Moneypit, I figured the experienced tech might use an impact wrench to break the plugs loose and then remove them with a ratchet. Whatever, the tech will likely have seen this situation before and know how to proceed to prevent stripping the threads in the block.

__________________
Fred Hoelzle

This ENTIRE post should be repeated, and repeated often. This is not an iron-head engine from the 60's, where you could not go wrong. Here is the equation:

Inexpensive torque wrench from Harbor Freight: $15
Other related tools: $20
Anti-seize: $4

OR

Easy-out to remove broken spark plug barrel from engine: 10-20
OR
Loaner car: $$$
Head removal and EDM work to remove thread, etc: $$$$

It's really simple. I had a POS Pontiac where I was in a hurry and actually paid a mechanic at a tune-up shop to overtorque the plugs and I broke the plug removing it when I tuned it up the next time, leaving the threaded part in the head. I dodged a major bullet with an EZ-OUT. Never again!

__________________
86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm

When I removed mine I am sure they were in there for 50,000 miles. I removed them dry and didn't know if I was unscrewing the plug or taking out the threads. They all came out okay but they were very stuck. I would now use PB Blaster for pennetration and let it sit overnight. Spray them again, wait a little and blow out the remainder of the fluid with an air hose. Use the T bar or however you will keep from breaking the plug. Good luck.

I had a mechanic tell me that some of his co-workers would remove and install plug with an air hammer. They didn't care if they cross threaded them because by the time they need to come out they won't be the one working on it. It is beyond belief. This is another reason to diy. I used anti-seize and next time they came out by hand (after breaking the torque).

__________________
Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si

When you get your plugs out it would not hurt to chase the head threads with a tap. Put some grease on the tap and run it in and out. Be sure that the piston is down far enough so as not to contact the tap.

I like this idea, but how do you remove all the dirt from the threads that's fallen into the cylinders before reassembly? Also to make sure you don't hit the pistons, would you have to turn the engine by hand for each thread you want to clean? At least one of the pistons is going to be at the top, surely.

I've always wanted to do my own plugs, but I've been terrified of breaking one of them and having to pull the head to get it out. Keeping the threads clean, using anti-sieze and torquing correctly would make me feel better about doing it.

Hello:
I had a simular problem with my 1992 300-E The spark plugs expand with head and cool and will tighten especially if the dealer cranks them down. first always change spark plugs on a cold engine. I used a little Wurth Penitrating oil spray to help loosed the seat the plugs are in. Make sure you you have the right socket for your plugs and use a firm slow preasure keeping the socket and rachet and socket stright not cocked.

I put a little anti seize on the plugs and a dap of dielectric silcone in side the plug boot so they will come off much easy next time.

Take you time :

Happy New Year
Bob Geco

take another week and start soaking with PB Blaster [available at most auto parts stores] Apply 3 or 4 times daily, hot and cold. Will come out easily.

Regarding anti seize, I use a little at the lower threads and wipe most off. Porsche recommends not using it because it inhibits electrical conductivity but I think it's a risk worth taking, if you don't coat the whole thread and crush washer. It does gunk up after a number of applications. I am considering buying a tool to clean the threads the next plug change.

Regarding cleaning the threads, you don't want to drop anything into the cylinder. Always blow out the plug area with compressed air before removing the plugs. You can clean that area prior to air as well. There is a tool that is filled with grease then compressed to slip into the plug hole. When released it engages the threads and can be backed out, catching any crud in the grease. It's like using a tap with grease, only better.

If you:
rhgtara@localne "take another week and start soaking with PB Blaster [available at most auto parts stores] Apply 3 or 4 times daily, hot and cold. Will come out easily. "
Then blow out with air and using a flex coupling of some kind and a T-handle or properly support it, if the threads come out a mechanic would most likely do no better. Most stuck fasteners come out better with shock, like an air hammer or sharp blow, but I would not personally use one on a head or aluminum part. Good Luck! MHO

__________________
Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si

smharr4,

Read my post where it says, "put a little grease on the tap". One you are not cutting new threads where there will be a lot of swarf, and two, what little sward and dirt that you will produce will be caught in the grease on the tap.

Tap sets consist of 3 taps, a starting tap, a middle tap, and a bottom tap. Each has varying amounts of taper on the starting end. The starting tap has the first 1/2 to 3/4 inch of its threads ground down so that it will guide itself into the untapped hole and prevent misalignment. The second tap is designed for existing holes and could be used to chase your spark plug hole threads. The bottom tap is designed to get threads as close to the bottom of a blind (dead end) hole as possible. It is used only after the frist two taps have done their job.

To tap out existing spark plug holes you can probably get the proper tap at a parts shop and not have to bother with the full set. Mostly you will never use the starting and bottom taps.

Here's some info on the "Back-Tap" tool:

By Dan Barnes
Photography: Dan Barnes

Not long ago, one of my shop mates salvaged a junkyard engine. The only problem found was that one spark plug hole was cross-threaded. The labor cost to pull the head and repair it with a hardened thread insert was more than the value of the car; a moment's carelessness by a mechanic or the owner was the end of the road.

Fortunately, my buddy was assisted by a highly skilled pro, and they were able to do the job properly, with the only cost being their time. But if the insert ever sticks to a spark plug instead of the head, it will be a big problem. I've seen someone doing the same job on an outboard motor. He was scared, because marine parts always cost two or three times what an equivalent automotive part does. Both situations would have been reduced to a minor inconvenience by the Starr Products Back-Tap, which was invented by a technician who had similarly "oopsed" a spark plug hole on a Jaguar XK120.

I have cross-threaded other holes, or been around when someone else did, and had the joy of correcting the situation with a conventional tap, trying to distinguish the true threads from the cross threads. With a lot of care, a little skill and some luck, I've always gotten it right, but I've almost never been sure I would at the beginning.

The Back-Tap begins chasing good threads at the bottom of a hole rather than damaged threads at the top. Machined from heat-treated 4130 steel, it compresses smaller than the thread's inside diameter to be inserted into the hole. A mandrel is drawn into the tool with a knurled nut at the top, expanding the threaded portion. Then the expanded Back-Tap is threaded out through the hole. Filings are brought up and out of the hole along with the tool; greasing it before starting ensures that even the smallest shaving is retained, as well as making it easier to turn. Go in steps, leaving the Back-Tap slightly compressed at first, subsequently expanding it until the damaged portion is clear to the same diameter as the rest of the hole. Tolerances on spark plug holes vary, and some may be too tight if the Back-Tap is fully expanded.

The Back-Tap is sold through tool trucks and NAPA stores, in 10mm, 12mm, 14mm (the most common spark plug size) and 18mm. The latter is extra short, for use on O2 sensor ports. Hopefully, your need for the Back-Tap is rare enough that it may not be worth having in your toolbox all the time. But if you ever need it, you now know it exists and where to buy one.


The Starr Products Back-Tap is a simple tool that will safely clean damaged threads in a cylinder head or exhaust manifold. It can save pulling a head, and maybe even save a whole car.

The Back-Tap cleans threads from the inside out by compressing smaller than the threads, as shown here. The mandrel is then drawn into the Back-Tap with the knurled nut at the top, expanding the threaded portion as in the main photo. By backing the expanded tool out from the bottom of the hole, the damaged threads at the top are restored.
Sources
Starr Products, Inc.
(360) 961-2671
www.starrproducts.com

FYI found this at Snap-on Tools

http://buy1.snapon.com/catalog/tools.asp?Group_ID=16723

A year ago I was able to buy a 14mm "Back-Tap" from a local NAPA store. The tool package had a KD part number of 3545. Now, neither KD or Snap-On lists a 14mm "Back-Tap", the most common thread size for Mercedes and other vehicles, in their catalogs. Wonder why ???.

Is there a size available for the Glow plugs as well???? Willie(Poor Soul)

As far as contaminants falling in the chamber, what's worng with blowing it out? With some air nozzles you can easily make an extension out of a length of copper tube. Just insert the extension deep into the chamber and blow. The turbulence should eject the particles out the hole.

__________________
95 E320 Cabriolet, 159K

OK. Thanks for all of the replies. Well, I decided to tackle this task myself and we got the plugs changed today. My father helped me. He did one side and I did the other. Seems as if that first plug on the left front was the tighest, but I did get it loose with the larger 1/2 inch drive wratchet. Luckily the others were not as tight, because I really do not think that would have fit in there. They all were very tight to say the least, but I managed to get the rest out with the 1/4 inch wratchet. Dad suggested spraying a little WD-40 or something on them, before we removed, but I was hesitant, because I did not want a smelly engine. The old plugs were old and was in need of replacements.

Thanks again for all of the tips/advice!