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 Gas Engine Standoffs 
Next, position the engine mounting plate or engine support flanges where it needs to be on the firewall and mark with a self-centering punch. For additional holding power, I drill the holes small enough that the chosen bolt size has to tap its way into the hole. The actual stand-off I make like this: I use a high tinsel strength bolt that will reach far enough through the firewall mounting holes and hardwood strips that a washer and nut will attach (even though I've caused the bolt to tap into the hardwood). The bolt: first: I chuck it into the lath and face off about two-thirds of the head; then, without removing it from the chuck, I drill a hole the full length of the bolt into which I will later tap threads to match the socket-head bolt that will eventually be the mounting bolt. I use a hand tapping machine I got from Harbor Freight and carefully advance the right size tap the whole length of the bolt. The remaining bolt head makes a nice flat "pillow" for the aluminum rod to rest on that actually determins the standoff distance. Next: I mount the bolts, usually 4, into the firewall. The one third of the bolt head that's left is kinda small to keep a socket on so I us one of the mounting bolts with a nut to use as a jam-nut. I screw the socket head bolt into the tapped bolt head and take it in about half an inch or a little more then run the jam-nut up to the head and crank it down. Now, you can use the socket head to drive the main mounting bolt into the undersized hole I recommended earlier. Once you have the main mounting bolt screwed all the way in, set the nut and washer on the back of the firewall strips. Now you can loosen the jam nut and easily remove the socket head bolt. Do this for all four of the main mounting bolts and you're done with the "hard" stuff. You can now chose the length of the stand off by choosing the length of the bolt you need to get it plus adding some length to reach into the firewall mounting bolt you've just mounted. If it makes you feel better, you could run the socket head bolt all the way through the mounting bolt and put a washer and nut on it too but a little Lock-Tite plus the lock washer on the head of the socket head and that should hold it ok. There should be a fair amount of threads screwed into firewall bolt and I use the smaller tap for the threads when I'm cutting so the socket heads screw in pretty tight. Use a good tapping lub and don't get in a hurry... remember, you're tapping into a fairly hard bolt. I remove the tap and clean the crap out the tap a number of times on the way to the other end of the bolt. Next I use the lathe again to cut and drill the needed lengths of aluminum rod... half to five-eights diameter depending on the diameter of the socket head bolt you use. The rod is carefully faced and drilled full length too and to a slip-fit for the size of the socket-head bolt you've chosen to use. The mounting procedure is pretty obvious... run the socket-head bolt through the engine mount (with small lock washers under the socket heads), slip the aluminum stand-off onto the bolt and screw it into the waiting bolts mounted in the firewall. Care needs to be taken here and it's a good idea to mic the lengths of the aluminum tubes you've made to be sure the engine isn't ultimately pitched at an undesirable angle. On the other hand, this is an easy way to build in whatever angle of pitch you wish. It takes just a few thousandths to achieve a fair amount of pitch at the prop hub. Be sure to mark the tubes for reinstallation position if you do this! This may seem a little involved but it makes for a nice clean installation and a safe easy way to mount and dismount engines as the socket heads are all that need to be removed and this is easily done from the front of the installation. The weight is minimal because a lot of the metal is drilled out and/or removed from the main firewall mounting bolt(s). Because of their inherent strength, the mounting bolts need to be only slightly larger than the diameter of the socket heads... enough for the threads plus a quarter to give an eighth all around the threads. For a more permanent installation, the main mounting bolts could be epoxied in place but this disallows the easy mounting of future engine installations should a major difference in size be desired. Try it, I think you'll like it. There's undoubtedly lots of ways to do this but this is the way I do it and I've never lost an engine. I don't have to ask you to let me know what you think because I know you will!!! --Bob Developed by Geistware of Indiana© ., 1999. |
