Ok guys, the compression in my 1969 280 SEL is excellent in 5 out of 6 cylinders, 150 lbs. for those but cylinder number 4 is down to 122 or so. I noticed that the spark plug in said cylinder is just a little wet. So...I'm using the Bosch W7-DC in the car right now, I'd like to use a hotter Bosch plug in cylinder #4 so could you recommend a good place to start. Thanks in advance.
Thom

You could try the W7DCO plug which has a wider heat range than the W7DC.

yal,
I'm curious. By wider do you mean hotter? And the DCO designation, do you know what it means? Many thanks.
Thom

OK, Thom, I'll jump over here! Have you run a compression test on #4 after adding a tbl or so of oil in the sparkplug hole? That SHOULD tell you if the compression problem is valves or rings, etc. If compression improves with the oil, it generally indicates a top end problem. Valve adjustment might help.

OOOOPPPSS!
 

OOOOOPPPPS!, I mean vice, versa! Improvement shows a bottem end problem! SORRY!!

Thanks Bill. Just for my own clarification, if there is no improvement with the oil then a valve adjustment might help. Was that a question? Egads, I'm talking to myself again. :D
Thom

Thom,
I have no idea what the O stands for the D is for 'tapered' and the C is for 'copper'. 7 is the heat range, you could go to 6 but I don't think this will help your problem. Maybe one of the techs can chime in here.

I think "o" is for high heat plug insulator.
They are used on engines like the 104 where the plug is recessed in the head and covered.

With Champion the higher the number is, the hotter the plug. Is this the same with Bosch? Which direction is hotter? Anybody? Thanks all.
Thom

I'll reference a different Bosch plug; however, the naming scheme is similar.

M103 motors use H9DC or H9DCO

The plugs with the "O" on the end have a slightly wider side electrode. It's been my experience that H9DCO plugs last a little longer than H9DC plugs.

Sorry Thom you're right the h8dco plug is hotter. Bosch use to go the other way with heat range and their numbers.
http://www.vintagesaab.com/sonett/ss/Spark%20Plugs/sparkplug.htm

Excellent research yal. I'm going to give the W8 DC a try. That should do it and since I'm only doing it in one cylinder I don't think I should have a per-ignition problem. Fingers crossed.
Thom

Just found the Bosch letter designation chart.
Letter "0" designates: "Special insulator/electrode."
Does not mention what's so special about it, but that's it.

Okay guys, here's what I did. As per my first post I went out and got a W9 DC (instead of the stock W7 DC) in order to keep the plug in cylinder #4 from becoming fouled. It is just a little bit wet and it's not like the cylinder isn't producing power or anything. I'll keep you all posted on how this works out.
Thom

So I get home with the new W9 DC plug and begin to remove spark plug #4 (the wet one) and lo and behold it isn't wet! What happened? The other day I had made a small timing adjustment and advanced it a tad. Obviously it was just enough to allow cylinder #4 to achieve a proper burn. I installed the new W9 DC anyway as an experiment and will take it out for examination in a couple of days. I'll probably end up putting the W7 DC back in but it's good to know that the engine is behaving normally.
Thom