Tech Tip: Spark Plug Thread Repair

Last weekend I had the great opportunity to fix a damaged spark plug thread hole on BSA a C15S (Scrambler) head

There was barely enough threads left behind for a spark plug to thread into the existing hole

Today I will go over how I fixed the damaged spark plug threads using a Heli-coil / Save-A-Thread spark plug thread insert repair kit

Considering there are other ways to repair a damaged spark plug thread hole, I decided to use inserts over welding because the spark plug hole was too close to the valve seats

With that being said, lets dive in.. 

How do spark plug threads strip?

Over the past few years I have found that are 3 common factors that could lead to damaged spark threads which all can be avoided if taken the proper precautions 

• Overtightening 

Arguably the most common way to strip spark plug threads is by overtightening the plug beyond what your workshop manual recommends for torque value

Have good judgement! 

• Lack of thread lubrication

Installing and removing a spark while the threads are "dry" will overtime develop damage 

It is important to add a small amount of thread lubrication like Anti-Seize to your spark plug threads to prevent galling 

How the kit works

As stated previously I will be using the Heli-Coil / Save-A-Thread insert kits

Each kit is designed to be used while your cylinder head is either on or off the bike

However, I highly recommend taking your cylinder head off to perform this job..

The kit (which can be found at your local Auto-Zone or O'reillys) comes with 3 steel inserts, special tap and a swaging tool

You may notice the tap included in the kit has 2 different diameters of threads with a large chamfer while retaining the same thread pitch - this allows the tap to thread into your existing 14MM spark plug thread hole while also cutting the larger hole larger followed by cutting new threads

Once the hole is tapped to the new oversized diameter, simply install your steel insert

I will use the supplied tap to screw in the new insert while adding a bit of JB Weld to the threads or green loctite

Lastly use the swaging tool to flare out the insert end

Expanding the top portion of the insert is critical to prevent the insert from dropping in the combustion chamber and preventing it from backing out

The repair process (5 steps)

Step #1

 Be mindful of the finished hole sized before using the pilot tool included in the kit - make sure that your inlet and/or exhaust valve seat is not too close for comfort to your spark plug hole 

If you have a head with damaged threads, you will need to purchase a spark plug thread repair kit from your local automotive shop or online parts outlet

Before starting the work I took some dimensions of the spark plug thread inserts I will be using So I can get an idea of which length of insert to use in the head

You might notice on some heads (especially smaller "CC" British bikes) you will find that that once you tap the spark plug hole to a larger size, your new hole could be very close and/or run into the valve seat

On this cylinder head, my spark plug hole was too close to my inlet seat, so instead of using the tool to remove material all the way through and expanding the hole diameter, I decided to go with the long insert and leave about .100” of material in the head towards the bottom to prevent the insert from entering the the combustion chamber and running into the valve seat (see photo above)

Step #2

I made a custom fixture out of 6061 to lock the cylinder head down to my angle plate on my mill

(Now to be clear, you can do this repair without a mill…)

Once I bolted down the head my main goal is to find the angle of the spark plug thread hole and to also find the center

When both where achieved I was ready to bore out the old threads

Step #3

I used a 5/8” end mill (not shown) to bore out the old threads

I set my depth to only go so far, leaving a .100” thick shoulder as we discussed earlier

Once done, I used the supplied tap in the kit to tap the new threads into the cylinder head

Step #4

Now that the threads are tapped into the head, it’s time to thread in the new insert

I will use just a small amount of JB Weld around the new inserts thread to help with securing the insert in place 

JB Weld indicates that there metal epoxy mix will break down once the epoxy reached a temperature of 550+ degrees 

If this is accurate, then JB Weld would withstand the heat that this head will endure under all conditions 

Using a new spark plug I will thread the insert into the newly tapped hole until it bottoms out, followed by removing the spark plug

Step #5

The last step, but the most critical

Using the provided swaging tool I insert the tool within the top portion of the insert 

Using a brass mallet, I strike the top portion of the tool to allow the insert to "flare" outward permanently locking the insert in place 

If all is done correctly, you should have a good, strong and reliable insert that will not come out 

This style of insert is designed to be a permanent fix

Food for thought

Now that we have covered and repaired this cylinder head, I wanted to cover 1 question that I always get

Here it goes...

• Why not use a conventional stainless steel Heli-Coil? 

Through experience I have found that your traditional wire-style Heli-Coils can "unwind" themselves when used in aluminum cylinder heads 

It appears that with heat and expansion if one were to remove a spark plug from a hot cylinder head, you might find that the Heli-Coil will back-out with your plug..

Thanks for reading todays post 

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 Ride safe!