How do I determine proper screw length...

[aarvig]

I am replacing a rivet with a pan head screw per Vans recommendation. I was told I would have to determine the proper length. Is there a magic formula for this? The material thickness I am dealing with is .3450 thick. I'll need a #8 screw.
Also, can I just use a regular Ny-Loc #8 nut on the back of this screw to hold it on?

[Rick_A]

You need to have a minimum of 2 full threads showing after you tighten the lock nut. Yes, a standard Ny-Loc nut is OK.

You probably have a bunch of #8-8 (1/2" long) screws that came with your kit. Start with one of these and if it's not long enough, order some longer sizes from Spruce.

[az_gila]

Start here -

http://www.gen-aircraft-hardware.com/default.asp

Pick the screw/bolt from the left and look up tech info -

Here is a typical one -

http://www.gen-aircraft-hardware.com.../pdf/an525.pdf

It will give the part number for a grip length. Pick slightly longer and use washers so the nut won't bottom out. A thin (-L) washer is 0.032 and a regular AN washer is 0.062 thick.

For a full grip, you probably need a 13/16 or a 7/8 long #8 screw ( -R13 or -R14) and one or two standard washers. If it's a rivet replacement you don't want any of the hole to be bearing on the threaded portion of the fastener.

The standard #8 nylon locknut will be fine aft of the firewall.

[aarvig]

Thanks guys. After the repair is completed the hole is measuring out at .1635. Is this OK for a #8 screw?

[az_gila]

Originally Posted by aarvig

Thanks guys. After the repair is completed the hole is measuring out at .1635. Is this OK for a #8 screw?

I like a #19 drill which is a 0.166

http://www.custompartnet.com/images/...rt-english.png

But if the #8 screw fits in the hole with a nice push fit, go for it...

[Auburntsts]

Originally Posted by Rick_A

You need to have a minimum of 2 full threads showing after you tighten the lock nut.

Hmmm. I'm pretty sure AC 43-13 says one thread, not 2. What the reference for 2 threads?

[Rick_A]

You are correct that 43.13 (Section 3, 7-37 & Section 4 7-63 f.) specifies a 1 thread minimum, but 2 threads is often recognized as a best practice.

Personnally, on non-critical items I'm OK with 1 thread but for all critical items (such as flight controls) I want to see 2 threads. I also never fly to the minimum allowed fuel requirements - but that's just me.

[Mike S]

Bend a piece of music wire to make a small "L" shape at the end-----small enough to fit through the bolt hole.

Put the wire in the hole, hook the bent end on the back side and mark the wire where it comes out of the hole.

Now, compare this with your bolt stock.

There is a fancy commercial unit with engraved length callouts available also, but I think it is for rivets not bolts. Really handy if you have one, but the wire works just fine.

[az_gila]

Originally Posted by Rick_A

You are correct that 43.13 (Section 3, 7-37 & Section 4 7-63 f.) specifies a 1 thread minimum, but 2 threads is often recognized as a best practice.

Personnally, on non-critical items I'm OK with 1 thread but for all critical items (such as flight controls) I want to see 2 threads. I also never fly to the minimum allowed fuel requirements - but that's just me.

A lot of it is based on how you measure "threads"...

The end of a bolt is slightly chamfered so you need to determine what is a thread. This is where two threads comes in since it allows one thread to be in the non-locking chamfered bit....

Fuller explanation, with specs., in this old posting -

http://www.vansairforce.com/communit...07&postcount=4

[aerhed]

Purty sure this thread exposure applies to all metal nuts. I think nylock nuts are only required to fully engage the plastic. That said, I still want some thread showing.