I just finished a 5" loco and used graphite yarn…works perfect! Would have prefered cast iron rings but not worth the hassle. There are modern alternative materials.

Make sure you get the right o-rings. The ones that are commonly available are for plumbing and don't last very long when hot.

Rumour has it that asbestos for lagging and wicks is out of favour now too, along with white lead and creosote.

Depends what your new engine is and what material the cylinder is made from.

For pistons I have used CI rings both bought and homemade, silicon & Vitron O rings and traditional graphite yarn, you can also get a version with PTFE in it.

J

Many full size locomotives and some ship and stationery engines had metallic packing in the piston rod glands . Many variations on how it was actually done but a common one was to have a stack of coned rings of soft whitemetal .

A good way of sealing small engine piston rod glands is to mimic the above using solid PTFE . In model size engines a simple PTFE tube with coned ends made to match gland bore and end fittings will work . I've actually done this successfully myself using Nylon but the PTFE would be better still . In either case make the tube an easy fit and tighten up gland a little under steam for an effective and low friction seal .