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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?

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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
    Apr 15, 2015 at 16:36
  • I agree with Brian that semi-synthetic should not have been used for startup. I would have suggested using a break-in oil which allows the wear which is needed for proper break-in. If the rings didn't seat correctly, you'll get an excess of blowby and a loss of compression, which means power will be way down. I'm going to leave this as a comment and hope that @DucatiKiller will talk to this as an issue or no with a bike. I know it wouldn't like it too well in a car, but things may be a bit different with a bike.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Apr 15, 2015 at 17:14
  • What surprised me a bit is that engine didn't smoked at all after startup, which makes me think that the seal somehow was already nice enough to prevent oil from entering the cylinder (not sure about the blowby, though, and yes, I checked that the oil pump works). I wonder, could it be that an engine that doesn't smoke consume excess oil? Or should I expect the smoke to appear later?
    – Anton
    Apr 15, 2015 at 17:43
  • It isn't oil consumption which is the problem, it's the fact you won't get the cylinder pressures you'd expect after ring seat of the compression rings.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Apr 15, 2015 at 19:07
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    @BrianKnoblauch - Please put your comments as the answer to close the loop on this question. That is how I'd answer the question, but would like to see you get the credit.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 31, 2015 at 12:07

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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.

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  • ...possibly needing to rebuild... :-)
    – Rory Alsop
    Aug 6, 2015 at 13:00
  • Well, most likely fresh rings and a hone job, probably not a full rebuild... :-)
    – Brian Knoblauch
    Aug 7, 2015 at 23:30
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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.

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  • What type of oil would they use?
    – Zaid
    Aug 6, 2015 at 13:23
  • You can buy oil specifically for breaking in engines. I believe Millars Oils in the UK do specific oil for the first 1000 miles of running in.
    – Steve Matthews
    Aug 6, 2015 at 15:40

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