David Schwarze writes,

> They are an  easy swap, yes.  If I'm not mistaken, 90mm = 1700cc,
> 93mm = 1800cc, and 96mm = 1900cc.  I had the latter on my bus when I got
> it, and the motor was in sad shape.  Upon examination, I decided that the
> 1900cc kit had *very* thin cylinder walls - too thin.  So, when I rebuilt
> the motor I went with the 1800cc kit (93mm, I think).  I don't know if it
> made a difference in power, because It never ran well as a 1.9.  If I
> had it to do over again (and I'm sure I will, shortly) I will swap the
> internals for 2 liter parts (rods, crank, pistons & liners).  It's a
> direct swap, and the extra displacement comes from a longer stroke, not
> thinner cylinder walls (I think the 2 liter cylinders are 91mm bore).

Stock bore and strokes for type IV engines are:

              1700cc    90 x 66
              1800cc    93 x 66
              2000cc    94 x 71
              
Heads can be interchanged between 1800s and 2000s (1800 valves are bigger) but 
1700 heads only fit 1700s. So, to make a displacement jump from a 1700 you have 
to change everything but the case to get to two liters, and change barrels, 
pistons and heads to get to 1800 (the best of the bunch, in my opinion).

The NPR big-bore kits for 1700s have very thin walls

Dave Kautz