Re: 1981 evinrude crossflow surface gap or not
Be very careful if you intend to switch from SG to conventional plugs. Reach and heat range are critical. Reach can get you in trouble as many conventional plugs have projected noses so they stick farther into the combustion chamber. You need to use a depth gauge to determine how much piston-to-plug gasket surface clearance there is with the piston at the top of the stroke. If your engine has a deflector on the piston crown, make sure that's taken into account in the measurement. This determines "reach". As a starting point in the selection, spend some time on the NGK or Champion web sites analyzing plugs for engines in your engine family. Generally this can be determined by displacement. Go back in years and forward in years to see if there ever was a conventional plug referenced. If there was, try it. WARNING: Do not install the plugs, dash out the lake and hammer down for 5 miles. Pay attention to every detail starting with ---- Starting! Better, worse, the same etc. As the engine warms, again pay attention to the little things that may tip you off that something is better, worse, or the same. Then try a few short part throttle runs where you accelerate to plane, but not using full throttle. If everything seems good so far, make a relatively long run at about half throttle. Chop the throttle and kill the engine as quickly as possible. Read the plugs. If they have the start of a nice light tan, you are now good to go for a "short" high speed run. Listen to the engine and be alert to any sign of a problem. A proper heat range plug will develop the tan color in a relatively short period of time. Until you are certain you made a good choice, check plug condition after every trip. Over time you may determine that a heatrange hotter or colder is necessary. The NGK web site has a very nice descritpion of how heat ranges affect engine operation. Most newer ignition systems can handle .035 - .040 gap. I happen to compromise and split the difference. On a recent outing I stuck the SG plugs back in my engine and could not wait to get them out again. In a nutshell, conventional plugs (in my application) starts better, idles better, gives me about 200 extra rpm and the engine just seems happier with them. As a final word of caution -- if you are mechanically challenged, by all means don't mess with the manufacturers plug recommendations. You should only depart from that recommendation if you know what you are doing and your operating conditions require a plug change. Having a properly tuned engine to start with will eliminate the need to even think about a different plug type.
