Fit Bolt & Normal M 20 Bolt

The fit bolts are locating pin cum fastener (two in one). It works very well as the first function (the pin) but is a weak on the seciond function (ie the fastening)

The Fit bolts pass through precision holes (reamed, microbored, etc depending on size). When these are tightened (torque/stretch) then there is just a bit of unequal stress distribution along the length due to the friction on the shoulders, and this bit may be a bit significant in the higher precision ones.

As a result, these are tightened to a bit lower stress values than the other ones.

As mentioned above, these are used where the locating and fastening has to be achieved simultaneously.

Examples (where we use these)- Gear to Hub fastening, Sprockets to Hubs, Couplings etc.

Additionally we have uses of Body bound Studs (Fit Studs with thread & Nuts on both ends) which has a bit of better stress distribution.

An important thing to know is that "fitted" bolts (even those which are blued) aren't really fitted when tightened

As soon as load is applied, the bolt stretches. The additional length has to come from somewhere, right? It comes from the diameter of the bolt!

Therefore, certain joint slippage can still occur. In high torque-transmission applications such as those on major equipment drive couplings, this slippage can cause binding of the bolts into the holes during removal. This is a significant problem if coupling disassembly is on the Critical Path. I've seen some pretty ugly holes where the bolt had been dragged through the bore

Special "expanding sleeve" bolts have been developed to address this serious problem. They're quite fascinating. Initially, they're inserted into a bore as a "clearance" bolt. Then, a tensioner is used to draw the tapered-shank bolt through a tapered sleeve. This action expands the sleeve and the hole is filled completely to become a true fitted bolt.

In regards to the above, here's a link describing these Expanding Sleeve Bolts