I'm running a DA-100 and am going to change the plugs and was wondering what is the recommened gap for the CM-6 spark plug. My engine starts and runs on one cylinder and then it will kick the other cylinder in and run just fine so I thought this might be a good place to start. Any thoughts.

# 11 xacto knife blade or 20 thsth

I used to get out the feeler guages, but now as stated above I just use a #11 X-acto blade. Works perfect and everyone has them.

There is no difference in performance of any engine with a spark plug gap of .020-.050...Try it and see...
A CM6 with no side electrode at all will run an engine using an RC EXL or C&H ignition...Been there, it DOES work...

They are rather powerful and have a very strong spark.

Most of them come out of the box with a gap of .018" since that's what they are tested at. Smaller gaps reduce the resistance through the ignition coil, reducing heat. Smaller gaps also require less voltage to bridge which permits a more complete coil charge cycle at higher rpm. I run them at .017"-.018".

For the average guy that might like to play with plug performance they might find that some trial and error work with indexing and cylinder head electrode depth can give a little extra over the engine next door. For those with a machine shop the sky's the limit, but it gets pretty involved...

So is there a common source for plug shims? For plug indexing? A very important step to getting full performance from an angled plug. This is SOP for any race motors but I've never seen shims for the little plugs.

No one makes shims for CM6 plugs...Our little gassers don't know the difference...

Originally Posted by rcign

No one makes shims for CM6 plugs...Our little gassers don't know the difference...

Too bad, with an angled plug, getting the plug pointed in the right direction can make a big difference in how much and how fast the charge is burnt. Everyone seems to squeak and squeal about a hundred rpm and plug indexing is such a no brainer.

Jody tried plug shaping (not sure about indexing) recently and didn't see any difference. I think it was a DL-50.

Originally Posted by rcign

There is no difference in performance of any engine with a spark plug gap of .020-.050...Try it and see...
A CM6 with no side electrode at all will run an engine using an RC EXL or C&H ignition...Been there, it DOES work...

there may not be a performance difference but a wider than designed plug gap will affect the reliabilty of the coil. like TOM said the wider the gap the more resistance thus causing more coil stress. .018 i believe is what da recomends. proper gap will give you a more complete and efficient burn time.

Ralph which way should the opening in the plug gap point? Towards the intake or Exhaust?

Dunno, I would think toward the intake...We tried it on our GT80 race gassers, no difference we could measure...
Why would a .018 gap on a small plug work better than a .030 gap on a large plug ????Same ignition, same battery...
Any ignition that won't handle a .040 gap isn't much of an ignition..The RC EXL ignition will eat away at the gap on a 1/4-32 Rimfire spark plug after about 300 hours...Been there.....The ignition now has OVER 600 documented hours on it in a small four stroke gasser...

Ralph,

There is a source for a steel washer/shim with a 10mm i.d. MSC carries them but they are marginally thicker than .010".

Index the open end of the electrode to towards the boost port, which is normally directly opposite the exhaust port.

bottom line is DA says that their spec is .018"-.020". the ignition system on my suzuki sport bike is way more advanced than any toy ignition we run on our plane engines and yes it is just about the same gap. must be a crap ignition. in all reallity i really would like to know why you [rcign ]think or have tested why such a large gap is needed.