I am putting together a Sig Somethin Extra. I am at the point of mounting an Os46fx and I have run into snag. Being the first time of building an arf, I don't know the correct way to mount the engine to the mounting arms. I have the arms on the firewall, but I don't know if they are correct. If I put them on one way, the engine sits high. If I turn them over the engine sits lower in the fuselage. Which way is correct for these plastic(I think) mounts? The second question is about placement of the engine on the arms. From what I've read, the spinner backplate should be about 1/8 to 1/4 inch in front of the nose of the plane. However, if the engine is placed all the way to the rear of the arms, the engine (spinner backplate) still appears to be to far out in front of the nose. Third question- how do I actually attach the engine to the arms? I know the engine has 2 flanges with holes, but do I drill through the arms with a drill bit and then put some certain size bolt with washer on top and a matching nut on the bottom of the arm? I can't tell if the flanges have threads in them. And last if the firewall has the correct thrust angle built into it, how can I be sure that when I put the engine on the mounting arms that it will be the same as the firewall angle? Thanks.

First, the holes in the engine flanges aren't threaded. You can either drill completely through the mount arms, and bolt the engine in place with washers and nuts as you indicated, or you can drill and tap the engine mount (which is glass-filled nylon) for 4-40 bolts. That works for me, but some people don't recommend it, preferring bolts and nuts.

Second, placement of the engine on the arms should be more a function of balance than distance of the spinner from the nose of the plane. If the engine sticks out more than 1/4", so be it--it's not as important as a well-balanced plane.

The thrust line of the engine (how high it sits) is indicated by the bottom of the engine flanges. On my SSE, this is almost exactly at the most forward point of the fuse sides. Your pictures of the mounted engine look correct. As for right thrust, if the engine sits square on the mount (i.e you drilled the holes nice and even), the engine will be correctly aligned. A little off isn't anything to worry about.

I agree with Nickj on all counts-except I'm one of the group that prefers to bolt the engine to the mounts with socket head bolts and those nylon insert nuts. I don't take chances with the vibration created from these engines. As far as the engine sitting on the mounts high one way and low the other, try to line up the height of the carb with the centerline of the fuel tank. That will determine which way the mounts should be. Good luck on the plane-and check those low rates!

You've gotten excellent answers, so let me add a couple of more details.

Drilling the holes in the mounts can be a nightmare. To mount the engine strongly enough, you need to use bolts that are a close fit through the holes in the engine's flanges. And then the bolt holes through the mount should give a tight fit as well. What that results in, are holes throught the mounts that're very accurately aligned with the holes in the engine flanges. And since we're working with very hard stuff (bolts, flanges, mounts), any misalignment makes it real hard to get the 3rd and 4th bolt through the holes after the 1st two are placed, unless all 4 holes are very closely aligned.

If you try to clamp your engine to the mounts and use it's holes for a drilling jig, you run into two problems. It's really difficult to clamp that sucker to those oddly shaped, and suddenly very slippery mounts. And if you do accomplish that, a power drill can really trash the holes in the engine flanges. Not good. Very not good! And most drills can't work close to the engine and go squarely through the holes anyway. Thank goodness.

So you're stuck having to mark the centers on the mounts. Some engines come with templates for the job. Kewl if you got the template. HOWEVER.... measure it to see if it's a good match with your engine before you use it. And if you don't have one.......

You find out soon enough that it's hard to keep the engine steady to mark the holes. Use a touch of any kind of glue that works. Glue the engine to the mounts, then mark the holes. And now you find out that a fat pencil won't get through the engine holes and reach the mount. And then you find a thinner pencil and it still won't square up on the side of the engine with that exhaust port sticking out. So you find an even thinner pencil (I've got one from 1970something, I've saved and used for only this job.) and discover it won't mark the motor mount material. So you pull the insides out of a ball point pen.

And discover that it's still fairly hard to get a nice round mark in the center of each hole. Those exhaust ports are a real pain.

But you finally get marks where they ought to be. Now you discover another problem. If you don't get all 4 holes to go through the mount at very close to 90degrees, the bolting down will still be a pain.

I've got a drill press. I've got to be honest and suggest that the primary reason I bought it was to drill motor mounts squarely. And after I bought it, I discovered that most motor mount don't fit easily into a drill press vice!!! And one day, I was rebuilding my front porch railings and had some 4x4 post scrap. And it was accurately square all around. I took the time to set it up to bolt my most frequently used motor mounts to it. And now I use it to hold any mounts that I wish to drill on my drill press. I keep that block of wood with that pencil.

There is an excellent little tool that's sold in most good LHSs. It's a "hole center marking tool". It actually has a small drill as it's marker. And it's got a tapered piece that centers the thing in an engine's bolt hole. You just have to make sure it's straight up for it to accurately mark. It now sits with my block of wood and pencil.