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  1. #1

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    Pro 785 motor break in , need advice on hours\ fuel

    just finished rebuilding my 785 motor ( WSM 750 pistons , crank checked , etc ), How many tanks or hours until " broken in ". Have a race labor day weekend . Also what fuel would you use ? , have a hot seat head with 2 dot domes , and oil ratio ? 32 to 1 ?
    PS , have the stock head , would it be better to break the motor in with that ?


  2. #2
    thetolleyman's Avatar
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    The core idea is to heat cycle the engine several times, with increasing range of throttle and duration after each warm up, followed by a thorough cool-down each cycle.

    Something like this is what Polaris recommends, SBT's recommendations are a bit less but similar:

    For break in, you need to add a little oil to the gas tank. Or if running premix, use 32:1.

    Run it for a min. and let it cool.

    Run it for 2 mins. and let it cool.

    Run it for 5 mins and let it cool. Vary the throttle position.

    Run it for 10 mins, varying throttle position.

    Run for 15 mins and let it cool. Varying throttle.

    After that, keep it under 3/4 throttle until the first tank is gone.

    After first tank, check the piston wash. if it looks good, then have a good ride.

  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    I agree with most of that suggested break-in procedure, but you do want to mix in short bursts of heavy throttle with the 'varying throttle' process.

    The idea is to force the rings to fully bed (controlled wear) into the cylinder walls. This requires combustion pressure, which requires throttle. The flip side is that during the early stages of break-in there is a lot of friction from that same wearing in process. That is why these is extra oil ratio during break-in.

    The varying throttle prevents any sort of odd wear pattern from forming in the cylinders and allows the friction heat to abate and the controlled wear process to, well, be controlled.

    The cool down cycles are in part to allow the new pistons to acclimate and heat cycle harden. After each cool down cycle the pistons and rings are closer to a perfect match with the cylinders.

    As you work through the break-in process with each longer cycle you want to apply more throttle, more often, but still with plenty of time spent varying the throttle throughout the range. By the end of the break-in process you will be applying full throttle for significant amounts of time, yet continuing to allow the RPM range to vary in between.

    Note that the entire break-in thing is about the pistons, rings and cylinders. The engine bearings do not require break-in and do not 'wear in'. The bearings just slowly wear as the engine gets used and eventually they wear out, hopefully hundreds of hours later.

  4. #4

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    What fuel would you use with the Hot seat head ? Been looking at VP fuels , would 107 octane be good ?

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