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fit tightly or perfectly to the case? I've read this many times but am not sure of the reason. Could the pinblock move(obviously disasterous) if it doesn't fit tightly?

It does not need to. Baldwin pinblocks were just screwed to the wooden frame, and held perfectly well. A good, tight fit to the plate is important, though. If it slips there, everything goes haywire.

You can say that again!!!

It doesn't need to. It is far more important that there is a great/perfect fit between the pinblock and the plate. If not, then under the strain of the wires, the pinblock can slightly shift forward in different areas and misalign the tuning pin holes to the plate.

You can also have cases where the pin-block only contacts the plate flange at one or two places-then when tuning-the block can pivot on those points actually making some section of strings go up in pitch.

If the block moves or distorts, obviously the tension on the affected strings changes, and they go out of tune. Total tension for all the strings on a piano can go upwards of 20 tons - that's like the weight of 40 or 50 mid-size grand pianos.

If the fit between the block and plate was not accurate, you'd get point loading and localized crushing of the wood. You might get lucky and it would crush to a stable end point, but maybe not. In that case, it wouldn't stay in tune very long.

Several manufacturer use a CNC machine to cut both the edge of the pinblock and the flange on the plate that it abuts. This creates a "perfect" fit. I saw it being done when I was at Samick's Indonesian factory and I am told it is not uncommon.

It is mainly a design problem: with pianos that have the pinblock affixed to the wooden frame of the piano, the plate needs to be lowered into piano, checked/marked, and then whole metal plate removed to access the pinblock to make the adjustments. That process takes time, so there is often a economic/financial limit to how many times workers are willing/able to pull the plate to ensure perfection with the fit, hence more instances of tuning and other pinblock stability issues with that type of construction.

When the pinblock is affixed directly to the plate the process goes much faster, as there are no in-and-out of the piano in that process. CNC machines make the process much easier, and most likely 'good enough,' but perfect it's usually not. I have no objections to this as a first step to get a very large amount of the preliminary work accomplished.

Point-load crush is never seen on high-quality pin-blocks. Since you can only change tension one string at a time there is time for the whole thing to pivot and warp slightly. Remember each pin holds quite a point load without crushing. Crushing around the tuning pin is cause by tuning.

You have a point load of about 160 pounds at each pin, and no crushing there. The block acts pretty much as a uniformly loaded beam, which collects all those 160 pound loads -- maybe 230 of them. That's what adds up to significant tonnage that has to be transferred to the plate. If there is insufficient area in contact between the block and plate, the wood will crush. How far will it crush? Probably just far enough to increase the contact area due to a poor fit until there is just barely enough area to hold those tons. Maybe a few thousandths of an inch. But the tuning would be quite unstable until that process runs to completion.

OTOH, if the design and construction result in enough area to stay well below the crush limit, you'll have a far more stable assembly.

I did a little structural engineering on wood framed buildings using Douglas fir. IIRC, the load limit perpendicular to the grain was about 625 PSI. I'm sure pin blocks must do a lot better than that. Given that the pins don't crush it, I'd guess it might be in the 1000 - 1500 PSI range. But you clearly do need enough square inches. If my 1500 PSI guess is right, you'd need about 27 square inches.

I also did some foundation repair, lifting a two story house with a 20 ton bottle jack, and got a good feel for the crush limit of Doug fir.... ;-)

The idea that the block pivots or rotates a little makes sense if there's a gap at one bearing point. It's a beam with insufficient support there, so it'll deflect.

There are 30 or so big wood screws holding the block to the plate. Some of the movement from flange gaps is torsional with the axis of rotation parallel to the key level along a line from bass to treble. The tuning pins are slightly over .250" diameter so the tension of each string is distributed over that area. There is a fair amount of end grain maple that is against the plate flange.

I haven't seen point load crushing at the face of a mis-fit pin-block, and I have replaced more than a few mis-fit blocks when I rebuild pianos.

I'd expect it to be very small, on the order of a few thousandths of an inch. So, no big visible splinters. I'm not surprised that it would be impossible to detect visually.