I was attempting to change my plugs today. I started with the driver front and it will not budge. I was afraid of stripping it out. I know these cars have aluminum heads. Should I try again and use extreme force, or should I let the shop do it? They said they would change them for $20. (labor). Plugs cost me $20, so I would have $40 in this task if I get them to change.

I think the plugs are original. It is ludicrus.

For $20, let them get dirty!!!

__________________
1988 W126 420 SE

beware of fundamentalists!

LOL. Yeah, but the negative is that he cannot do it until Friday and knowing them, they will have it until Monday. They have done me like that in the past. I will take something in on Friday morning, then they get to working on other cars, then mine sets there all weekend, as does everyone elses. Since this is a "quick" repair, I guess they may get it done fast.

I feel like a fool, because they are ridiculously easy to get to, yet I still cannot change them myself. I do not know why there are in there so tightly. I may try again tomorrow and hope I do not damage the heads.

Try to remove them with the engine hot.

I prefer changing plugs with the engine cold. If room temperature plugs are installed and torqued to spec on a hot engine the aluminum head will shrink as the engine cools and the plug heats up and expands, which will combine to increase the head's grip on the plug, especially with taper seat plugs.

If you can get them out, let the engine cool to near room temperature. Then install the new plugs and torque them to the minimum spec using a torquewrench and the published spec range. It may be in your owner's manual. The spec is in my owner's manual - 10-20 NM (7-15 lb-ft) for the M103 taper seat plugs. Look in the index under "spark plugs".

For stuck plugs it's best to use a T-handle and apply equal force to each side, so you don't place a bending moment on the plug, which can break off the insulator. If you use a conventional breaker bar, support the pivot point with your free hand to mininmize the bending load that you place on the plug.

Also, use a large 1/2" drive T-handle or breaker bar and put a shock load on it - like an impact wrench. A shock load is more likely to break the plug loose than a steadily increasing pull.

Duke

Thanks. Well, the engine was rather warm, but not "hot" when I tried to remove it. I did not even attempt the others after the first one failed to turn. I used a 1/4 inch ratchet, so perhaps I should use a larger 1/2 inch ratchet? Unfortunately, I do not have a "breaker bar" nor a torque wrench. I have changed plugs in a lot of cars and I have always just got the plugs finger tight, then used the socket/wrench to snug them in- never had any problems doing that.

Thanks for that tip. I am off again tomorrow, so I may give it a whirl again if the weather co-operates. It was 66* today, but 41* forecasted tomorrow, so it may have to wait. Back in the 50s and 60s after tomorrow, then I am not off again until Tuesday. Go figure.

I learned a long time ago that buying the right tools saved way more than their value in damage I created by using the wrong tool, especially when dealing with aluminum engine components. I would never attempt to change spark plugs with 1/4" drive socket components, and I always torque spark plugs to the proper specification. Taper seat plugs, in particular, need to be properly torqued. Many overtorque them which is why they are sometimes tough to remove or take the aluminum head threads with them.

Look up Harbor Freight on the Web and buy some tools including torque wrenches. A lot of their goods are made in China and pros may look down on them from their Snap-On tool boxes, but for the amateur/occasional mechanic you can't beat their value.

Duke

Remember to use some Never Sieze on the threads when installing the new spark plugs. That should eliminate the removal problems next time. I'd try a 1/2" breaker bar to remove the stuck plugs with the engine warm. A 1/4" ratchet is undersized for the job.

__________________
Fred Hoelzle

I had the same problem with two ML's, but simply put a short piece of pipe (12" or so") on the end of the breaker bar, which supplied sufficient additional torque to break loose the plugs... they were not all tight, just a few of the 12 on that car. As suggested in previous replies, put some anti-seize lube on the new plugs when installing them. If you can't loosen them with the pipe extension, then I would take it to the shop. The other thing you can do is spray the plugs with a high level solvent; I forget the really good new one that is better the old types ... and let it sit overnight. That could also help a lot.

Ben Carter

__________________
Ben Harrison Carter
1999 Mercedes ML320 87K
1992 Mercedes 300CE 89K
1995 Corvette 29K -- Sold Dec 09
1989 Mercedes 420SEL 99K -- (Sold 4/08)
1968 Mercedes 230S (106K) (Sold 9/06))
1976 Mercedes 450SEL 130K (Just sold - 06)
1961 Mercedes 220Sb (sold years ago)

The following product has worked well for me on rusted,corroded,or gunked-up threads:

WURTH-"ROST OFF" spray,300ml(10.1 fl.oz.)
Art.-Nr. 08902-U

I fully agree that 1/4 inch wrenches are too light for spark plugs. I use both 3/8 and 1/2 inch wrenches depending on the space I have to work. A T-handle is also a very good idea if room permits.

I use a little never-seize on threads when installing plugs.

A very good product for loosening stuck threads is PBlaster.

__________________
1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog

I agree with NOT using anti-seize compound on spark plug threads. There is always some leakage of heavy fuel components up the threads and if you combine this with anti-seize, it can make a real mess. AT MOST I might put a very thin film of anti-seize on the seats of taper seat plugs.

I've never had a taper seat plug come loose from an aluminum head when torqued to a little above the minimum spec, and I've also never had one come out tough or strip the head threads.

I use conventional copper core plugs, and pull them for inspection at 15K miles and usually reuse them for another 15K.

I would be reluctant to use a "100K mile" type spark plug for fear that they would not come back out without difficultly or stripping the thread.

Changing plugs on my M103 and Cosworth Vega (both have aluminum heads and use the same geometry plug, but the CV uses a resistor type and my current plug of choice is the NGK TR5) is so simple that I will continue to use conventional plugs, pull them at 15K mile intervals, and replace them at no more than 30K intervals, and torque to little more than the minimum spec of the range. At the rate I am accumulating miles on both cars it will take years to add 15K miles, but even if I was driving 15K miles a year, I would follow the same procedure.

Duke

I have always used anti-seize on spark plug threads, I have never had a problem. Be it GM, Ford, Jag, Toyota, Volvo, Mercruiser, or MB.

I wouldn't go nuts and dip the entire plug in but a light coating works like a charm.

The problem is the heads are aluminum and the plug threads are steel so they corrode in place. Be carefull or the threads will come out with the plug!

Get a 3/8 drive or if you have the room 1/2 is even better. Also spray the plugs with a good oil like PB Blaster, that usually helps stuck ones along.

FYI thats why their is a time and mileage intervial for changing plugs. Most of the new platnium plugs are 100k miles or 4 years. The reason they say 4 years is so you can get them out of the heads without stripping the threads.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Thanks. Well if the old oil was any indication of other things, these things probably have a lot of miles on them. It does not miss or anything, but it does have a slight splutter at idle and I want to change them.

I may give it a whirl with the 1/2 inch drive and see if that works. Like you said, I do not want to strip the head.

86560SEL, your best bet at this point would be to have an experienced tech remove the plugs. In your attempt to save a few bucks you're liable to create more and expensive problems. As mentioned previously an experienced tech with an impact wrench can likely remove the plugs without stripping the threads in the block. In the future you can change the plugs on a routine schedule.

__________________
Fred Hoelzle

I don't recal ever seeing an experienced tech using an impact wrench on spark plugs.

__________________
1989 300 SEL that mostly works, but needs TLC