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  • #1

    Piston ring questions

    Due to low compression, 120psi on 1,2&3 and 95psi on #4, I pulled the motor apart.

    Found poor valve seats and deposits so I had the shop reseat and clean everything and all is better on the top end, sooo........here's my question.

    The cylinder bores measure 2.816" (71.52mm) well within spec, so can I keep the piston rings or should I get new? Pistons look good and are in spec as well.

    I don't want to cheap out and not get new, but I don't want to throw away money if it's not needed. Also, I didn't know if it was best to put in new rings if the valves are done.

    Thanks for the help,
    Jack
    Tags: None
  • #2
    Hey Jack,

    I was taught that if you did a head/valve job, that it would put more pressure on the rings, and could increase blow by if they are a bit worn? Since they are in spec, you could probably get by with the originals. However, you didn't state whether you did the oil in the cylinders compression test to verify whether the low comp in that 1 cylinder was due to the valves or rings!? So.. you could hone the cylinders and put in new rings.....for peace of mind!

    Did you or have a shop measure the bores all along the stroke in the bore, not just at the top??

    Hopefully Ratbyk or one of the other mechanical GURU's with much more engine building experience and knowledge will chime in?
    T.C.
    T. C. Gresham
    81SH "Godzilla" . . .1179cc super-rat.
    79SF "The Teacher" . . .basket case!
    History shows again and again,
    How nature points out the folly of men!

    Comment

    • #3
      Did you take the rings offa the piston and place them in the cylinder and measure their 'end gap"?
      Unless the piston shows signs of wear (Rubbing on the skirt... the bottom part)and otherwise checks out ok, I'd leave the pistons alone.
      Blow-by is caused by worn rings... too much gap. (or not seating properly, or being siezed in the grooves)
      Oh, be careful taking the old rings off to measure them, as they are quite brittle and snap easily.
      If you do go for new rings, put a new cross-hatch pattern on the cylinders so that they can wear themselves in properly.
      "Damn it Jim, I'm a doctor, not a mechanic!' ('Bones' McCoy)

      Comment

      • #4
        Piston to bore clearance is one of the deciding factors. Clymer's calls for a range of 0.0016" - 0.0018" That's not a whole lot of clearance! Also, the limits for out-of-round and taper are 0.004" and 0002" respectively. Finally, piston ring end gap needs to fall between 0.008" - 0.016" If the pistons and bores fall within spec, you could get away with new rings if you can find them.

        On my current restoration project, I did not bother even starting to look for rings or doing a lot of measurements. I went straight to the 'small' big bore kit from Wiseco and had the cylinders machined to 'just big enough' for the new pistons.

        At the same time, I had the valves and seats done, including shortening up the valve stem to compensate for the valves sitting just a bit deeper in the seats. None of this was cheap, but the engine should be good for another 100,000 km when I'm done.
        Ken Talbot

        Comment

        • #5
          TopcatGr58.... I've posted a related thread in the general forum related to this, called "Valve job increases blow-by?" I would appreciate your thoughts and comments. Anyone else that wants to play is invited, also.
          "Damn it Jim, I'm a doctor, not a mechanic!' ('Bones' McCoy)

          Comment

          • #6
            correction

            Sorry, I meant Member's Lounge, not general forum.
            "Damn it Jim, I'm a doctor, not a mechanic!' ('Bones' McCoy)

            Comment

            • #7
              Shop measured the bores in various places and it all fell into spec. As far as the valve work, I learned the hard way about leaving old rings in and reseating/lapping the valves. Made for a great mosquito fogger. But that was a high mileage car.

              I am heading out now to check piston size/clearance and piston ring gap.

              For my compression check I just used a screw in type with a vaccum gauge. I let the motor turn over 8-10 times. Checked each cylinder twice with the same results.

              Thanks,
              Jack

              Comment

              • #8
                Hey Jack,

                The testing technique says that IF you have low compression values, you then squirt a touch of oil inside the cylinder, then repeat the test. IF the pressures rise, then the oil is helping the rings to seal better, and reveals worn rings. IF the values DON'T increase, then the leakage is from the Valves cause the oil won't help seal them. So..just repeating the values dry was not as diagnostic as adding a touch of oil. However, it's a mute point now, cause the engine is apart. But sounds like if your measurements are within the specs Ken stated, then you should be good to go back without honing/crosshatching or new rings!
                T.C.
                T. C. Gresham
                81SH "Godzilla" . . .1179cc super-rat.
                79SF "The Teacher" . . .basket case!
                History shows again and again,
                How nature points out the folly of men!

                Comment

                • #9
                  Updates

                  Here's the latest.

                  Piston ring gap is around 0.7mm. Clymers calls for 0.2-0.4mm with a maximum wear of 1.0mm.

                  Looks like I am somewhere in the middle.

                  Comments??
                  Jack

                  Comment

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