roadtrip

My question is about where to position the engine on the motor mount in relation to distance from the firewall and the front of the motor mount. Does it make any difference, other than with the balancing of the model? I ask, because I have a motor mount for an OS .60 size engine and miss drilled the holes on one side of the motor mount. What effect, again aside from balance would I get if I relocated the engine, forward or backwards on the same motor mount and drilled four new holes? How will this effect the strength and vibration qualities of the mount, etc? Also, a friend gave me an old plane of his and I found that the motor mount bolts were smaller than the motor mount bolt holes in the engine being used, but fit the holes perfectly that were drilled into the motor mount. It didn't appear to effect the engine performance, as he has been flying this plane with the same set up for a couple of years, now. I would feel better if I replaced the bolts with bolts that fit the engine mount holes and redrilled the motor mount holes, even though the engine didn't have any problems with this set up before. What do you think?
[X(]

LouW

Extra holes drilled in a motor mount will weaken the mount some. Depending on the location, it may or may not be significant. If the mount is metal, the strength should still be satisfactory. If it is plastic, I would consider the following. If you have room to move the engine backward (toward the firewall) and drill four new holes, the effect should be minimum. If you have to move the engine forward to drill the new holes I would consider replacing the mount.

Regarding your other question, it won’t hurt anything to drill the motor mount holes to a size to match the ones in the engine, but since it has been flying with the smaller bolts for several years there probably isn’t any problem keeping things like they are. The only advantage I see in re-drilling the holes in the mount and using larger diameter bolts is that there is less chance of accidentally introducing some side thrust by mounting the engine a little crooked.

DarZeelon

Bob,

Most glass filled nylon engine mounts cost only $3-$6, and if your mis-drilled mount is of this type, it is best to discard it and get a new one.
If it is an aluminium engine mount, a good inspection can tell if it is ruined.
When buying such an aluminium mount ($14-$40) it would be best to get one that is already pre-drilled for your particular engine. Another possibility is to get a hole locator, like [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXK378&P=7]this one[/link].

Another possibility, is to get a mount that has slots instead of holes, or a "press down" bar, that can be used to mount just about any engine.

Vibration damping mounts do isolate your model from the engine vibrations, but the fact that the engine is allowed to vibrate more freely, could end up in broken headers, exhaust stacks and even engine mounting lugs.

I would keep away from most of them, especially with two-stroke engines.