sizes but I'm sure the kits will provide the necessary sized drills and taps?
 

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As above I suffered similar with a twinny in my old 156,used one of those collar type kits matched to the spark plug size in my case was m10 I seem to recall. The tap required comes in the kit,some high strength stud lock was used when screwing collar in. Never gave any issues far better than helicoil.
 

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So armoore if the spark plug thread is 12 mm across the thread do I use M12 tap to make the hole bigger for the timesert to go in? I'm so confused😩😩
12 mm is what you end up with after using the over-size tap. Sometimes they're not a common size but in this case it will be 14mm or very close, the insert then takes the thread back down to 12 mm.


The link I included show the basic kit, the other how the timesert is done professionally.

The kit can be used in situ as stated by symon using grease in the flutes. I would add that if you bring that piston up to TDC that will help if you then use a vacuum cleaner with a small diameter nozzle to remove any that may have escaped into the chamber. Then clean the threads before inserting the insert. I would use high strength Loctite to ensure the insert doesn't wind out when the plug is changed next! Also use a touch of copper grease on the threads of the spark plug. The stripping of the threads may have been caused by the threads binding previously and/or not torquing them correctly, they don't take much.
 

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Discussion Starter #10
Thanks for all your help fellas I've ordered the kit that Symon posted hopefully it will have the correct size in there👍🏼
 
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