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Results 1 to 13 of 13
  1. #1
    Join Date
    Sep 2012
    Location
    Indianapolis, In. 46254
    Posts
    5

    Default Piston to cylinder clearance?

    I have been reading through 4 pages of threads that came up in the search tool for piston to cylinder clearance. I have a Johnson 150 made in 1980. I need to bore at least three cylinder's and I have located some used pistons. Trying to find what everyone is running. I have seen threads that say .004 and others that say factory reps go for .009 that's a pretty big difference. I currently have .007 on the three that I hope not to bore.

    Thanks for the help, Randy

  2. #2
    Join Date
    Oct 2011
    Location
    maplevalley WA
    Posts
    8,002

    Default Re: Piston to cylinder clearance?

    Get all new pistons, rings and wrist pins and have the cylinders bored to spec all the same the tighter the better IMHO.

  3. #3
    Join Date
    Oct 2011
    Location
    maplevalley WA
    Posts
    8,002

    Default Re: Piston to cylinder clearance?

    http://www.marineengine.com/newparts...&Engine=&Model= Have it bored .030 over you have two choices .020 or .030 either way get the pistons and the machine shop will know what to do.

  4. #4
    Join Date
    Sep 2012
    Location
    Indianapolis, In. 46254
    Posts
    5

    Default Re: Piston to cylinder clearance?

    Thank you for that info. I run a fabrication business and I do my own machine work. I have been rebuilding engine's for many years and I do realize that getting pistons and boring all the cylinders larger would be a good way to go. What I am looking for is what is the minimum and maximum that people run. The manual that I have says .004-.0075. I am at .007 on two cylinders. .006 on another and the other three are currently bored for .020 over pistons and I am installing .030 pistons in those cylinders. The .020 over cylinders also have a .006-.007 clearance on them, but the pistons are scored. I plan on re-ringing and installing the pistons that I have as that would save buying about $600 worth of pistons.

    Thanks for the help, Randy

  5. #5
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,775

    Default Re: Piston to cylinder clearance?

    Obviously you are aware that even a new piston is 0.010" smaller in diameter at the area of the piston rings.------------The clearance per the book is measured at the skirt.--------------So you have to look at the overall picture as extreme wear near the rings will mean a " noisy engine " due to piston slap.-----------I also understand the economic consideration too.------At 0.007" clearance the engine will certainly run fine as I have seen them at up to 0.010" and still running OK.

  6. #6
    Join Date
    Oct 2011
    Location
    maplevalley WA
    Posts
    8,002

    Default Re: Piston to cylinder clearance?

    I would be concerned about end gap on the rings these motors work real hard and compression is everything when it comes to performance. Spending the dime now and your chances are alot better for the extended life of the motor. You can get away with mismatching pistons in a jeep but out on the water is it worth the risk?

  7. #7
    Join Date
    Sep 2012
    Location
    Indianapolis, In. 46254
    Posts
    5

    Default Re: Piston to cylinder clearance?

    Thanks for the replies....I do understand that you measure a piston at the skirt and that they are smaller at the top. I believe that I will be fine with the pistons that I have, but was looking for what clearance you guys run. Once the new rings get here I will measure the ring gap and see!

    Thanks, Randy

  8. #8
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,775

    Default Re: Piston to cylinder clearance?

    A lot of this depends on how much boating you do and where.---------------Small lake or river is totally different than going offshore where you might not see land or another boat.----Also depends on whether you run 1 or 2 hrs a month or 6 hs a day.

  9. #9
    Join Date
    Oct 2011
    Location
    maplevalley WA
    Posts
    8,002

    Default Re: Piston to cylinder clearance?

    Water is water you cant just walk away it dont matter if your out for five minutes or five hours. If you can get the compression within 5% you should be just fine the less you use a motor the harder it is on that motor!

  10. #10
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    744

    Default Re: Piston to cylinder clearance?

    It depends on what type piston you use, If say your finished bore is 3.500 you should have .003-.004 piston clearance(piston skirt would be 3.496-3.497) if a Wiseco you want a .001 -.0015 extra clearance as these need to go thru several heat cycles to harden up, a on a cast you want closer spec's. When I bore my customers engines I will usually bore them to .0005 undersize and use Sunnen hone to to finish cylinder and they will be in the .003-.004 range. Make sure when you bore it you go past the cylinder liner to bottom of block or it will leave a step in bottom that piston will catch on...

  11. #11
    Join Date
    Sep 2012
    Location
    Indianapolis, In. 46254
    Posts
    5

    Default Re: Piston to cylinder clearance?

    Thanks for the info faztbullet. That is the what I was looking for. Now I have a question about ring gap. The part number for the rings that this website shows is
    385663. This has been replaced with 396504. That ring fits many engines up through 1987. The book I have shows that a ring gap for my 1980 engine is .007-.017. I have bored my cylinders at 3.530 and put a ring in and I have a clearance of .023. This is on a brand new Johnson ring set 396504. When I compare that part number on boats dot net I see that it fits up through 1987 and that 1987 engine should have a ring gap of .020-.033. I believe that I have the correct rings, but thought I would get an opinion on it!

    Thanks, Randy

  12. #12
    Join Date
    Sep 2012
    Location
    Indianapolis, In. 46254
    Posts
    5

    Default Re: Piston to cylinder clearance?

    This is not my first rebuild......Numbers are numbers and I am just looking for the correct numbers. Seems like I fall into the second set of numbers on the piston ring gap of .020-.033?

  13. #13
    Join Date
    Oct 2011
    Location
    maplevalley WA
    Posts
    8,002

    Default Re: Piston to cylinder clearance?

    I dont have the specs for your engine but for up to 40 hp same year I am seeing .007-.017. .020-.033 seems extreme to me but I would think as long as they all are the same or close the compression would be more even my concern would be if it effects pressures in the crankcase would be problematic IDK.

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