Hi,

I am about to change the spark plugs on my UK 1989 300SE. I am a bit confused about the spacing. What should it be? and also, sorry to be really dumb but how do I do it?

Thanks to everyone for all the great help

Oliver,

If you purchased Bosh plugs, they are already gapped. If not or you want to check, I think it's .32mm. Do a search, I know I've seen this many times.

Remember to grab the wire boots(not the wire) and twist before you pull.

Good luck, your enthusiasm is refreshing.

Thanks! It is the Bosch plugs I have ordered, so that's a relief!
So, if I understand this correctly, I hold the base of the cable and twist to remove, and then I use the socket to remove the plug.
I'm a bit nervous, but I have been wanting to do it for ages.

Thanks again

88-91 300-SELs use a Bosch H9DC or H9DC0 plug. Std. setting is .032; however, there are some on this list who believe a better idle is attainable using a slightly wider gap. I haven't experimented with widening the gap yet.

The H9DC0 plugs have a slightly wider side electrode and in my opinion, last a little longer than H9DCs.

Oliver,

While the Bosch plugs come pre-gapped, it is a good idea to check the gap before installing the plugs. You never know, could be the box had been dropped, and it just happened to land on the electrode end, now it is now longer gapped to spec compliments on gravity and inertia. You can pick up a gapping tool for a song, throw it in your toolbox and use it for ever.

There are a number of style from the simple siding ramp gapper, to the fixed loop type. All you need to do it determine what gap you want, then place the proper sized loop between the electrode and center post, if gapped correctly you should feel just the slightest resistance as both points make contact with the gapping tool. If no resistance is felt, gently close the gap, if you cannot get the proper size loop in the gap, gently open it up. Some tools have a "H" shaped prong for opening the gap when necessary.

It is more difficult to explain how to, that it is to do. You will learn the proper "feel" shortly, that only experience can provide.

Oliver,

Grab the rubber boot that fits over the plug itself and twist,then pull,

If you have trouble getting the boot off, buy the proper tool to remove them, then it's easy. It grasps the metal shield. MBZ makes one that I just bought one for $30.00 and it is money well spent as you won't damage anything. When you have them off, be sure to coat the insides with a thin layer of silicone grease to help you the next time. A cotton swab, lubed with silicone is the way to go.

oliverp
 

Oliver, you've received enough instructions on renewing your engine's spark plugs to fill a book.
One mention as to gap, it was referred as 32mm, that's not quite right, it is 0.032 inches.
as for type of spark plugs, use the plain jane copper cored plugs, they say they perform the best in our Benz engines, they do for me anyway. But I have the Jacob's Omni Magnum Ignition System.
A hint in removing and installing the plugs. Have two types of spark plug sockets at hand. One plain deep socket and one with the rubber insert for holding the plug in the socket.
When removing the spark plug from the cylinder head use the socket with the rubber insert. This socket will bring the loosened plug out of the bore.
When installing the new plug use the plain socket and when torqued properly this socket will come free of the plug whereas the other socket many times stays with the plug and the extention comes free of the socket.
Remember to appy anti-seize compound on the threads and silicon lubricant on the rubber insulator.
Now you can write two books.
Happy Trails Pip Pip from the Midlands, Houston!!!
Donald, El Cheapo

Thanks people!! I'll tell you all how it went.

Cheers

What about spraying some anti-rust as I noticed a tendency to rust on the plugs?

ymsim
 

Anti-rust or anti-seize, I'm not sure where you saw rust!
I use a Loctite product. "Hi-Temp Anti-seize Compound with Nickel".
I apply it to the seat and the threads.
As stated before I apply silicone lub to the porcelan portion of the plugs to provide ease of removal of the plug cable.
An anti-rust won't withstand the high temperature the threads of the plugs experience. I would imagine that would be no higher than 200 degrees F. Considerring the rubber insulators don't carbonize except with time and temp and you can hold the plug bare handed. I just never considered the temp before.
Possibly some else could elaborate on the temp.
Good Luck oliver and ymsim.
Happy Trails Pip Pip from Houston!!!
Donald, El Cheapo, Yank
Book Number Three

Donald
 

I just had my plugs changed last month and when it was extracted, the heads had some rusting on it. Quite a fair bit actually. So, we got the plugs replaced, and in doing so, sprayed some anti-rust over it.

After reading your comments, you might be right that the temperature in there might just totally wipe of the anti-rust.

Now its left me thinking why were there rust in the first place? Is it common???

:confused:

Growing up along the Gulf Coast I remember rusty plugs. Combination of salt air and not changing them soon enough. Mostly it was just surface rust on the exposed portion of the plug.

What do you mean by "the exposed portion" of the plug? The rust I noted on the W126 plugs before they were changed we around the head pin ...

Perhaps its vapour + combustion + 1 year = rust?

The six flat sides of the plug are basically exposed. The threads are in the head and the porcelin is under the rubber boot.