Construction manual
Per Vans construction manual, section 5, common to all kits:
"For AD3 rivets, a total material thickness between .016 [.4 mm] and .032 [.8 mm]
must be dimpled. Material thickness between .032 [.8 mm] and .040 [1.0 mm],
should be dimpled, but a countersink may be used if necessary. Finally, for a thickness
of .050 [1.3 mm] and above the material must be countersunk.
For AD4 rivets, .050 is the minimum thickness that may be countersunk.
Metal thicker than .040 is difficult to dimple, so it is common practice to machine
countersink any material thickness that is more than .040. There are a couple of
reasons for this. First, although RVs are designed so that a countersunk joint is
acceptable, the interlocking nature of a dimpled rivet line is stronger. Second,
dimpling leaves a thicker edge for the shop head of the rivet to form against,
meaning that the metal deforms less and there is less chance of "working" rivets later.
This recommendation is meant as a guideline, not an absolute rule. There may be
instances where countersinking 0.032 stock makes the job easier, and certainly can
be used. One example is when a sheet is sandwiched between two other sheets.
In this case, the sheet receiving the rivets manufactured flush head is dimpled, the
sheet underneath is machine countersunk to receive the dimple, and the third sheet
is left full thickness. When countersinking the second sheet you must go slightly
deeper than you would for a rivet. See Figure 1."
There are however situations where the plans call for machine countersinking .025, e.g. fuselage sides for 3/32" rivets where skins overlap and the underlying skin is machine countersunk at the longeron and the overlying skin should be dimpled if it is also .025.
