2509C Spark Plugs

[glmoore0001]

The car is a 1971 250C.

I ended up buying MB plug 0031591203 but I cannot seem to find a hard answer on plug gap anywhere.

I am still trying to buy the data manual and the service manual.

In Glenns MB Repair and Tune up Guide it states a gap for the 250SE (no listing in specs for 250C) a gap of .35mm or .014". The Haynes Manual that came with the car calls out .50mm or .020".
I do not think it matters, but the car has an electronic ignition unit installed.

I have searched this forum for "Plug Gap 250C" and "Spark Plug Gap 250C" and received 4 posts, 2 posts concerning the gap of 280C/CE and 2 posts not relevant to the gap.

Every time I look up 250C in a browser it all comes back as modern C class Mercedes.. frustrating. If I put in W114 and I get mostly info on 280c and ce.

Does the .35mm gap look correct?

After doing the conversion between mkp and ft/lb, the Glenn Manual suggests an equivalent of 18ft for plug install.

Owners manual states a special tool is needed to change plugs. Is this referring to a boot pulling tool? After seeing a photo, other members believe the plug wires to be original MB wires.

Thanks.. Gary

[Mike D]

With an electronic ignition module installed (Pertronix, Crane/Allison XRXXX, etc.), there is no gap (the first "gap" you mention).

That spec. is for the Kettering Ignition style of controlling spark, consisting of a mechanically operated set of contact "points" operating via lobes on the distributor. Mercedes added a separate transistorized unit which reduced voltage to these points, adding longevity to their use. This unit can lead to confusion. A quick check by removing the distributor cap will reveal if you have "points".


Spark plug gap should be between 0.024"- 0.028". If you do have an electronic control module installed you can increase the gap to 0.030" - 0.040" for a more intense spark.

Torque on plugs is 22 ft/lbs.

Look for specifications of the engine you have, NOT the model. You have a M114 engine in a W114 chassis.

[glmoore0001]

Originally Posted by Mike D

With an electronic ignition module installed (Pertronix, Crane/Allison XRXXX, etc.), there is no gap (the first "gap" you mention).

That spec. is for the Kettering Ignition style of controlling spark, consisting of a mechanically operated set of contact "points" operating via lobes on the distributor. Mercedes added a separate transistorized unit which reduced voltage to these points, adding longevity to their use. This unit can lead to confusion. A quick check by removing the distributor cap will reveal if you have "points".


Spark plug gap should be between 0.024"- 0.028". If you do have an electronic control module installed you can increase the gap to 0.030" - 0.040" for a more intense spark.

Torque on plugs is 22 ft/lbs.

Look for specifications of the engine you have, NOT the model. You have a M114 engine in a W114 chassis.

Yes sir, The car has a Crane Electronic Ignition installed. I understand there is no gap in the electronic ignition, I have a simple understanding of how it works. I just wanted to make sure the spark plug gap would not change with electronic ignition.

I am looking to verify spark plug gap for the 250C.

Is it recommended to increase those gaps for a hotter spark?

I am missing a bit at idle. I will check resistance on the plug wires when I change the plugs.

After that, I am going to check the valve clearances.

I was under the impression that the engine was a M130.932 which I also searched under,
but that may have been thru google, not a Peach Parts Forum search.

Thank you for the advice.. Gary

[Mike D]

M114, M115, M110, M129, M130, M116, M117 plug specs/gaps/ torque values are all the same.

Using the electronic ignition module allows you to increase the plug gap if desired. You don't HAVE to but I recommend it. As usual, YMMV depending on your engine conditions.

I prefer the "suppressor" plugs (NGK BP6ES) but using the Crane Optical sensor the Bosch ones you selected will work.

[cth350]

The 1971 250C got an engine change mid way through production. That is why Mike thought you had an M114. He is correct that for the question of plug gap, it doesn't matter in any material way.

-CTH