Can someone tell me the best way to find out where to drill holes on the firewall for the engine mounts. I have built several planes in the past but all have had the holes predrilled. The book says install motor. Period. This is a cap 232 with a YS FZ 91 4 stroke motor. The other little problem is that the distance from the firewall to the prop washer is 120mm. I do not have that much room because of the pump on the back of motor. I need about 8 to 10 mm more space. I guess the only problem might be the prop sticks out farther than it is supposed to and maybe the CG might be off some. Any Ideas.

First, you need to bolt the engine to the engine mount. You can even use C clamps to temporary mount the engine to the engine mount. Then measure the horizontal distance between the left and right mounting holes. Divide that distance by 2, which gives you the amount you measure out from the vertical centerline marked on the firewall. Then just line up the crankshaft centerline of the engine mount (the surface on the engine mount where the engine flanges sit on) with the horizontal centerline drawn on the firewall.

It may be possible to open up that firewall (cut a hole in it) , and allow the
rear of the engine to fit back into the recess.

FBD.

Well that makes sense. Do you have any ideas on the distance from the firewall to the prop washer. Is that a big factor here. I might be able to push the cowl out farther but won't know till the motor is mouted.

Yes but if I did that it might weaken the whole structure or cause adverse vibration. The part that you mentioned to cut would be at the very bottom. But good idea.

You can beef-up the firewall with some triangle stock, or fiberglass it if necessary.
You need to mock-up the engine on the mounts, and see what you have to do
(distance-wise) to get the cowling to fit properly.

Refer to what Richard L. said.

It may well be that your engine selection would be more trouble than it's worth. []

FBD.

The proceedure explained earlier " measuring side to side " works for the horizontal but wont help the vertical. Many Kits and arf"s have the engine located above or below the center line. How do you figure that out. Many do not have side profiles on the plans.

For the vertical, you measure the distance from the crankshaft centerline on the engine mount up to the top mounting holes and transfer that distance to the firewall. Then measure the distance from the crankshaft centerline down to the bottom mounting holes and transfer that distance to the firewall. The principle is the same.

I can't measure. So I do it the dummy way.
Put 4 screws that fill the engine mounting holes and that stick out the back of the holes by about a 1/2". Put 4 nuts on snug, make sure at least 3 or 4 threads are showing.
Press hand pressure hard. No good dents ? 2 people and a piece of carbon paper.


Puttin a point on the screw makes it 1 person again. Your choice.