Light Aircraft Association

I've noticed that a couple of screws and bolts are missing from my aircraft (two bolts securing the lower cowling, and three screws securing inspection panels underneath the wing).

Unfortunately, the aircraft builder didn't make a note of what size or thread the missing bolts/screws actually are. I suspect the two bolts securing the lower cowling were British Association threads, because I understand that BA threads were used a lot in aircraft. I'm guessing that the screws to secure the inspection panels would have been UNC or UNF.

My plan is to buy a small selection of screws/bolts that I think might be approximately correct size and thread, and attempt to screw them in until I find one that fits. However, other people have expressed their concerns that this might cause more damage by ruining the threads in the aircraft.

Are there any recommendations anyone can make as to finding out the correct thread size, or any other solutions to fitting new bolts/screws to the threads?

For what it`s worth, John Taylor specified BA bolts and screws on the drawings for the Monoplane.

Regards,
Keith Picton.

Depends how you attempt to screw it in. If it goes in for a turn or so then binds it is the wrong thread. Reverse. If it goes in ten threads and binds then it isn't but it is too long. If you only use the pressure you would use winding a watch with your fingers, a little common sense and no tools, you won't go far wrong.

Are there any other similar looking screws/bolts on your aircraft that you can temporarily remove and try in the threads? I would thought it highly unlikely the designer/previous owner would have mixed different threads.
Tony Nowak

Hi John
This is a case for knowing where you are i.e. checked your profile no location.
Re bolts, a case of trying till you find the right ones. Take one of the existing ones out and find a match that way.

Cheers

Mike G-BMET

Mike, I think you will find that John lives near Doncaster.

Isn't G-INFO a wonderful tool!

Not on our list of places to go, now Fenland and hopefully the new café at Boston is. Mike

tnowak wrote:Are there any other similar looking screws/bolts on your aircraft that you can temporarily remove and try in the threads? I would thought it highly unlikely the designer/previous owner would have mixed different threads.

The above is good advice. I would have expected other fasterners (tech word for nuts, bolts, screws etc) in the same area to be the same type.

Once you've got a sample then you need a set of thread gauges, a micrometer or vernier and an engineers information book (e.g. a ZEUS book) which has details of thread standard sizes: outer diameter size, thread turns per inch / pitch etc. You measure the screws, cross reference the size and then buy some more, probably from Light Aero ()http://www.lasaero.com so they are of aircraft grade so keeping your inspector happy.

The info in the ZEUS book is also easily found on various web sites.

My guess was right, and they were British Association threads!

I bought a 'lucky-dip' pack of BA machine screws from Namrick, and tried them via hand-tightening only. The screws just went in without any effort. Just to check, I also tried some metric screws, but it started to lock about one or two threads in - so I'm happy that I now know the correct thread.

I've not decided yet whether I'll order any more as spares, but I think I'll get some locktite to help stop the screws disappearing again.

Cheers for all above advice and help.