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Spark Plugs

It is a major component of the Ignition System.

Bosch spark plugs – what do the letters and numbers mean?

The nomenclature "W7DC" breaks down as follows:

W means 14mm diameter, 7 is heat range (higher numbers 8 or 9 are hotter plugs), D is the length of the thread (how deep the plug goes), C means copper tip (an S plug has a Silver tip, P is Platinum.)

Bosch numbering

A hotter plug (9) reaches operating temperature faster, is more resistant to overheating, and does not foul as easily...

W7DC is the Bosch plug most commonly recommended for the 113 engines. W7 is a colder plug for warmer climates and high rpm or highway driving.

W8's and 9's are hotter running plugs and might indicate for colder climates and stop and go traffic. 8's and 9's are used by some drivers in "normal" driving conditions with satisfactory performance and plug life so the heat range does not seem critical in most applications.

Bosch plugs

An "R" (e.g., WR7DC) indicates a Resistor plug. The additional resistor adds about 5-6 kOhm to the total resistance of each wire plus connectors and may require a stronger coil.

R might help suppress Radio RF static and since Bosch does no longer sell non-resistor plugs both Bosch and Mercedes now recommend these plugs. However, some argue that Resistor plugs are not good for these engines.

Function

Describe, in general terms the function of this component. Meaning what is it there for and what role it plays. Describe how it works, the inside mechanism. Use diagrams to explain.

Maintenance

Describe common maintenance procedures, and common faults that may occur. Describe how these may be diagnosed and resolved. Again, include diagrams, photographs and explanations. Where possible, include measures, tolerances, weights etc.

• Symptoms when it faults
• How to test if it is faulty - what tools to use
• How to fix / change

Spark Plug Condition (Color)

Old Yahoo content

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Dan Caron: I use W9DC or W7DC plugs. Sometimes I use platinum's in certain cases.

Naj: I use NGK's BP7EVX. I trust the '7' heat value corresponds to the W'7'DC ?

Dan Caron: Possibly, but I never assume that anything matches between two different manufacturers. In some cases low numbers mean hotter. The Bosch recommended plug for almost every older car is W7DC but I often use the W9DC if the engine runs a bit rich. There were actually four different distributors used on a 230SL , the last was the 051. The 250/280SL's used the 051 as well, but the last ones with aluminum housings were the 061 ( I think ). These units have a different dwell angle and also a different set of points which are set to 30 degrees dwell angle. If you set them to the regular 38 degrees that the earlier ones use it won't run properly. Later distributors also have speed control and two vacuum ports. Frankly, I don't like these units all that much. Too much electronics to go bad and eat a hole in your pocket. The 051 is the best all around unit for these cars. Matched with the right coil and wires, it's pretty hard to beat. I've been using W9DC for years with no problems. The 7's are the recommended one's to begin with. Which model do you have? A 280 SL should just drain back into the engine. Hotter plugs do in fact run at higher temperatures. The cylinder temperatures are transferred on to the spark plug and depending on how far the porcelain goes down into the plug will give you hotter or colder. Hotter plugs warm up faster and stay that way so that anything that forms on them is burned off. Plug gap is also important. I've seen manuals that say .024 or lower - don't do that. 032 or even a bit more is where I like them. Make sure you have a good working coil and no resistance in the plug wires. Terminal ends at the plugs only should be 1K ohms and no more.

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