rwh

Once again I have come to the point in the assembly (yes, it's an ARF) where I have to mount four hardwood cowl mounting blocks. I have tried many methods over the years, but nothing seems satisfactory. The blocks go onto a shiny fuel proofed firewall, they don't absorb glue very well, and they split easily when drilled in the direction of the grain.

Stick them on with CA? Forget it.

Epoxy, maybe good for one flight.

Drill and screw on? They will split and the block mounting screw will invariably hit the cowl mounting screw.

Throw the blocks out and make your own, but bigger? Done that.

Throw the blocks out and screw the cowl directly into the firewall? Done that too.

Here's what I'm doing on my curent project. I drilled little holes in the bottom of the blocks and in the firewall to give the epoxy something to hold onto. About 9 holes, 1/8 inch diameter in the block and firewall at each location. (Thanks, RCU, for that tip). Now I need something to hold the blocks in place, preferably with some clamping pressure. Anyone who has done this knows that this is a difficult spot to attach a clamp.

After fussing with rubber bands and other things, I discovered a workable technique. I used my big clamps to make a surface above the mounting blocks, then smaller clamps to hold a board onto the handles of the bigger clamps. Finally I used some foam and various shims to apply pressure to the mounting block while the epoxy set.

This seems like a lot of effort for this little job. Who has a better idea?

michpittsman

My Feedback: (11)

Tetherite makes some plastic cowl mounting blocks in more than one size that may work better. Prather used to sell some 1/4 turn fasteners with a captive screw that were used to hold down hatch covers on boats; never had a chance to use them on a cowl...JIM

MinnFlyer

My Feedback: (4)

Holy Cow! That's a lot of work!

I stick them right to the firewall with epoxy, no surface prep. Add a drop of Med CA with kicker to hold it in place until the epoxy cures. That's it! Never had one come loose even on big gassers.

Next, enlarge the screw hole, and fill it with a short piece of inner Nyrod. It will accept a #2 screw, and no more screws splitting the block (The nyrod can also be drilled out slightly to accept a #4 screw).

rwh

Hmmmm. CA to hold until the epoxy sets. That sounds like a good idea!

I've heard about the Nyrod idea, but never done it. I think I will try it on this project, I need at least #4 screws here. I suppose the Nyrod helps to prevent loosening too.

Thanks!

basackwards

My Feedback: (1)

I used this method on my U Can Do and it will be standard operating procedure from now on.

Get some rubber insert nuts from Lowes or Home Depot. These are a top-hat shaped rubber insert with a nut built into them. They work like a thermos plug. As you tighten the machine screw into them, they swell up and tighten themselves into the hole they are in.

Also pick up some light aluminum angle extrusion big enough to bore a hole in to fit the insert into. I think I used about 5/8" x 5/8".

You are going to build a small angle bracket with a large hole in one side for the insert and two small holes on opposite corners of the other tab for small wood screws. It's easiest to lay out a bunch of these and do the drilling while they are all together in the extrusion and then cut the pieces apart into individual brackets once the drilling is done. It's hard to hold those little brackets while drilling a 3/8" hole into them!

Mark the holes for the small attachment screws on your firewall so that the bracket with the rubber is about even with the side of your fuselage. This will leave the rubber flange sticking above the surface of the fuselage. Don't worry about a gap. The rubber will squash down a bit, and this leaves a bit of a gap between the fuselage and the cowl so you don't get rash. If you want them tight, set the brackets a bit tighter. I like to leave a slight gap so there is no rubbing. Now predrill the screw holes and epoxy and screw the brackets in place. I use four per cowl.

Match the holes in your cowl and attach with cap screws with rubber backed washers which you can also get at Lowes or HD. As you tighten down the cap screws, you sandwitch the cowl between the rubber insert and the rubber washer. If you made the holes in the cowl a bit oversized, you will have a cowl mounted completely in rubber with no metal connection to cause vibration tears or cracks!!!

You can build similar brackets for your fuel filler and remote glow connections.

This works great and never comes loose unless you want it to. Also the screws and inserts won't wear out, but even if they do, they all can be popped out and replaced.

This is a bit of work, but it's worth it. If you make up a dozen, you have enough for three planes and can reuse them as planes go away.

Good luck.

Basackwards.

c550

My Feedback: (16)

For cowl mounting blocks I like to use bamboo dowels drilled thru the blocks into the firewall. Epoxied in they stay there forevere

famousdave

My Feedback: (61)

I have used this method on many, many planes and not a SINGLE failure in 10 years.

1> use good quality hardwood servo rails or engine mounting rail 3/4" x 3/4" x 1/2"
2> scuff the firewall with a dremel or 100 grit sandpaper (I even do this successfully on fiberglass firewalls, like on the lancair ES from GP) where the wood will attach. Prior to gluing - wipe with ALCOHOL (not thinner or acetone, both these leave petroleum residue that will weaken epoxy).
3> attach the mounts with 30 MIN epoxy (30 minute allows more working time and it is stronger)
4> test mount the cowl and then drill 1/16" holes into the hardwood blocks where the cowl attaches. Use small screws to hold the cowl in place while you drill all the holes, putting a screw into each new hole before drilling the next one.
5> take the cowl off. Drill the 1/16" holes out to 1/8" carefully. Press in a 1/2" long piece of Nyrod insert (yellow part)and use CA to set it.
6> mount the cowl and set the original screws into the nyrod. They will never come loose in flight and remain tight after many removals. If after many uses if they are not tight enough, simply drill out the nyrod and install a new one and re-ca. Simple and replaceable.

If you want a longer screw - servo mounting screws are perfect for this.

I have used this on 1/2a all the way up to 33%+ planes. It works well and every time.


DP