Yes there have been massive changes to production engineering and tollerances on modern engines which are better than a blue printed gold star motor. But modern engine parts are made for modern engines.
Running a rod from an over square modern buzz box in a long stoke slow reving bearing pounder will have it falling apart in no time flat. Rods are designed to handle the forces generated by the lighter pistons, in a faster reving better cooled engine.
Most water cooled pistons are very thin and even worse for us, round , as the barrel they are running in is water cooled and kept at a relative stable temperature thus there is little dimensional change from dead cold to full heat, and better cooling allows it retain its roundness .
Our barrels are anything but round and change dimensions drastically and non uniformly to boot.
Modern engines have substantially better oil delivery systems and many of them actually have metering jets to control the oil flow to again tight tollerances.
I would be estatic if I could find some good substitute pistons to fit my A7s,. I have 4 in parts, 2 being the early longer stroke engines and every piston that I have found so far has either been given away or flogged on evilbay as I was not happy with any if them.
What I was trying to say in a rather clumsy way there is more to the selection of an alternative part than " will it fit in the hole".
Even things like valves can give grief as again , car valves run in water cooled, constant temperature heads with in some cases regulated oil flow and valve stem oil seals. They do run at any where near the temperatures that our bike valves do. They are also designed for a much lighter duty cycle and are made from substantially lower grade steels than our original valves.
Other things like modern oil seals, yes go for it, better materials & better made , sealed bearings , fine solid state electrics , big tick.Belt drive primaries , what a good idea, clutches grafted from Yammies or Suzukis great
