There is an article in VirtualIndian by Tom Cotten that gives some good tips, one is the anvil to support the nipple from the inside. AND checking for leaks is essential, both before drilling and after riveting the inlet nipples.
A grinding fixture (dummy thread insert to protect the threads and a plug that slides in that, with a rim slightly wider than the nipple rim) can be made to grind the cylinder surface perfectly flat and perpendicular before.
Note that threads is never sealing as there is always a gap between the male and female threads (if dry, without sealing compound). it is the rim that must be tight, and rivet. A few threads on the nipple close to the rim must be absent, as seen in one of the pictures, in order to avoid threads pull up and distort the cylinder rim surface when the nipple is torqued down.
One way torqueing the nipple down is to use a turned down muffler tube expander. The cylinder can be heated a bit before threading the nipple in but keep the temp below the temperature that would settle the thread sealer.
But don't overdo when tightening down! No hammer tapping. Cylinder threads can strip! it is fine shallow although wide threads. One time medium torque to shape the threads if new, then back out and torque again a similar or maybe medium-hard ompfh should be enough.
Picture shows different eccentric anvils and a special clamp to pre-shape and compress the rivet butt before final fine-tune riveting. The rivet should fit snug in the hole and protrude 1,5 times the diameter before forming a shallow dome about 1/2 height of the rivet diameter. A tall dome is not better. The inside head is deforming as well so keep the anvil tight during hammering. A drop of anaerobic thread locker before putting in the rivet wouldn't harm, in my opinion.
If the attempt is unsuccessful, the rim on the nipple can be ground or turned thinner in order to turn the nipple a bit deeper, to shift the position of the already drilled hole, and drill a new hole in the nipple.
http://virtualindian.org/11techleaktest.html
P.s. A medium-hard ompfh is a calibrated elbow torque on the “uggadugga” scale. In German Torque it is almost one Guttenuff.
