Fat Plug?

[MisterCinders]

So I went to change the plugs on my 1978 GS750. I picked up 4 shiny new NGK B8ES plugs. Three of the plugs installed perfectly.

When I tried to install the inner plug on the right, it would not catch the threads. After much cursing and fumbling, I examined the old plug. It is marked B8ES, but the threaded part is significantly larger in diameter.

The old plug threads right in, but new plugs are much too narrow to fit that plug hole.

Why would one plug hole be bored out like this? Where the hell do I find a plug to use for this big hole?

[Adler]

As far as I know, B8ES's only come in one size... Maybe you could take a pic of the removed plugs?

[silverramsrt]

The plug hole could have been repaired in the past. When the threads get messed up you drill it out and put a heli-coil or threaded insert in there to bring it back to the right size.

Check the plug for this insert.

[tkent02]

I bet you pulled a Helicoil out of the head.

[MisterCinders]

Awesome. :-(

There is some sort of threaded thing around the old plug, but that sumbitch is not coming off.

Guess I will run with the old plug until I obtain a helicoil.

Thanks.

[bwringer]

Helicoil is a brand name for a certain type of thread repair insert. A Helicoil resembles a spring. They work OK in some limited applications, but are a very bad idea anywhere you might need to remove the part again, such as a spark plug, or where you need sealing, such as an oil drain plug or spark plug.

Some people have taken to using the word Helicoil as a generic term for any thread insert.

What you have is a solid spark plug thread repair insert that was improperly installed, and came out with the plug. Some are retained simply by a thread locking compound, some are swaged into place, and some have pins or tabs that are driven into place after installation to retain the insert.

This brand is the one most commonly found in auto parts stores. It comes with an "expanding tool" you strike with a hammer to expand the insert and lock it into the head. These work pretty well as long as you follow the instructions and take your time.

http://www.midwayautosupply.com/showproduct.aspx?productid=30423&affiliateid=10050

These kits are also sometimes sold under the Helicoil brand name, even though they are not the usual spring-type Helicoil.

http://www.thetoolwarehouse.net/p-1491-helicoil-5334-14.aspx

Another common type, retained only with locking compound -- rather inferior, since the cylinder head gets too hot for locking compounds to do much good.

http://www.handhelditems.com/2129-3434-14mm-spark-plug-thread-insert-p-59061.html

One of the best. Very expensive kits, so it's rare to find a shop that uses these:

http://www.timesert.com/

Another good type of insert, also hard to find and expensive tooling.

http://fulltorque.com/sparkplug.htm

What you need to do is examine the old insert closely to figure out exactly which type and brand it is and obtain a new insert of the same type. You should then be able to thread it into the head, secure it properly, and then install your spark plug normally.

BTW, B8-ES spark plug threads are 14mm diameter X 1.25 thread pitch. As it happens, Suzuki used the same threads in the oil drain plug, so you can also use a spark plug kit to repair a drain pan.

[kochic]

This is what i use for spark plug hole.
Marc

[mighty13d]

Those are exactly it. You'll have to insert another one. Make sure you use the tool to swage it or lock it into place (per the instructions) and don't forget locktite.

[MisterCinders]

OK - Picked up one of those Sav-A-Thread kits.

The instructions pose two problems for me.

First, they call for some kind of silicone that I don't recognize to lock in the insert. Is this Loctite? I thought Loctite was bad with heat, but don't really know what to use here.

Second, the instructions warn against installing the insert without pulling the cylinder from the engine because metal bits may fall into the machine from the tapping and locking the insert into place. I don't think I need to tap (much) since the hole is already tapped, but I would need to swage the new insert into place. I suppose that could also create some metal shavings, etc. Do I really need to pull apart the head to fix this f***ing plug hole?

TIA

[tkent02]

It would be safest to remove the head. I had a car engine develop a bad rod bearing within about five minutes of doing this, I used a small tube attached to a vacuum cleaner, wiggled it around in the cylinder, though I had gotten all the pieces, but apparently not.

[MisterCinders]

Ugh. I figured that was the answer.

The current plug is still good, so I will punt that fix until winter. I have a nagging fear that I may need to remove the head anyway to explore valves and piston rings.