I had the same problem on my 88. I went down to the auto parts store and got a thread chacer and some cutting oil to catch the savings. It fix the problem.
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88 240DL a.k.a The Blue Tank

If a sufficent amount of threads remain I have used a rethreader tool(Jaw Manf. Co.) to realign the threads followed up with a thread forming tap to repair thread form afterwards .I would use a wicking sealant( high temp product by Loctite) when installing the sparkplug. Alignment is essential to this process being succesful. If your threads are really bitched up and little base metal is available to work with then the best solution is to take it to a machine shop. Mike G

Here's another www.timesert.com
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john,, 1985, 245:Ti, 251k mi..so.california,,,1974, 145, 230k mi. ..

Since you said you watched the video I had to go see what I had never seen..I have a backtap tool but never saw the video..It was exactlly the way it works.. The site you saw also showed the tool part numbers in KD, MAc and Snap-on, which if you have ever worked on many cars you would you know they are the most respected and long established tool marketers in the business. They do not sell junk,, believe me as a MAc KD and Snapon tool owner I have thousands invested in tools and these are the only ones I will use.. Well there might be a craftsman or two but guess who makes craftsman?? Anyway yes it works rather well..But be sure to greese the tool before you use it, not only to help lub the threads but to catchthe metal shavings you might have..I always use a small tube that I made that attaches to my commercial vacuum I use in the shop for inserting into the cylinder after I have used any tap in a spark plug hole. It is best if you keep the piston near to top dead center with enough clearance to insert the vacuum tube..Sorry for the long post but I get carried away talking about this thing since it was long over due to be developed. I have pulled many a head off to repair a muffed up plug hole.. Anyway good luck if you order one or catch the friendly snap-on man driving down the road..Max
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Max..1989 244 DL 5 Spd., Bilstein HD, Turbo Swaybars, Poly Bushings all round, Turbo Wheels, Black leather interior, Electric mirrors, LED dash and gauge lights.