Well my 601XL upgrade is now complete. The post-upgrade test flight occurred yesterday, March 23, and things went great. The sun was going down so I went up for three quick touch and go’s. The first thing I noticed was no oil can popping noises any more. I only got to 100 MPH so it will be interesting to see if they are gone at high speed also. In my short flight I did not notice any changes in flight characteristics, but further flights are needed to explore the edges of the envelope.

The upgrade took me about 4 months over the coldest and snowiest winter we have had here in the Midwest for some time. I imagine it would have been taken a lot less calendar time had I been able to work in warmer weather and not had to wait for the upgrade kit. I flew my last flight on November 16 and started the teardown. The upgrade kit arrived just before Christmas and I finished up all upgrade tasks on March 19. The hours worked totaled up to 150. This is only slightly higher that I had originally anticipated. Some of this time was spent incorporating a few nice-to-do upgrades of my own. My airplane is 35 pounds heavier, so I estimate that the upgrade added somewhere around 31 of those pounds.

A lot of these were already discussed in previous discussions, but here are a few lessons learned:

• Drilling out over 2000 blind rivets without screwing up the holes is a daunting task and patience is required. The solid rivets on the spar pieces need to be tightly clamped on either side in order to remove them or the multiple layers of aluminum will separate and firmly “grip” the rivet in place. Adapting a punch to an air hammer or rivet gun is much better than whacking away with a hammer and punch. This is especially important when working alone, which I did most of the time. Who else is going to come out a 20-25F and work away in three layers of clothing?
• After applying the upgrades and before placing the center spar into the fuselage, test fit it over each wing spar to make sure the fit is not too tight and the bolt holes line up perfectly. It will be lots easier to make corrections now than when you are trying to wrangle the wings into the fuselage. After I riveted my center spar with the correct shim thickness (.665) in place, it sprung in a little bit and made it a very, very tight fit over the wing spars. I un-riveted the top row of 5/32 solids, re-shimmed to .665 + a .016 sheet of aluminum, re-drilled to 3/16 and re-riveted with AD6’s. This provided a much better line-to-line fit over the wing spar.
• The instructions call for the counter weight arm to be parallel to the third rib 327.3 mm outboard. Who are they kidding with ".3?" The intent is to pick the closest existing rivet hole to that point. I think mine was around 329 mm, so what. At that distance, and parallel to rib#3, it will put the weights right into your wingtip, which interferes, so don’t do it. Either place the arm perpendicular to the aileron leading edge, or move the arm in-board one or more rivet locations. The main thing is to lay it all out before drilling to see how it fits your airplane.

• The nose ribs are a real pain to add upgrades and still line up with the existing nose skin holes. Since they are so close to the end of the wing, I re-riveted the nose skin back in place along the bottom and used the actual nose skin to align the ribs. You will also probably experience edge distance issues for the AN3 bolts that go through rib angles 6ZU1-4 as well as the 6ZU1-3 angles to the original spar 5/32 holes. I had to make some of my own angles with wider sides to fit better.

What do you need to purchase that the kit either does not include, or is short on?

• Definitely buy the small right angle drill recommended by Zenith. McMaster-Carr has it. Also buy several extra drill bits, as they are unique to the drill. I wish I bought this when I built my airplane the first time
• Of course buy extra solid rivets to practice on before riveting the real thing.
• You will need a few –16 (1 inch) 3/16 rivets for the ones that hold 6ZU1-4 in place. So if you want, buy a ¼ pound of –16’s and cut them down to the required length for practice ones as well as the ones you will drill out because you screwed a few up. I did.
• You will need lots of extra A5 Avex rivets. I think I had enough A4’s. Not sure how many are needed as I had a bunch extra that I bought during my original build, but I definitely ran out of A5’s. A wild guess would say at least 300 or more are needed.
• The long bolts for the counter weights were way too long for me. I had to buy some that were a lot shorter than the ones the kit supplied. I did not feel comfortable stacking ¾” of washers to make up for it.

Here are a few pictures.

How many pulled rivets will you need? Exactly this much. It does not look like a lot, but it is:

You will spend a fair amount of time on the ground. Wear safety glasses for falling debris!

Teach the next generation a few skills other than Xbox and Wii:

The wife wants to know if the upgrade will ever end:

It was a happy day when the wings went back on:

The Model-T doubler does not look so bad: