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  1. #1

    Compression after rebuild.

    Hey guys new to forum I just rebuilt a 95 speedster with twin 657x engines and I have a few concerns. First off I had the starboard engine compleatly rebuilt by company x and the portside I had the top end built by company y but did they assemble process myself with a new crank and seals. I did all the basics like replacing fuel lines cleaning carbs ect... After breaking in the motors I did a compression test and the motor I rebuilt came out at 145,145 psi, the motor that company x built came out at 120,120 so I called company x and he said that the correct compression was 6.4:1 at 130 psi for each cylinder. I was always told that correct compression for a 657x is 150,150 is he wrong? I also did an oil test to see if compression jumped and it did I got 145 145 with a little oil added to the spark plug hole would that tell me that the rings are not seated yet? For break in I followed the rules and ran it at 32:1,premix only I blocked off the pump and looped the rotary cav lines. I have around five hours or so on the motors with around 15 gallons of gas ran through it. I broke the engines in with the 1/2 on 1/2 off process slowly working my way up to 3/4 throttle never going wot and varying the throttles. I took it on two 2 outings at about two and a half hours each I know I might need a little more time to compleatly break in but I just need to know if what company x said about the 130 psi compression is correct or if he is just trying to get one past me. Sorry for the long post I just needed to give all the info. Thanks for the help!

  2. #2
    Derek45's Avatar
    Join Date
    Feb 2014
    MO. Mark Twain, LOTO
    150 over 150psi, Spraying Stabil or adding oil in cylinders will always up the compression. just keep tabs on comp. and run it, it should go up, if it falls you have an issue with that top end

  3. #3
    wkuadam07's Avatar
    Join Date
    Feb 2013
    Bowling Green, KY
    I have opened up a couple of reman engines that had lower compression like that... usually the head has had the dome milled out. Install a stock head, or have the one on it milled and your compression will come right up. The risk you run by doing that is voiding your warranty if you still have one...
    The reman engines that i use rarely come up to 150 psi but run perfectly and last forever if taken care of. They usually have 130-135psi compression. My speculation is that they lower the compression to help the motor last longer and run more reliably.

  4. #4
    Myself's Avatar
    Join Date
    May 2007
    North Central Arkansas
    I just fixed some electrical and fuel issues on a a ski with an SBT engine in it. It only reads 110psi but runs great.

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