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Standard piston size on Briggs 18 HP

Discussion in 'Mechanic and Repair' started by kernopelli, Apr 4, 2008.

  1. kernopelli

    kernopelli LawnSite Member
    Posts: 20

    Hello Men, I have been lurking in the background for awhile. Great site. I am getting ready to rebuild an old B&S 18 HP I/C opposed twin. It is a Model # 422437 0719. I bought a B&S manual and have scoured the internet but can not find what the standard pistons are supposed to Mic. out at or how much piston to cylinder clearance I am supposed to leave.

    I know the standard bore is supposed to be 3.4365-3.4375 on all the 422400-422499 motors (as well as most of the other old opposed 18 horses). I have measured my cylinder bore and am a little oval shape with the higher numbers coming out right at +/- 3.4375. I don't want to hone my cylinder out any more than I have to and it "looks" like .10 over is all I'll need but without knowing what the standard piston measures how do I know FOR SURE what to hone it to and how big of piston I need to order OR how much piston/cyl. clearance am I supposed to have?

    Any input from the Gurus would be greatly appreciated.
  2. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    If you are right at reject size at the widest point of the out of round a 10 over piston should be plenty depending on the shape of the bore (scratches/gouges and taper).

    What numbers did you get for cylinder tapper ?

    I have always bored ALL brand engines using the new oversize compression ring, I determine the size needed by cylinder condition/out of roundness and tapper.

    If ring end gap reject is .030" I hone until I have .012/.014" ring end gap on the top/center and bottom of the cylinder. Hone a little then check then hone some more.

    If the cylinder is tapered concentrate more in the smaller diameter area to
    remove the tapper.

    This may not be a "By the book" method since the "Book" doesn't give piston specs but it has worked for me on countless engines of any brand for 17 yrs. with no failures.

    Good Luck
  3. kernopelli

    kernopelli LawnSite Member
    Posts: 20

    The small number was 3.4366- right at min standard bore specs but that was at the bottom of the cylinder right at the end of the ring travel.

    The ring gap reject size is .30 (the ones in the motor were showing about .95... just one of the reasons it was smoking I'd guess). My manual only gives the reject size and nothing on the minimum size, is the .012/.014 that you mentioned the gap size you try to end up with or where you start trying to finish hone?

    Most people I have talked to have told me they usually go for .10 piston to cylinder clearance, so I am going to go with that since I can't find what Briggs recommends.

    I'm going to go ahead and order a .10 over. The cylinders don't have any heavy scratches or gouges ( in fact, they are as smooth as can be). I know that a standard piston is supposed to properly fit a 3.4365-3.4375 bore. Is it reasonable that I go ahead and true and clean up the bore, shooting for something on the small end of 3.4375-3.4385 and then file the rings for correct gap if need be when I get the piston/rings or should I just wait until I get the parts in and go from there. Pretty sure I already know that answer but want to hear from someone who has done this before.

    Thanks for your help. This is my first time to do the honing myself, I've always had a shop do it for me so I'm a little nervous.
  4. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    Just to let you know, I'm a Briggs master service tech.....

    From the above description you were not measuring cylinder out of round but cylinder tapper, So what did you get for cylinder out of round ?

    Not try but end up with, Your manual states to change from the course stones to the fine when you get within .0015 of your desired size.

    You can do this how ever you like, I explained how I've been doing it without any problems. Just remember the rings are sized to fit the piston, If the cylinder is sized to fit the rings (which I wouldn't recommend filing) the piston will follow.

    I would wait till parts are in hand.....
  5. kernopelli

    kernopelli LawnSite Member
    Posts: 20

    You had asked about taper so I mentioned that reading at the bottom of the cylinder. The oval I spoke of originally was 3.4370 taken at 90 degrees from the area that read 3.4375.

    I measured the cylinder at six points and took multiple readings at each to make sure I was getting consistent numbers. I should have given the full details.
    I agree it would be best to have the parts in hand but since I am a novice wanted to see if other options were as viable. I appreciate and respect your advice and what works for someone that has done it many times is the route I will take too.
  6. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    No problem.....

    Good Luck [​IMG]

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