How to use helical thread inserts (including tips for spark plug threads)

Helical thread repair guide.

Here is a guide on how to fit helical thread inserts. Including spark plug  inserts  which use a slightly different method.
I’ve not had the need to do the plug treads on on a K series engine but I've used them on many other engines.
So whilst I cant give you a guide tailored to the MGF/TF, I can give you a generic one.

It's also worth noting that there other types of thread repair inserts other than the helical type discussed here, these are much stronger but a fair bit more specialised to industry and heavy vehicle use (like trucks, earthmoving equipment,  locomotives, and ships) I may do a how to to these someday, probably next time I have occasion to use them if I have time. There has been to odd occasion where I've used then on cars.....rarely though!

For normal threads you can get a kit in the individual size you need to repair or you can get a kit with a range of sizes.
 
Spark plug kit with single size kits

 
A metric size range kit


For spark plug threads, you need to get the spark plug kit with the installing tool, the thread tap and the insets, the kit I have also has a insert removal tool, but while useful you probably/hopefully won't need it. You will also need a tap holding tool/wrench.

DO NOT attempt to turn thread in the tap using a shifter ( sometimes known as a crescent wrench or adjustable spanner )  the turning pressure needs to be evenly applied to the tap holder from two sides. to cut the thread straight and true.

 

There are a variety of tap wrenches available
 Most of these will not be much use trying to cut a thread in the confined space available in the engine bay of any car, let alone our cars!

 
A selection of tap wrenches,  the two handers will be useless in our engine bays, the smaller one handers at the front will be more useful, note that there are chuck types that are really handy as they can be used with your 1/4" square drive socket extensions and Tee handle for reach into awkward recessed holes and just plain bastards of places!. the is also some special 3/8" drive sockets (The row of black things in the rubber holder) these are very useful
The large two handers I included in the pic are there to scare you, along with an example of the sort of size tap I regularly deal with. Note the size 10 ezysyout  in the big two hander I use these to remove broken bolts up to 65mm dia bolts!

 
Chuck type tap wrenches can be bought in a tap and die set or as a stand alone set, featuring a reversible ratchet tee handle and 1/4" drive extension bar
 
To reach into those bastard hard to access places!


 
With normal helical thread inserts you will need to drill out the damaged thread with the special sized drill provided in the kit, use a good cutting fluid to lube the drill.
Don’t use rubbish like WD40!

 
Suitable cutting fluids for drilling note. there is one especially for drilling alloy
The one labeled Tap Magic is suitable for both drilling and tapping

 
Thread cutting is best done with the proper compound, the stuff in the tub is the best, stuff, but only ever for hand tapping never ever use it for drilling the smoke is nasty toxic shit!

Then with the tap provided, once you've applied a suitable thread cutting compound you carefully and slowly start to cut the thread, turning it a few turns forward, then half a turn backwards (to break of the swarf) then another couple of turns forward again repeating this process until you have cut deep enough. If you feel too much resistance building up, back off the tap possibly even remove it for cleaning, then continue from where you left off.
Never force a tap, if it breaks you are going to be in a world of pain!
You may find the tap has bottomed out and can’t go any further as it's in a blind hole!
So don't continue to force it!
Once done, unscrew the tap from the thread.

Now take your thread insert and place it upon the insert tool with the 90 deg drive tang in the slot of the insert tool (it needs to be screwed in drive tang first) screw the insert into the threaded hole, until it is below the hole surface, now some kits have a dedicated tang breaking tool, place this into the hole and give the breaking tool a short sharp whack with a hammer to break off the drive tang, to recover the broken tang from the hole you can poke a piece of wire that has a dob of grease on the end into the hole to try and fish it out.
And for those kits that don’t have the dedicated tang removal tool, you just withdraw the tool enough to disengage the tang from the slot in the toll, then turn the tool 90deg and with a short sharp whack from a hammer, break of the drive tang.

For spark plug threads you don’t need to drill the hole, the special tap in the kit has a pilot thread on the end to use what’s left of the existing plug thread to get you started, You use grease to lube the tap, generously filling the flutes with grease ( the grease is to trap the swarf  in order to prevent it dropping into the combustion chamber. )
Also you can use compressed air to blow down the plug hole to try and blow any crap out of the combustion chamber if you like, it won’t hurt to try.

With alloy headed engines the swarf will be soft so if a tiny bit escapes recovery it shouldn't be the end of the world, However with cast Iron headed engines the swarf will be hard and damaging to the bores if left behind but stray bits can be recovered with a tiny magnet on a telescopic stick.
My bore-cam has a tiny magnet attachment, that's handy for this, you can see what you're trying to pick up.

 
Note the pilot thread on the special tap for threading spark plug helical inserts

Screw in the insert, with the insert tool, then withdraw the tool, now here is the tricky bit…breaking off the drive tang without it dropping into the combustion chamber! Your kit may have a dedicated tang breaking tool, if so put a dob of grease on the end, insert it into the hole and give it a short sharp whack with the knockometer…..hope and prey to the deity of preference if you must, that the tang is on the end of the breaking tool when you remove it!
For kits that don't have a dedicated breaking tool, take note of the index angle of the tool before you withdraw the inserting tool and load up the slot and end of the tool with grease and with the slot of the tool 90deg (this is why you took note of the index angle) from the tang give it a short sharp whack as above, and as above you want to find the tang in the grease on the end of the tool when you pull it out.
Otherwise and you’ll be using a bore-cam (endoscope) and your new friend and fishing buddy, the bit of wire with the dob of grease on the end, to recover the lost tang! What you don’t have a bore-cam?….You will soon by buying one if you loose the bloody tang!

It may be wise to make sure you make a blood sacrifice to appease the angry car gods by skinning a knuckle or two whilst carrying out any of the aforementioned procedures!

If your kit came with a insert removing tool (the arrow headed tool in the pic up near the start of the thread ) you can use this to remove any suspect helical inserts that either you or some previous person buggered up.
 

That is a superb and very comprehensive How To...awesome..in fact awesomer.

I doff my hat to you as I say Thank you.

....and just cos you are a Mod I'm not sucking up!
Genuine praise.

As always that is a brilliant " How To "  Cobber. Thank you.
John 

Very much appreciated. Thank you.

Great how-to! I've used these on aircraft, propeller brake mounts etc, and they're invaluable!!

That, my friend is a masterful piece of work, instructive & an enjoyable read.
Thank you.
Mike