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I've finished rebuilding the engine of my motorcycle (4 stroke 650cc single-cylinder). For now it runned only once varying between 1000 and 2000 RPM for 5 minutes with a couple of short bursts after fully heated.

I used Motorex Semi-Synthetic Formula 10W40 oil for that. Now I'm reading various sources of information about how further break-in should be done and am a bit afraid because everyone seems to agree that rings will never seat properly with modern semi-synth oil. It is advised to use conventional oil during first maybe 1000 miles of engine life.

Does anybody have an experience of succesfull break-in with semi-synthetic or synthetic oil? Should I replace the oil with a conventional one immediately? How likely is that these 5-6 minutes with a semi-synth oil have already caused trouble?

Nick C
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Anton
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2 Answers2

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I'm told that the bulk of the break-in is done in the first few minutes of running and that no synthetic of any kind should be used. You're in "test pilot" land now, all bets are off... If it was me (a person not afraid of blowing up an engine), I'd switch to conventional and do a lot of WOT/decel runs to try to beat in the last of the break-in that I could and then just see how it goes, expecting higher than normal oil consumption and possibly needing to rebuild it again shortly.

Brian Knoblauch
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I know that Fiat, with the old FIRE engines (that's Fully Robotised Integrated Engines) used to run them up to 14,000 RPM for 20 minutes and if they passed that trial, they'd put them in a car.

Steve Matthews
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