By Matthew Neumann • Originally published in Stunt News, July/August 2009
Editor’s note: This article was originally published in Stunt News and has been reformatted for digital readability. Original terminology, measurements, and technical details have been preserved where possible.
Bolt holes. These are a personal bugaboo of mine. I am one who, if at all possible, likes to eliminate them, or at least put them in such a place as to not make them too noticeable. However, bolt holes are a fact of life for our airplanes. There will always be some sort of hatch or other item that needs to be attached with some kind of bolt. So, how do we make these as attractive as possible? Keep reading and I will show you what I have done.
Here is a picture of the exhaust stacks on my 2002 plane. The full-scale Stuka has a fairing of some kind over the front of the first exhaust pipe with an opening in the front of the fairing. I just put the screw hole where the front hole of the fairing would be. This helps to “hide” the bolt hole even though it is in plain sight.
One idea is to incorporate the hole into the paint scheme in such a way that it blends into the surrounding area. Think of how you are going to paint the plane. Would it be possible to move the hole slightly one way or another to be able to incorporate it into the paint scheme somehow?
A way that I have handled this for the cowl bolts on my 2002 plane (which I jokingly call my “Frankenstein bolts” because they come in from the side) was to incorporate the bolt hole into the front of the painted exhaust stack. I still had to use the method of dressing off the bolt hole that I will describe later on in this article, but at least the hole is not as noticeable as it would have been had I just left it out in the middle of a panel.
Here is a shot of my cowl with the bolts snug enough against the balsa wood to hold the cowl in place but not so tight as to crush the balsa. I then drew a line around the bolt heads so I could use this as a locator for the outside diameter of the aluminum tubing.
Another idea is to put the screw hole attachment points on the bottom of the plane. However, there are some applications for which, no matter how hard you rack your gray matter, you just can’t come up with a good way to hide the bolt holes. The cowl mounting bolt holes on my Mustang is one of those cases.
Ok, so now what? No matter where you put the holes, you still have to dress them off so that they at least look nice. Here are a couple of different ways to do this depending upon the situation.
Here is a side shot of the nose of my Mustang. No matter what, I just could not come up with a good way to hide the bolt holes on the bottom. So I had to just put them on the side. Although, I did not do it here, some people will paint the heads of the bolts the surrounding color in order to make them less noticeable.
First is the type of hole that has some depth to it. On my 2005 and later planes, I figured out an attachment system for the cowl that allowed me to put the bolts on the bottom on the plane where I would prefer them: out of sight. The method described here would fit any bolt hole situation that has a depth greater than the thickness of the bolt head.
The first thing to do is get some aluminum tubing that is the smallest diameter that you can find that will still slip over the bolt head. Now, I am assuming you are using socket head bolts. You are, aren’t you? If not, get some and use them! These are much easier to work with because you have much less chance of the tightening/loosening tool slipping, not to mention the fact that the tool itself will fit much easier inside the tube.
Now that you have the tubing, the next thing to do is to bolt everything together. This is to ensure that everything will be aligned properly giving you a better chance of getting the hole location correct. Now mark exactly where the bolt head is by tracing around it with a pencil. A dull pencil is actually a good thing to use here because it will produce a thicker line which will show you how much bigger you have to drill to accommodate the aluminum tubing.
Another method would to be place the tubing over the bolt head and gently press down in a twisting motion making an impression with the tube—builder’s choice.
This is what it looks like when the bolts are tightened up and the pieces of tubing glued in. Note that the tubing at this point is cut slightly long. The tubing goes all the way down to the bottom of the hole and touches the washer imbedded underneath.
This is how the end result should look. The tubes have been carefully ground off and finished with a sanding block and 180-grit sandpaper. You may have to take the end of a dull number 11 blade and clean out the inside of the hole. Notice I said dull blade. If it is not dull before, it will be after you clean out the hole. This is one of the few cases where a dull blade is actually preferable to a sharp one.
Remove the screw now that you have everything marked and drill a hole all the way down to where the base of the screw head will sit—preferably against a washer. You need to seat it against a washer. It is unwise to have the screw head push up against bare plywood or worse, balsa wood. Over time, the screw will wind up pushing its way through the wood making a hole the size of the outside of the screw head. Not good.
Make sure one end of the tube is square and cleanly cut. If you used a tubing cutter, the end that the tubing cutter was used on will need to have its end opened up so that the screw head will slip through the end of the tube. Now put the screw back into the hole and tighten it.
Insert the tube with the squared off end over the bolt head and mark the tube where the end of the hole and the tube meet. It is usually best to mark all the way around the tube for clarity sake. Cut the tube off slightly long. Then simply glue the piece of tubing into the hole as a liner (with the washer already seated at the bottom). You can use instant or epoxy glue for this task.
When the glue is dry, you can take a motor tool with a grinding attachment and grind off the excess tube so that it is just barely too long. Be careful with this, however. The grinding action heats up the aluminum tubing and can soften the glue to the point where the tube will want to move. If this happens, just stop, and the glue will cool off and reattach everything. The last little bit can be then sanded off with a sanding block and 180-grit sandpaper.
If you don’t want to use aluminum tubing to line the bolt holes, there are two other choices I might suggest. One is brass/copper tubing, although you need to be aware that this material is actually harder than aluminum so it will therefore be harder to shape to the final size. The other material would be a carbon-fiber shaft of the appropriate size. This would probably be the best material if you can find the right size because it is the lightest, easiest to sand, and the glue will stick to it the best. Also, when grinding it down with the motor tool, it won’t heat up and melt the glue.
Here is the hole drilled and the washer placed in the bottom of the hole. I am using this method for the tip weight box lid.
Here is the assembly with the “greased” up bolt head. Notice the large gap around the head. This is about to be fixed properly.
Now, what do you do if you have a hole that is shorter than the bolt head? You could still use the method above but because the length of the tubing is so short, it will be much more difficult to accomplish. Here is a method I used on this type of hole.
The first thing I do is bolt everything together again. Mark the location of the bolt head as described above except I am not worried about making a thin line around the hole this time. I then drill out a much larger hole than necessary. I make it big enough so I can put a washer in the bottom of this hole.
I then cover the head of the bolt with a thin layer of petroleum jelly. Notice I said cover it with a thin layer, not glob on a bunch of it. Everything is bolted together again with the now “greased up” bolt head.
The next step is to mix up some epoxy putty and put it in the oversize hole with the bolt in it. The Vaseline is used as a mold release agent. Do not pile up the putty over the head of the bolt. Try to keep the putty about the same level as the surrounding area. Remember, I said this method works best if the hole is the same or shorter in height than the bolt head.
Here is the finished product. Notice that we now have a clean edge that matches perfectly to the size of the bolt head that we are using. You can see about how thick the putty is from the oversized hole that was made earlier.
If you happen to get some putty in the hexagon hole in the head, just use an old knife blade and pick the putty out of the hole. With the “greased” up head, it should just pop out very easily. Once the putty is dry, you can do some sanding with the bolt head in place to get a rough leveling out job. Once that is accomplished you should then be able to easily remove the bolt revealing a nice round crisp hole. Some light touch-up sanding may be necessary at this point.
Well, that is how I make nice neat holes for the bolt heads in my planes. The method is not that difficult and it really does not take that much time but it really dresses off those areas nicely. They not only add to the looks of the plane, but they hold up over time with many removals and attachments of the pieces. The 2002 plane is seven years old with over 2,500 flights on it and it still is in fairly good shape in these areas.
It is all in the details. Until next time…
This article first appeared in the July/August 2009 issue of Stunt News and has been preserved as part of the PAMPA digital archive project.
