Everything2Stroke - That precise piston diameter
  • That precise piston diameter

    I just finished up a bore job on an old early 70’s R5 piston port 350 Yamaha. The pistons came from HVC Cycle in Nebraska. Give them a call if you ever need some old Vintage parts.



    This set is one of the few 2-stroke Japanese cylinders which appears to have removable liners. I think I’ll have a set to repair and reline soon.

    My thoughts came up about how to accurately measure piston sizes when doing a bore job. With all of the different suppliers and brands it’s tricky to figure out where to measure the largest diameter.



    Some pistons have a mild taper, some have a belly in the center and some a fairly straight.

    What works for me: While finish honing the cylinder, I stop at around .0015” clearance and rinse things off. The fine hone filings are enough to jamb up the new piston.





    While stroking the piston up & down in the bore, a certain resistance will be felt. It probably would be good to paint the pistons with Dykem layout dye, but I didn’t on this project. After a short time you will see the largest diameter point from the wear marks. Each piston needs to be measured since production runs may vary in diameter tolerance. Sometimes you’ll find the largest diameter isn't where you expected it to be. When the dial gauge is set you’ll have the correct dimensions to fit the parts accurately. That final skirt clearance fit is one of the most critical measurements in your engine.



    Just got his photo from a happy customer in North Carolina.




    Wash your Hands
    "Old Dog"
    JT
    This article was originally published in forum thread: How It's Done - Trade Secrets inside. started by John Tice View original post
    Comments 12 Comments
    1. volfan537240's Avatar
      volfan537240 -
      Nice info JT. Where's a good place to buy a mic to measure most pistons (60-80mm maybe)?
    1. John Tice's Avatar
      John Tice -
      Go to www.cavcosales.com You can buy a whole set of mic's for a good price. I purchased all of my cylinder machinery from Dan at Cavco. The only thing that killed me was the freight. The net is a wonderful place to purchase tools, buy a good set for a lifetime.
      JT
    1. volfan537240's Avatar
      volfan537240 -
      Thanks. Ill look into it.
    1. Morph0906's Avatar
      Morph0906 -
      Another good read. Thanks
    1. garrywarber's Avatar
      garrywarber -
      I have been told piston/cylinder fit is critical, and many have A, B,C, and D fit designations. What are the manufacturers aiming for to use this designated fitting code? It would be nice not to have to buy piston/cylinders in sets!
    1. John Tice's Avatar
      John Tice -
      The size designations A, B, C, etc are for very slight dimension increments for fits in plated cylinder bores, I’m not sure how much the increases are. I only use my micrometer & dial bore gauge to figure out the proper skirt clearance.



      Another thing to pay close attention to is all of your parts are at the same room temperature. Many times the cylinder will heat when extra amounts of material are removed while honing. The BDC end or any thin portion of the cylinder may warp, “bridge in the exhaust”. It’s a good idea to let your parts set over night to allow the temperature’s to settle. The next morning is the best time to check the final fit. The Skirt Clearance is about the most sensitive dimension in your engine which you can control.
      .1mm = .004”
      .25mm = .010”
      .5mm = .020”
      .75mm = .030”
      1mm = .040”

      Don’t’ be confused, you might split the cost of a set of measuring tools with your friends. Lastly, with a bridged exhaust port, always drill lubrication holes.



    1. garrywarber's Avatar
      garrywarber -
      And the .0015 is the final finished clearance? That's what, about .025 of a millimeter? I cannot do those numbers; I need to think in thousandths! One and one-half thousandths of an inch, and it does not seize? I am learning a whole new engine with the 2-stroke! How badly does a looser fit hurt performance? I do not know if my Limbach engines (Stihl 090 chainsaw cylinders and pistons) have a single exhaust port or if it is bridged. I will find out in a while...
    1. John Tice's Avatar
      John Tice -
      garry: .0015" = .0381mm assuming that 1" = 25.4mm. If you send me a request via standard email, I'll send you
      my handy motor-math spread sheet
      www.john@smallenginemachineworks.com
    1. garrywarber's Avatar
      garrywarber -
      Thank you John, but I just did a quick and wrong in-head guesstimate. I will stick to thousandths of an inch and buy tools in that. What I could use right now is some reading recommendations to improve my skills. Have any favorites?
    1. John Tice's Avatar
      John Tice -
      My best advice is to enroll in a community college machine shop class, otherwise shop on line for used text books. It would also be a good idea to purchase a copy of Machinery's Handbook. Take a look at "How it's Done" for a few good ideas.
    1. garrywarber's Avatar
      garrywarber -
      Quote Originally Posted by John Tice View Post
      Go to www.cavcosales.com You can buy a whole set of mic's for a good price. I purchased all of my cylinder machinery from Dan at Cavco. The only thing that killed me was the freight. The net is a wonderful place to purchase tools, buy a good set for a lifetime.
      JT
      Thank you for the link John! I will be dreaming through that site for some time. It's really great to find a down to earth and practical 2-stroke guy also. I envy your 2-stroke "cars!"
      Garry
    1. John Tice's Avatar
      John Tice -
      Hello Garry: Anyone can do machine work, being a detailer really helps, you need to start with education. Read anything that you can to learn. We now have Mr. Billy Alvarsson on board www.bimotion.se who is the example of a good education. Also, you might get a job with a cycle shop who does some cylinder work, anything you can do to (as they say, rub elbows) with someone who can teach. Read through my thread, “How it’s Done” you can pick up many things, Mr. Bell, “2stroke tuners”. Much can also be picked up by following E2S.
      Stay tuned

      PS: I’m not the car guy although he is in my end of town
      .

      E2S: the place to go when you need to know.
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