Spark Plug Hole #1 problem

[hands_aus]

Hi,
As I was supposed to be going on a MB Club run on Sunday, I decided to clean the plugs and do my usual fluid checks on Saturday afternoon.
Afterwards  I went for a test drive.
The cars' (1967 early 250sl) engine was missing and idling roughly.

The plugs were covered in black soot after about 7 Km.
I re-cleaned them plus lubricated the warm up regulator plunger and checked the injection pump oil level.
All the plugs came out easily both times. I usually use a piece of fuel hose to start/remove the plugs.

On the 2nd clean #1 plug would not go back in.

I have attached a couple of pictures of the problem hole.
The black gunk is anti-seize compound not cleaned off yet.

It looks like TWO pieces of metal have broken off the head.
The broken end is like a BURR.
The plug seems to start in the hole but it just binds.

I have not pushed my luck with this.

Any one know what seems to going on?

Thanks

[JimVillers]

Bob .... That looks like a HeliCoil that was used by a prior owner to to fix a stripped thread.  If it is no longer usable, you might be able to remove the coil with some needle nose pliers and then repair the hole with a new HeliCoil or some other thread repair technique (there are several from which to choose).  This might be a time to consult a machinist if you are not comfortable with the job.

[hands_aus]

Hey Jim,
Thanks for replying.
This is #1 plug hole which doesn't have very good access.
Will the head have to come off or leave the head on and remove all the injection pump lines etc?

I was thinking that the sharp burr might be able to be smoothed enough to insert the plug and drive the car to a mechanics workshop.
What do you think?

[ja17]

Hello Bob,

Yes, as Jim has pointed out, this head looks like it has an insert in it already.  You should be able to remove it, clean the threads and insert a new one without removing the head. You may have to remove some injection lines and do a little research on the type of insert used.  Many mechanics will not attempt to repair spark plug holes without removing the head. The fear metal shavings damaging something else.   However I have always been able to repair these with the head in place. Soft aluminum shavings are not as dangerous to deal with as are steel ( as far as damaging the cylinders or pistons if they fall into the engine).

Coat any thread taps with grease so that most shavings stick to the tool instead of falling in the cylinder, then crank the engine over with the spark plug out to blow any shavings out of the cylinder. Replace the damaged coil with the same type if possible so you will not have to re-cut threads. Do some research on the different types of spark plug repair hole repair fixes.

Most likely your head has the standard "heli-coil" type insert.  The most popular special tool used to repair spark plug holes, reams the hole and cuts new threads at the same time. You will need to figure out what insert style was used on your head.  After cleaning up the threads, install a new helical, using some lactate to keep it in place. Keep using anti-sieze on this spark plug and hopefully the insert will stay in place. These fixes will work  fine and can be permanent in most cases.

[hands_aus]

Hey Joe,
I have been busy, sorry for the late response.

One of my massage clients is a diesel mechanic. He had a look and said it was a heli-coil.
Friday he called and said he had found something that might help.

It was a 14 x 1.25 THREAD SAVER. pic attached

He applied grease to it then after cleaning around the hole, he wound it in then expanded it slightly and wound it out which cleaned the gunk etc out of the thread.

The spark plug wound in quite easily.

One thing he advised was to start the plug by hand then wind it in with the fuel hose.

Thanks again for your help