I don't have any Enya engines, but if it's old enough to use a clamp-on (factory) muffler, I'd expect 3mm major dia. x .6 mm pitch, from OS and ST experience from that era. Somewhere along the line, that size became non-standard, and more modern engines use 3m x .5 pitch. For metric, that means .50 mm (almost .020") from peak to peak, unlike the way SAE threads are measured. If you have the old screws, you should be able to scale them (.0236" for .6mm pitch) to get a general idea. The 3M x .6 is almost dead nuts on a 5-40 thread, and most engine tuners routinely put 5-40 screws into the old ST and OS engines, generally without anything other than lubing up the 5-40 screws a bit.
If you have an old screw, measure the OD...3mm would be just under .120". Check that, then try meshing a 4-40 thread screw alongside the old one. If it's .6mm pitch, it'll be almost a perfect mesh, but the diameter is wrong. DO NOT use a 4-40, it will be very loose...but it would screw into the female thread and wobble around. The 5-40 screws aren't real commonly used, but they are available from a good industrial fastener store or online from various sources, such as McMaster-Carr.
There are some sources for "3M x .6" (apparently the proper designation) screws, but personally, I would use 5-40's because they'll use a 3/32" hex wrench and I won't have to dig out the metric set...tho I carry both in my field tool box and have several sets at home in the shop.
Steve
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