This Q&A is now closed and replaced with a new Q&A based on the latest upgrade drawings at

To all 601XL and 650 builders and aircraft owners. If you have a specific question regarding the upgrade, post it here and I will consult with Chris Heintz so that we can give you the best possible answer. Please be specific about your questions.

When posting a question or replying to an existing comment, PLEASE add it to end of the last page.

Zenith Aircraft is upgrading its 601 XL, so a lot of questions are answered on the step-by-step Blog Posts on this main page.

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The retrofit kit will include the 5/16" bolts. If you have 3/8" bolts, they are fine. However, people who have 5/16" should not open up to 3/8".

Since I have never driven a rivet that size and have no tools to do such, I'm pretty sure I don't want to learn on the wing spar. I'd rather use the bolts. So Two questions..

1 When you say rivets are preferred, is there a reason?? Do bolts have downside??

2.Can bolts be used in place of all of the large spar rivets we have to drill out, including the center section?

Solid rivets are excellent fasteners as they fill the hole and by doing this, its better for fatigue etc. Also, solid rivets are significantly lighter.
Yes, you can replace all the solid rivets with bolts, but the bolts have to be installed properly. Preferable into a tight-fitting hole, and torqued close to the max. allowable torque value.
Will a proper size reamer be included with the kit so, at a minimum, the spar bolt holes through the added material will be sized correctly for the new bolts. Simple drilling can produce a hole that is too large for a standard size bolt?
Louis, at $352 USD, no tools are supplied in the upgrade kit.
Not to be obstructionist, but has anyone flown a 601 with the upgrade package installed? Do we know that this isn't causing a different problem?
Reply to Eric,
As an Aerospace Engineer involved in the design of airplane and missile parts, we had an often used expression, "every solution has it's problems", and it was generally true. You make the best decisions you can based on the available information using the best technology accessible and the problems will be minimized. I would like to think this is what Zenith has done and as a result, any residual problems encountered should be very minimal. We need to move forward because the existing "problem" is unacceptable. The folks at Zenith have a lot at stake and are doing their very best. The problem may simply be a slightly greater empty weight.
Hi Mathieu

When I built my XL I dimpled the wing skins and most of the fuselage.

Will it be acceptable for me to countersink the top of the L angle doubler that attachs to the top of the wing spar so that my wing skins can sit flat against this surface?

Eddie Seve
Yes, there is no problem to countersink the top "L" extrusion since it is 1/8" aluminum.
Good question. The only item that could affect the way the aircraft flies is the UK LAA aileron counter balance weight modification. We are using this modification as the LAA did flight test the aircraft with it installed. The German flutter specialist did confirm to us that there is no increase in potential flutter with the upgrade kit. As a general rule, the stiffer you make the aircraft, the less it is prone to vibrations / flutter.
I am building the CH 650 from a kit. I am ready to start on the wings, so I am in a good position to incorporate the upgrades. It appears that a few items you are specifying for the 601XL are already in the original 650 kit. It also appears that the upgrades are mainly pointed toward reworking the 601XL.

(1) Will there be two upgrade kits? One for the 601 and one for the 650?

(2) Will photos, videos, and detailed instructions be supplied with the kit? It looks like I will have to mark up my original plans very carefully so that I don't put something in that has to be replaced (unless revised 650 plan sheets will be available soon.

(3) I have seen comments about adding reinforcing around servos, etc. I assume these will be added to the final drawings (maybe sheet #5 ?).

Jim Snyder
CH 650
Will the solid rivets, AN bolts, NAS bolts, A5, AS5 rivets, etc. be included with the kit?


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