DIFFERENT LENGTHS OF BOLTS AND WHY
The majority of climbing holds are hand shaped. That means every hold has different measurements. That includes the thickness of the climbing hold where the bolt passes.
To add to the organic nature of climbing holds, it is also a goal to have the head of the bolt as near to the face of the hold as possible. We don't want the head of a bolt protruding on the front of the grip because you would feel it when grabbing the hold.
We also want the bolt as near to the face of the hold as possible. The more material we can get between the washer and the back of the grip increases strength.
To sum up, the thicknesses of the climbing holds we are dealing with are rarely the same. They might be in one pack, but very unlikely in bulk packs and especially when considering an extra small hold to an extra-large hold.
DIFFERENT LENGTHS OF BOLTS
One would think that a bolt supplied with a climbing hold will exactly be flush with the back of your wall. That is rarely true. In the climbing hold industry, we expect the bolt to protrude out the back of the panel. THIS IS WHY YOU SHOULD NEVER HAVE YOUR CLIMBING WALL PANEL FLAT AGAINST ANOTHER FLAT SURFACE. YOU SHOULD ALSO EXPECT THE BOLT TO PROTRUDE OUT THE BACK OF THE WALL.
To add to all this fun with bolt lengths, climbing hold bolts are available in 1/2" lengths. The common lengths are 1-1/2", 2", 2-1/2", 3", 3-1/2"...
So as an example of a common issue, let's say you have a climbing hold that has 1" of material behind the washer to the back of the hold. That's common in a large-sized hold. Your wall is the standard 3/4" plywood. That means if you use a 1-1/2" bolt, you will have 1/2" of thread sticking out the back of the hold. But you need 3/4" because you HAVE TO HAVE ALL THREADS OF THE BOLT ENGAGED WITH ALL THE THREADS IN THE T-NUT. The reason is strength. Climbing holds exert an immense load on the bolt/t-nut combination. Anything less than full-thread-engagement can fail. Another way to say this is, if you don't pass through all the threads of the t-nut, you run the risk of shearing off the threads inside the t-nut. That means you rip the hold and the bolt off the wall. That is a zero fun scenario.
EXAMPLE OF BOLT PROTRUSION
The correct length of bolt for the above example is a 2" bolt. 1" to pass through the hold, 3/4" to pass through the panel and t-nut. You will now have 1/4" of thread sticking out the back of the t-nut. If your panel is up against a flat surface, the bolt will push against it. The undesirable result is that the bolt won't tighten down anymore, or you push the panel away from the wall—both situations you have to avoid.
The answer to all of this is FUR YOUR PANEL OFF THE WALL AND EXPECT BOLT PROTRUSION.