Roll-bar holes too big: threads loose

I'm about to reinstall the windscreen and would like to solve an issue I have.
When I drilled the roll-bar, I did not look properly at the manual and mistooked, enlarging the holes to #30 instead of #27 to accept a #6 screw thread.
Threads now are done, but screws are a little loose on them. I don't want them to unscrew once airplane will be completed.

So, in order to fix the problem, I thaught at:
- enlarging holes on roll bar and windscreen to accept #8 screws (preferred solution at the moment);
- welding roll bar holes and redrilling them to #27 (but...it might be difficult to drill on a welded point);
- leave all as it is and use a lock-threader sealant.

May you please help me with the decision?

Thanks.
Camillo

A number 30 drill bit is smaller than a #27. If you used a #30 by accident, the holes aren't oversized.

Camillo said:

...May you please help me with the decision?

Click to expand...

It is unclear to me that you have a problem, but if the #6 screws are in ill-prepared, oversize holes here's my take: This is close to being a non-event. I would then prepare the holes to accept #8 screws. Later those screws will be captured and completely hidden underneath the fiberglass fairing you will eventually lay up. When that happens, there is no chance those screws can ever back out. Keep that in mind when you go to countersinking. Give the screw head a decent countersink to seat in but you do not have to generate a full #8 countersink just to attain perfect surface flushness. One more thing. Do not overtorque those screws to pull out a possible gap between the Plexiglas and the roll bar. Take care to avoid any preload and local deformation of the windshield. You accomplish that by shimming any and all screws along the rollbar that may require it.

Rick6a said:

It is unclear to me that you have a problem, but if the #6 screws are in ill-prepared, oversize holes here's my take: This is close to being a non-event. I would then prepare the holes to accept #8 screws.

Click to expand...

+1. Move to #8s.

Yep - Those are now going to be #8's. No problem!

Thanks to all. And...yes, #27 is bigger than #30. Maybe I don't remember well. But I'm sure screws are loose. I will check again. Thanks.

Some ideas to consider before plunging ahead... If the #6 screws are loose with a #30 pilot hole, you need to figure out why that happened, before opening the hole up for a #8 screw. The #30 hole might have been made oversize by the drilling technique, causing less than 75% depth of thread; depth of thread is how much of the screw's thread is engaged. Hand drilled holes are usually oversize as much as 0.005, and in addition the holes can be oblong and/or bell shaped. If the tap isn't held perpendicular to the tube, it will also cause less than 75% depth of thread. If the tap isn't aligned with the hole axis- the tap wobbles as you turn it by hand- the depth of thread is reduced by the sweeping arc of the wobble removing too much material. You can buy small hand held tapping guides to keep the tap square to the hole but you'll need a way to shim the guide to account for the tubing curvature. Try practicing on a spare piece of tube to get your technique right because you've only got one more shot going to the #8 screw. Finally, when opening up the hole for a #8, consider drilling a size or two smaller, then ream to the final size; reaming also requires the reamer to be square. Good luck!

I made the same mistake ...

.. used an 1/8" bit when I should have used something smaller for the tapped holes. I'm sticking with #6 screws but will install them with a dab of pro-seal, as others have recommended in previous posts.

Why not just sika the plexi and loose the screws? Better for many reasons. If I could do my plane ove I would not use screws on the plexi.

Since no one else has mentioned it...do not go near acrylic with thread locker (loctite). Even the smallest amount on the windscreen will cause major problems.

Missing the boat?

Google rollform taps.
http://www.monstertool.com/tap_drill.html
The way aircraft threads are made.
1/8" = 6-32 #30 will work fine
roll roll roll your threads------
Look mom no flutes!