I am about to mount an engine on a P-51 and not sure how to make the prop shaft align with the cowl. I have a few ideas, but am curious to get more.

Hmm! You could mount the cowl then eye ball the center where the prop shaft goes. Maybe even tape some string on the opening to mark the center. But if the firewall has right thrust and down thrust built in you have to account for that. All is not lost if your off on your first attempts. Just plug the screw holes with a wood dowel and expoxy then sand smooth and retry.
I always mark the firewall engine center prior to gluing. Just did all this a few days ago on a P-38.
Edwin

I would mount the cowl first without the engine and then poke something in through the hole to mark the firewall where it aligns with the centre of the cowl. Granted you do need to account for engine offset/right thrust etc. But this would be a way to start.

Mount the engine mount, then the engine, then the cowl. After you have the engine mounted in the mount on the firewall measure the distance between the firewall and the drive washer on the the engine. Now tape the cowl in place on the front end of the plane. Set the cowl in place so the the front of the cowl where the engine shaft comes out is 1/8th of and inch closer to the firewall then the drive washer. Follow me? This should give you enought clearance for the right thrust. Do not line your cowl up with the right thrust of the engine, it will not look right. You may have to take the engine out of the plane to mount the cowl if the engine doesn't fit in the cowl.

Now after you have the cowl taped on the front end drill 3 holes on each side of the cowl threw the cowl and the plane. Take the cowl off the plane and drill the holes out ON THE PLANE large enought to fit some yellow nyrod in the holes. CA the yellow nyrod into the holes on the plane. This will give you something good to screw into instead of screwing into wood.

Get your cowl and put some rubber gromets into the holes. This is a good method to vibration mount your cowl. Esp if it is a plastic cowl.

Now put your cowl back onto the front end of the plane and secure the the cowl to the plane with some #2 button head screws. There you go!!!! a nice vibration mount cowl setup!!!!

Email me if you need some help.

Blackdog

With right thrust, you cannot mount the engine on the center line. I take a yard stick and tape to the center line of the fuse, hold the engine and mount in place and mark where the prop hub will be centered, eyeballing from the top. Have never done this for up or down thrust but most plans come with a drawing of where the thrust line is, the same theory could work.

Drill the mount holes appropriately.

Mount the cowl after the engine is mounted. Remember to measure the distance from the from of the cowl from the firewall and ensure spacers are taken into consideration if needed for mounting the engine mount.

Ed

PS for 140-120 4 strokes I figure 1/4" round about from center for right thrust, and 3w100-150 3/8" from center if you just want close.

When I put together a firewall from a kit with designer recommendations for right and downthrust, I like to install the firewall angled right and down. Often this feature is built into the kit; but if not, then I build it in myself. Then there is no funny business with washers or shims. If the design is off a bit for my choice of engine, then I trim with shims and washers from there. This can only be determined through flight tests.

So, if the firewall is angled, and you center the engine mount on the face of the firewall, your prop hub some four inches out from the firewall is going to be off center from datum by some fraction of an inch too far down and too far right. To compensate, you shift the mount relative to the center of the firewall by the appropriate distance up and left. You can work out these distances with pencil, paper and protractor or drafting machine or CAD software or trig tables or trig functions on your calculator, depending on the vintage of your schooling.

If the cowl supplied with the kit assumes no offsets, then the spinner mounted to your propshaft will not line up with the ring of the cowl. Now one of two things will be awry--either the cowl will fit the fuse at an odd angle, or the spinner will interface with the cowl nosering at an odd angle. The only way to fix this problem is make a new cowl, by mounting the engine with proper offset, then fitting a plywood dummy nosering on the propshaft, then building out and shaping from firewall to nosering so that everything is harmonious. This is a lot of bother for the average sportflier, but very absorbing to the scale purist who has lots of time and patience at his disposal.

I don't know what size your plane is but the most positive way is as follows. If your engine is on motor mounts, mount it. If it requires right thrust, ca your spacers under the left motor mount base. Now Hang the plane vertical, place the engine on the firewall and put the cowl over the engine. Locate the cowl and tape it in position. locate the engine to center, remove the cowl and mark the mounting holes. WAALAA perfect location. If mounting directly to the firewall ca the spacers to the back of the engine and do the same. If its a big plane hang it outside from a tree if you have to, I do this all of the time with large planes. Ken....The "BALSA BANDIT"