MIMF

I was looking at moving my bolt on necks to threaded insets and bolts rather than wood screws. It looks like people are using 8/32 bolts. Is it me or is that to small? What are you using? Thanks

I think that is way to small. I use 1/4". If you only use two bolts, in reality, all the load is carried on the bottom bolt. It better be strong.

If your using them on a fender style neck, they worked fine on a repair for me.

If you're talking electric necks in a pocket, I think four 8/32's are fine. If it's an acoustic type heel, then you need two 1/4".

8/32" = 1/4"

I assumed that Andrew was talking about an 8-32, which is a small bolt (.164" diameter) with 32 threads per inch. That is half the diameter of a 1/4" bolt and a lot weaker.

Fun with bolt numbers. If a 1/4" bolt is Grade 2 it has ultimate tensile strength between 60,000 and 80,000 psi, which translates to around 3500 lbs. A 8-32 bolt, at .164" diameter, has a UTS of around 1500 lbs. Two of them can pick up a car (a little car, anyway). They both can create over 500 lbs of clamping force each, with between one and two foot lbs of torque (12 to 24 inch pounds, or really a small amount of torque). The greater issue is probably getting the threads of the bolt, or an insert, to grip into wood. Sleeve or barrel inserts would work. The other big issue is keeping the bolt head from just sinking into the wood it's clamping. My first archtop I used two of those furniture assembly bolts, 1/4", into threaded inserts I tapped and glued into the neck heel. Did both bolts up to around 5 ft lbs. Six months later I needed to do a neck reset. Not only had the bolts heads sunk into the wood and then loosened, the cheeks of the neck heel had sunk into the sides of the body, literally squashing the wood of the sides. I did the math, and I was clamping that poor neck onto the body with around 2400 lbs of force - no wonder the wood got squashed... Now I use one 1/4" bolt and do it up with two fingers and a thumb on the allen key.

I learned in engineering school that the strength of bolts is much less than the full cross sectional area. The bolt will fail first by having the threads strip off which is a smaller area to consider. Not really relevant to the discussion, but all the same.

If you're talking about a Strat® type 4 bolt neck with a chrome neck plate just use the size bolt whose oval head best fits the counter sunk hole in the neck plate. An 8-32 has a pretty small head diameter. A 10-28 might not even be large enough. If you're not talking about a Strat® type bolt on neck, then never mind.

8-32 is a perfect bolt if you are retrofitting a strat/tele or similar guitar with 4 bolts. Standard threaded inserts work well. I like the ones with the hex drive as it makes them easy to control as you put them in. I use stainless bolts but only because they are most readily available at my local fastener store. As Brian points out above, the force that can be exerted is pretty astonishing. Comparing bolt force numbers with holding force numbers for inserts in various materials, it looks to me like the threads in the softer inserts will strip or the inserts will rip out of the wood long before the bolt breaks. I experimented a bit on some scraps (maple and fir) and I was actually not able to exert enough force with a Phillips screwdriver to strip the insert or the bolt, nor was I able to make the insert come out of the wood. 4 of them together is way more than enough. I'm sure two bolts would be more than sufficient to hold against string tension, but the clamping force would not be distributed as nicely over the whole joint.
Size-wise, the head of an 8-32 bolt is just right for a standard 4-bolt neck plate. I use flat head but oval is fine too for this kind of neck plate. I suggest a Phillips-type drive as that tends to prevent over tightening but I have used some hex drive ones too.
I converted three teles and a strat in the past month and they all seemed much improved to my ear and hands. I see it is a very inexpensive and easy upgrade that has many advantages and, as far as I can tell, no disadvantages. I will certainly never go back to sheet metal screws, which is what the standard neck screws are. I've repaired so many stripped holes over the years from those things.