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I bought a new motorcycle. The manual says that for the first 500 km, I should not twist the throttle more than half. And for the first 1800km, should not twist it more than 75%. Why isn't it allowed to ride a new motorcycle too fast?

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It's my understanding with the current manufacturing technology it's not necessary anymore, on new cars, now there may be differences in motorcycle engines as I am not as familiar with those. With that being said we ask the customer to come back in 500 - 1000 miles for a free oil change on the engines we build and install. We do this for a couple of reasons, we change the oil getting any of the contaminates that may have been the result of the rebuild and break in, it also allows us a chance to double check our work and make fine tuning adjustments.

Here is a related excerpt from a popular mechanics article

The ritual of breaking in a new car is part of the body of knowledge we refer to as conventional wisdom. It’s not necessarily wise, and the technology of building a modern automobile has evolved to the point where a lot of “wisdom” is obsolete. Few cars specify a break-in procedure anymore, simply cautioning you to avoid extreme acceleration or extended idling for the first thousand miles or so, and there’s little in the way of extra service up front.

Engine Cylinder Walls

Piston rings don’t rely on their spring tension to seal against the cylinder bores. Instead, combustion gases work their way between the rings and the piston and force the rings outward. During the first few minutes of engine operation, it’s important that the throttle be opened pretty far at lower rpms to provide this high pressure. Otherwise, the rings won’t burnish the cylinder walls properly, and the engine will have high volumes of blow-by—which means excessive oil consumption and shortened engine life. If you’ve ever seen the car jockeys who drive new cars off the end of the production line into the storage lot, or the transporter drivers zipping up and down the car-hauler ramps, you’ll realize that this all-important step has been performed for you many times. If you’re installing a new engine, simply give it a few seconds of wide-open throttle in a high gear. For the first thousand miles, avoid constant speeds and throttle settings. If you commute in normal stop-and-go traffic, you’ll be fine. I advise against cruise-controlled sojourns across Nebraska.

Bearings

The admonition to keep engine revs low for an extended break-in period stems from the days when bearing and crankshaft manufacturing tolerances were far less rigorous and lubricating oil wasn’t nearly as good. While modern engines are assembled to much the same design clearances, the tolerances are much tighter, meaning the variability is smaller, greatly reducing the possibility of a tight spot. Redlining a fresh motor is generally a bad idea, but there’s no reason you shouldn’t drive normally. I would, however, avoid top-speed testing, drag racing or towing heavy trailers for the first 1000 miles.

Oil

I customarily change the oil in a new engine after about 20 miles, and again at 1000 or so. That 20-mile oil, you would think, would look pretty much like fresh oil right out of the bottle. Wrong. It usually looks more like metal-flake paint, iridescent with tiny particles of metal worn off rubbing surfaces inside the new engines. After a few hours of operation, this completely normal phenomenon slows down as the rings, camshaft, lifters and bearings burnish their respective mating surfaces.

Read more: New Car Care – How to Take Care of a New Car - Popular Mechanics

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Larry - you say break in isn't required, and yet your comments and quoted article both say it is. I know it is only an extended break in for high performance engines, but a break in is still a good idea. –  Rory Alsop Jun 23 '12 at 19:38
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@RoryAlsop It's my understanding with the current manufacturing technology it's not necessary anymore That is my understanding based on current literature, but I see nothing wrong with doing it because it causes no harm. I'm not sure if we do it because we always have or because it gives us a chance to check our work and catch problems early. The popular mechanics article pretty much says the same thing, it's not required anymore, but here are some good tips. –  Larry Jun 23 '12 at 22:35
    
my non performance engine from a seat leon tdi had a 600 mile break-in period –  Mauro Jun 24 '12 at 6:15

This is actually the same for any new engine- while it its bedding in high revs can damage engines. For those first few thousand miles, minor differences in tolerances between components can be gently worn down.

This is more of an issue with highly tuned or closely machined engines, but no engines should be run hard until worn in.

Admittedly some cars have their engines bedded in before you get them, and some have wide enough tolerances and low enough tuning that they should be fine.

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I would also add that a lot of motorcycle technology ages well, and a lot of parts can be used for decades. For instance, my 2011 motorcycle’s engine has new top end (valve assembly, cylinder etc.) since mid-2000s and fuel injection, but the rest of the motor has been used since at least the early 80s. I am sure they improved quality over the years, but no one is going to drastically re-engineer the industrial process to produce basically same old engine. And so manual required break-in and oil change at 1000 km. Many manufacturers reuse aggregates for a long time. –  theUg Jun 24 '12 at 17:17

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