commando balance shafts?

Time Warp said:

What is the minimum mileage expected with these pistons given the low return on the stock bore and piston

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Not sure what you mean here.....the bore life can be more than enough for most users, unless something else reduces it to scrap.....

john robert bould said:

But doesn't the balance shaft "pull" the crank in the opposite un-balanced directions, which suggests lots of power loss?

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Something seems to have been lost in the translation here ?
Balance shafts don't 'pull' the crank in any fashion.
It just adds its own vibe, that counteracts the other unhelpful vibes.

Think of a giant railway loco (steam engine), with a dozen or more rotating wheels and shafts.
Each with their own imbalance.
All adding up to one synchronised SMOOTH fifty ton ride down the rails.
Balancing things was sorted out centuries before infernal combustion engines came on the scene....

The average steam locomotive running at speed down the tracks beats the daylights out of itself and the track.

Perhaps in your part of the world...

Some of the better locos did a million+ miles between major overhauls.
If you read your railways history, railways used to have big investigations into some terrible accidents.
Far more rigorous than modern day aircraft investigations.
Solving all the problems is partly where modern day engineering and metallurgy was born...

Oddly enough, the best steam train I have ever been on was in India.
Went to sleep with my head against the window.
Cheap labour means the track can be made purrfectly smooth...

Somewhere it is mentioned that China and India between them are taking delivery of a new steam loco every day. !
Steam is not entirely dead, it seems.

But we diverge...