Spark plug with 2 different size threads.

[Olazz]

Well, after fitting a 123 ignition, I went to change the spark plugs and put in a new set.

Currently have Bosch WR7DC and wanted to change to NGK6BPES.
All well until I came to no 3 plug.. Came out alright but just couldn't get the new plug to take the thread.  Went on to do 4,5&6 and came back to no 3, but Still no joy.

Head scratching and muttering, I looked at the head thread and it seemed ok with little or no damage.
Took out no 4 plug and compared head threads...ah there seemed to be a little different. So to the dustbin to retrieve the old plugs to have a look see.

Found one plug of the six had a "double thread". The top half of the thread was much larger than the bottom half!
It was the only plug that would fit in that cylinder...

Is this normal for these cars? or do I have a head thread problem...

If so, is this a special type of OEM plug from Bosch... (done a Google but find nothing)

Anyone got any suggestions please.

TIA

Lazz

[waqas]

It looks like a "head thread" problem. All threads should be identical, and it seems a previous owner made some 'modifications' in order to repair stripped threads.

My best advice is to put in a helicoil insert. This can be done by drilling a slightly larger hole and tapping with the correct helicoil outer thread. There are ways to prevent metal pieces from remaining in the combustion chamber, but they escape me right now.

[Olazz]

Here is a picture of the offending article

http://img.photobucket.com/albums/v688/Derfs/Bosch%20Plug/IMG_2003.jpg

[waqas]

It looks like there already is a half-length insert, which came out with the plug. If you grab the base of the larger thread, you might be able to unscrew it from the plug and place it on the new plug.
Beware that the threads are continuous at the transition point. Take as many photos as you can of the transition point before removing the insert from the old plug.

I have no experience with this sort of setup, so others may chime in and point out any potential issues.

[Benz Dr.]

  Hey Waqas,
it's a thead insert alright. I probably wouldn't drill anything until I had a closer look but they often come out with the spark plug if thy're not installed correctly. It's about the only time I might consider using Loc tight.

[ja17]

When you install the new insert, make sure it is the correct length. Is sounds like someone had installed an insert which was not long enough.

[stickandrudderman]

Yes, it's an insert.
FWIW I never use helicoils, I use a Wurth system called "time-sert". They are excellent and very reliable, although the kits are very expensive.
I also have a kit called a "Big-sert". This, believe it or not, is another wurth kit with the specific function of repairing spark plug threads that have already been repaired before!
It is possible to do these repairs in situ but the only way to guarantee that no swarf is left in the engine before start up is to do it with the head off.

[Olazz]

Thanks for all of the replies gents!

Not what I wanted to hear and not good news as I was supposed to be cathching Euro-tunnel for the start if my Italy trip @ 7.45 this morning.
Having just spoken to Roger Edwards, taking the car up there to have a Time-sert installed and hopeflly I'll be on way tomorrow, assuming no other problems.

Thank you all again, lekker stuff!! 

Lazz

[ja17]

I have repaired many of these with the head on the car.  Use a lot of grease on the thread cutting tool. This will collect most of the shavings.  After the procedure, crank the engine with the spark plug out to blow any debris out of the cylinder. The shavings are soft aluminum so any damage is unlikely.

[Olazz]

The guys at Roger Edwards were brilliant today, can't speak highly enough of them.

At short notice they downed tools and took my car in straight away and got to work.

Who ever had done the job before had really "bodged it" and trying to fit a Time sert was not easy.
The repair was successful enough however to allow me to make the trip to Italy leaving tomorrow...... however, the French are now threatening strike action at the chunnel so may still be sitting this side of the continent tomorrow!!

Not entirely happy knowing there is a temp fix, so will more than likely have the head removed and a permanent solution found upon my return.




 

[jaymanek]

A thread insert would probably out live the car so dont lose any sleep!

[Benz Dr.]

Thread inserts are fairly depenable but they do come out sometimes. The only permanent fix is to weld the hole shut then drill and tap it. The head would have to be removed to do this of course.