I purchased a 40 trainer builtup construction and a OS 46FX motor. The question is should I use the hardwood motor mount beams and how do I seal the motor mounting holes from fuel or should I use a afteramrket motor mount.

Using the old fashioned hardwood beam mounts is usually pretty much a thing of the past what with all the good plastic and metal mounts currently availble. Reinforce the firewall and use a plastic or metal mount held on with machine screws and blind nuts. If anything happens in an argument with runway you can remove the mount and replace it much more easily than a beam mount. If the beam mount gets cracked or oil soaked you will have to disassemble the entire front part of your fuselage to replace it. The removable mount may cause some damage to the firewall in a crash but that will be easier to repair than removing and replacing a beam mount.

ballgunner has some good points, however before discounting beam mounts out of hand as “old fashion”, I have used them in one out of two of my latest designs. First, don’t use anything but maple or oak. Anything else will crush under the screw loads.

Beam mounts extending to the second or third bulkhead make an incredibly strong structure. It is generally lighter than a firewall mount, especially when the added beef required around the firewall is considered. The beams aft of the firewall provide rails that are convenient for mounting fuel tanks and other equipment. Beam mounts can be tailored to an exact fit and location for the engine. They also provide convenient anchor points for cowling brackets. On the down side, changing to an engine with a different width may not be possible without a lot of rework. Regarding the gaps where the beams penetrate the firewall, cut as accurately as you can then seal the gaps with epoxy.

Like a lot of other things, the choice is a tradeoff of several factors, and a little personal preference thrown in. Of the eight airplanes I currently fly, which range in size from .061 to .80 four stroke, three have beam mounts and the others have firewall mounts.

Finally, if the kit you’re building is designed for beam mounts I would use them. They add strength, and if you leave them out it is usually necessary to modify the firewall structure to take the additional load. An experienced builder has no trouble doing this, but if you are just starting out, you will fare better by sticking to the plans.

Thanks good points from both. If I do use an aftermarket mount I would still build the beams in but cut them off at the motor side of the firewall this would keep the structure intact. I would still have to beef up the fire wall some.
Thanks again.

A good way to beam mount an engine to put the beams out against the fuse sides then get a piece of
1/8" plywood or circuit board - bolt it down to the beams after cutting a "U" in it & bolting the engine to the sheet. This is replaceable/ repairable in case of excess ground effects.

If you build in the beams and then cut them off and use a firewall mount you are just adding weight. No advantage to that.

Louw is absolutely correct on the use of beam mounts. My choice of words "old fashioned" was poor. I too design and build airplanes with beam mounts. Oak and Maple are the only two acceptable woods for beam mounts. If the model you bought was designed for beam mounts then by all means use them as directed. My comments were directed mostly at ARFs which almost always have firewall engine mounts. The choice of engine mounting styles is usually left to the designer or builder. Nothing is engraved in stone and as you can see from the comments the differences all have advantages as well as advantages.

I'm going to use a glass filled motor mount.
Another question.
After I coat the motor compartment with epoxy and cover the plane with monocote what should I use to paint the motor compartment with?

Just my 2 cents. I recently had to rebuild my engine compartment after a nosedive. The original beams had split and were useless. As I had a firewall mount available I decided to use it. I still wanted to maintain at least some of the original integrity so I cut and sanded until the firewall mount met level with the beams. So far it has worked like a charm (guess we'll see what happens to the firewall in the next meeting with destiny).

On a rebuild, use anything that works. You shouldn’t have a problem.

As far as painting the engine compartment after coating with epoxy, any fuel proof paint is OK. I usually use the same butyrate dope I use on the rest of the airplane. Since you have used monocote to cover the plane, Lustrecote will match the covering exactly and is quite fuel proof. Some use Rustoleum, which they say is fuel proof. If you use something else, it would be smart to make a test piece and test for fuel resistance before committing to it on your model.