I would not use solder because I'm not sure about the heat tolerance. I would only stick with w$ in the original core. There is a reason they chose copper. Also, there is a small spring inside the plug as well I left that in ensure maximum contact. What I did was just crack open a Bosch plug and cut the core to the proper length. On a side note, I have have recently switched back to the RS12YC plugs as there is no documented facts that it causes any adverse effects tho the ignition system. 2 separate.independent mb mechanics confirmed this for me and neither of them has heard of mb publishing anything regarding the use of resistor plugs. I did not put the resistors back in I just got new set. Good luck

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1990 190E 3.0L

Ok so after leaving the last set of resistor plugs RS12YC in the car, I did not have any issues with driving it for 40,000+ miles. Went in a few days ago and bought another set, but I was still curious about the non-resistor conversion and so I went to a hardware store and purchased a foot of solid copper rod the same diameter or extremely close as the internal resistor of the plug and I cut it down to the same length. Anyways, converted the plugs to non-resistor plugs with the solid copper rods and drove around with them for about a week now. I can't say if its because the other plugs had about 40k+ mile on them or if its because these have no resistance to them, but there is a definite noticeable difference in the smoothness of the driving, especially the acceleration. I also notice that when I'm on the freeway doing 70 mph it feels like I'm going slower.....or rather like I'm not putting a lot of effort in to keep speed. Hard to explain. Before it was like I had my foot on the gas quite hard just to maintain 70 mph and the engine even sounded like it was revving harder, but now its so smooth and effortless. Funny thing is, the car didn't hesitate with the resistor plugs and it always started right up and idled smoothly So I didn't notice the difference until after putting this last set of non-resistor plugs in. From now on I will just do this conversion to keep using non-resistor plugs, plus the RS12YC are only about $2 vs H9DC0 at the dealer which run for about $7-$8. A foot of the copper I got, which actually has 3 solid copper rods (1 white sheathed, 1 black sheathed, and 1 bare rod) was only .90 cents. Several members have stated that the non-resistor plugs are readily available online, but to this day I have yet to find any readily available which are direct replacements for the m103. I have seen most manufacturere numbers contain a "R" somewhere in the part number which denotes that the plug qis a resistor type plug.

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1990 190E 3.0L