I've never used an OS .25, but have one and am getting ready to give it a try. What size bolts/screws do you guys use to mount them? A #4 machine screw looks very wimpy. A #6 won't fit through the mounting holes. I generally like for the screw to completely pass through the mount and use a nut to retain it. Do I go with the #4 screws/bolts or is there a better size? I'm ok with metric sizes if needed.
What are you mounting the engine on? Typical wood profile, 4-40 will work fine. I prefer "T" blind mounting nuts. I also prefer socket head bolts. A wood mount will continue to compress over time. Socket head and Blind Nuts will be easier to get to, to tighten up prior to a flying session. On the resin mounts, you can tap into them and use sheet metal screws, #6 or larger, however it is better to tap for the bolt and back up with a lock-nut. I am not afraid to drill out an engine's mounting holes to assure a well mounted engine.
4-40 will adequately handle up to a .35. From the .35 to a .75, I use 6-32, and above that 8-24 or 8-32. Once I hit the 26cc, I go for a #10 bolt preferably black, but in actual, whatever is handy. I always use T-nuts when the nut is not handy on the flying field. Nuts and bolts will always loosen for the darndest reason at the darndest time.
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More so on wood and wood firewalls as the wood compresses, even when prior epoxy soaking is accomplished.