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Hi guys

Rebuilt an engine and took it for a ride. Everything is fine. It has new pistons and rings and barrels (professionally rebored) and all kinds of renewed parts. I babied it around. How long would you usually do this, e.g. 100 miles, 200 miles, 300 miles, 400 miles? I want to know when I can use the higher revs again, without impacting the engine's longevity (as I'd rather not rebuild it again). Thanks

The rings need pressure to shove them into the cylinder wall, allowing them to wear in together, thus creating a good seal.  The worst thing you can do is "baby" the break-in process.

You will do well to drive the bike in an assertive city driving regime.  Use a good bit of throttle, accelerate quickly from stops, use engine braking when slowing, no lugging the engine, and no constant speed running.  Keep rpm's down to 3/4 of redline for the first few hundred miles.  Retorque the cylinder head, and check valve clearances, after a couple of hundred miles.  If the head nut torque is loose, check again in another couple hundred miles, until the torque stays constant between checks.

Kawasaki back in the day recommended the first 500 miles be ridden at less than 4,000 rpm, but I don't really know why.  I think you want to avoid easy riding or constant revs so that you don't glaze the cylinder walls.  I would not worry too much about going over 4,000 rpm, as anyone back then couldn't resist the temptation on a Z1, and most of those engines survived just fine.  Just don't test your redline! 

Change the oil after the first 500 miles and check your oil drain plug for metal.  

 Modern boring and plateau honing techniques makes much of the original factory running in advice unnecessary , especially rev limits and mileage periods which can actually reduce overall power once fully bedded in..
The key is not to baby the engine or sit at constant speeds labouring in high gears but also not to over rev and just let the bike pull naturally .
Also make use of engine braking to push the rings into the bores.
The factory running in advice was mainly to protect the machine from mechanically unsympathetic idiots and reduce unnecessary repairs under warranty..

Yup. Ride it like you stole it. 

If you want to read more on the subject of break in you may find this link of interest. Not saying I agree with all that is written at the link, but I did find it an interesting read.

mototuneusa.com/break_in_secrets.htm

Many thanks for all the helpful advice. 

I've been building up a few miles over the past couple of days, and at this rate should be at the 500 mark within 2 weeks.
I think I get the general gist of what you're saying. 

1) Don't touch the upper third or upper quarter of the available revs.
2) Use brisk acceleration but not crazy acceleration (ragging).
3) Use engine braking as much as possible.
4) Whatever happens, do not let it lug - keep it in the happy accelerating zone.
5) Do not take long journeys at constant-ish speed.
6) Don't baby it.

This is the regime I'll follow. I actually found a great Youtube video which summarises all of these points, but I can't find it at the moment.

I think I'll take a pass on the Mototune guy.