Many folks know that I've pulled back from participating in the big Internet RV-building groups, preferring instead to depend on the advice of a smaller group of favorite RV builders. They sure can come through for you, which is why I recommend people join a regional building group and look for help on a one-to-one basis. That's not a knock against the Internet groups, but for me -- presently -- human communication with people I know, or at least think I do, is quite rewarding.
On Saturday, the Minnesota Wing of Van's Air Force held its quarterly meeting. Alan Jesmer from Precision Airmotive flew in to speak to us, indicative, I think, of how valued the experimental market is by more and more companies.
If you're an RVer, you probably know that Precision Airmotive recently sold its carburetor line because it could no longer afford the liability insurance across its 11 product lines. It seems a couple of guys flying a Cessna 150 on a hot day in Florida, tried to do a touch and go and forgot to retract the flaps. The plane crashed and, according to Jesmer, the lawyers has recently realized his company had $50 million in insurance coverage and so they were sued, again. It costs a lot to lose a product liability case; it costs a lot to win one, too.
Jesmer spoke for a couple of hours about electronic fuel injection -- his company makes the Silver Hawk EX,which is geared toward the experimental line. By the way, a service bulletin for these was issued last week. He also talked about his company's version of FADEC -- The Eagle EMS. Given I'm at the stage of ordering an engine, I'm getting bigger and bigger headaches the more I learn; each seems to be proportional to the amount of money mentioned.
Doug Weiler does a great job with the Minnesota Wing. I'd guess there were about 80 or so people who showed up. It was a good reminder that most people aren't regular posters on the bulletin boards and these face-to-face and smaller groups are really a fabulous way to get help.
Last week, Doug was good enough to help organize my accumulation of information on how to drill the engine mount to the firewall on an RV-7A. There is no way to clamp the thing on and really keep it from moving while drilling. And I was concerned about predrilling the holes (that are predrilled in the firewall but not in the gussets and angles behind it except in quickbuild models.
I had, however, drilled the holes open to a 3/16" diameter, and then got concerned since several of them would be slightly off center given the varying tolerances of the manufacturing process of the engine mount.
So Doug solicited advice on my behalf and I got some really good suggestions, some of which I provide here:
First, my buddy, Warren Starkebaum, sent me a Word diagram on how he clamped the thing. He advised against drilling the holes ahead of time.
I don't have the clamps necessary here. I used a pipe clamp, however, to semi-hold the mount in place by putting a 2x4 across it and then clamping it to the rudder pedal tubes.
Moving on, Bruce Fiedler of Janesville, Wis., sent this tip:
I used 1/4" bolts in four points to attach the mount to the firewall. They held it very firmly. I then just removed one 1/4" bolt at a time (starting with the one that was the closest to being centered) and used the mount as a drill guide. I used my battery electric drill and lots of Boelube. When the first hole was enlarged, I inserted the finished bolt in place, tightened, and removed the next 1/4" bolt and had at it again. Got the four corners done and then did the middle tow on the bottom (which needed about 1/8" spacers made for a final fit.Dick DeCramer has some additional thoughts:
Reduce the size of the holes in the motor mount by using drill bushings to a manageable size bit...say, 1/4" or less. Drill bushings are available through tool suppliers and usually come in sets which have a shaft with a center hole already drilled in them. The bushings are designed to be fit inside each other reducing the interior guide hole to very small dia. center hole in steps which will drill easily and remain centered in the appliance (Motor mount in your case.) If you know some Northwest mechanics they probably have a set from work & nice guys will loan them to you.
However, since the average guy seldom uses drill bushings they are difficult to find so I improvised by finding a tube (pipe, steel tubing, etc. at the hardware store.) with the outside diameter the same as the motor mount bolts. You need to find 2 or 3 tubes that will fit snugly inside each other. I am estimating about (AN6 bolt)
or 3/8" O.D. tube with 1/16" wall thickness would then need 1/4" OD tube to slip inside of this tube and if that second tube also had 1/16" thick wall would then be able to insert an 1/8" drill bit into the 2 concentric tubes and the drilled hole would be exactly in the center of the Motor mount hole.
Clamp the motor mount to the firewall at 3 points and insert the nested drill bushings into the 4th corner bolt hole. Using the bushings as a guide, drill the first hole at 1/8" (as in my example), remove the innermost bushing leaving the larger bushing hole of 1/4" and now drill the hole with a 1/4" bit. Remove the last bushing and use the motor mount itself as the last guide and drill it to 3/8". Debur the hole, insert the AN6 bolt and repeat for each hole until all four are drilled. Do not drill the hole full size in one operation or you will likely catch and tear the stainless firewall as it is too thin not to mention spraining a wrist or thumb. I hope this helps
Bob Barrett of Sartell, Minn., (who I also saw at the meeting) had more:
You need to probably drill the upper left and upper right pilot holes with out the engine mount in place. Measure each of these holes at least three times and then recheck once more. Then center puch them and drill #40 or #30 pilot hole and verify that they are in the correct location. You will probably need someone to help you hold the mount up to the firewall to verify that the pilot holes are in the correct place. You can put a light in the fuselage just behind the firewall and then the light should show threw the engine mount hole if you have them correctly placed. If they are not correctly place you can drill another pilot hole and if the first pilot hole needs to be closed you can close it with pro-seal. If the pilot holes are correct just enlarge them to 1/4" and then use hardware store bolts and washers to hold the mount in place and drill pilot holes for the rest of the engine mount holes. Usually you can find some tubing as a spacer for your drill bits when drilling the pilot holes with the engine mount fastened to the firewall with temporary bolts.
In the end, I incorporated a little bit of everything that I was told. The bushings idea was great, so I ordered four of them from McMaster-Carr -- two with 3/8" outside diameters to fit inside the engine mount holes. The smallest it reduces to, however is 1/4", so I ordered a handful of 1/4" outside diameter bushings that reduced to a #40 and a #30, figuring I could slip it inside the other bushing. Unfortunately, I learned after spending $45, that they don't fit inside each other. Rats. Anyone want to buy some bushings?
So the only bushing I was able to use was the one that reduces to 1/4". The other bushing that had to be shipped from the New Jersey warehouse (I would have used it to step drill the hole from 1/4 to 5/16") never showed up.
So after the meeting yesterday (those meetings are always good motivation), I tackled the task. For those of you who haven't done this yet, here's what I did:
First, armed with some more advice from Ken Scott (who also said it's OK to use the engine mount as a drill guide) , I think I did the right thing in drilling the four holes out to 3/16". You can do thisif you measure ahead of time and can be sure that the outside diameter of the hole doesn't go beyond the 3/8" hole boundary in the engine mount. Yes, it's likely to be off center when you go to fit the mount, but this isn't the problem I'd thought it would be.
Once those are drilled, pick up three 2" 3/16" bolts from the hardware store along with three large washers and some nuts or wingnuts. Push them through all but the most offcenter hole, attach the washers and nuts and tighten finger-tight. Now, move the engine mount slightly to try to center -- as much as possible -- the hole in the engine mount over the hole in the firewall. You can't see this because the washers will block your view, but you can estimate it. See above photo, obviously. All of these photos can be enlarged by clicking on them, by the way.
Draw a circle around the engine mount on the firewall with a Sharpie, so you can be sure the thing doesn't slip around (Note: You can also do this before adding the washers and nuts if you have a second pair of hands.
I then put the 1/4" OD bushing in the open engine mount hole and used a 1/4" bit and lots of Rapid Tap (cutting oil) to slowly enlarge. I had concerns that the drill bit would wander into the open hole if it was off-center (see photo above), but with the bushing, this was not a problem.
It takes quite a long time to get this hole open -- at least 10 minutes in my case.
I had also bought some 1/4" bolts and inserted one here (above photo), figuring I'd expand all the holes to 1/4" and then go back and enlarge each one to size. I decided, however, that I would finish each hole fully before moving on. Since I didn't have another bushing to increase the hole size, I had ordered a "T" drill bit from Avery and went directly to it.
This, too, was a pain in the neck because it's a very large bit, but it allowed me to use the engine mount holes as a drill guide. On the last hole, I tried stepping up via various bits but they were cheap Harbor Freight bits and it caught the edges too much. Again, lots and lots of oil and lots of cleaning the bit and hole before reoiling.
Once I was through with the "T" bit, I reamed it with a 3/8" undersize chucking reamer I also ordered this week from Avery. I think I probably should've gone to a "U" bit, but the "T" bit was OK. However, I found it easier to pull the reamer through the hole rather than the reverse.
I cleaned up the hole, and then inserted the bolt, washer, and castle nut and tightened. I then move to the next hole. In my case I started with the lower right, moved to the lower left, the upper left, and the upper right.
I still have to drill the two holes in the bottom middle. If you've added a doubler for a Facet pump (I did, even though I'm not going to use it. I'm going with fuel injection, but I didn't know it at the time I built the firewall), then a nutplate on the forward side interferes with the engine mount. So a spacer will have to be put in between the mount and the firewall. Some small filing of the nutplate will also be necessary.
Many thanks to all who volunteered information. After I finish the mount, I can begin to get the wheels assembled and get the plane up on its own three legs. That will be an exciting moment.