Looking at the two extremes, in one ideal case, where there is no clearance, the pressure will approach infinity as the piston approaches TDC.
In the other extreme, as the cylinder becomes infinitely long, the pressure approaches zero; ie: no compression is taking place.
I seem to recall that diesels use a relatively long stroke, with associated slower maximum rpm that a square engine, ie: a square engine having a bore that equaled the stroke.
Looking at a Detroit Diesel 671 engine, the displacement per cylinder is 70.93 cu in, with a 4.25" bore, and a 5" stroke, 18.7 : 1 compression ratio.
This is not nearly as long a stroke as I had thought was used.
So with a diesel, the key is the air inside the cylinder being compressed to the point where it reached the ignition temperature of diesel fuel; recalling the old thermo class days, and the PV=mrt forumla (I think is the correct forumula, or ballpark thereof).
So perhaps we can use the 671 data and work backwards to see what the clearance would have had to have been at TDC to get 18.7 : 1, and reach the ignition temperature required.
Obviously as the clearance goes down, the pressure and compression ratio go up.
So if Peter's forumula is correct, then a Detroit 671 would have a head clearance of 0.267", which seems reasonable.
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