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Lapping in a cast iron piston

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  • Lapping in a cast iron piston

    I'm building the Crusader .60 and have just reached the point where I am instructed to "lap the piston to the cylinder..." Being relitively new to the hobby, I have yet to lap anything. Do I do it with the lathe running at a low rpm or is it strictly a hand opperation? Know any sources for "rottenstone" or a commonly available substitute? I've been reading this BB for a month or so now and really enjoy the shop wisdom and (most of )the humor. Thanks.

    Todd

  • #2
    If it were me and I have limited resources. I would turn down a piece of wood about a tenth (.1) smaller than the ID and cut a slot in it lenghtwise. I would put in emery paper and wrap it around the wood (use the correct direction cause you'll be using the lathe) Then chuck the cylinder in the lathe and hold the wood (like a broom stick) with your hand and spin the cylinder kinda slow. I would use decreasing grits until i'm down to about 220. I'm not sure what grit a cylinder hone is but once that has been determined then use the same grit paper. From past experiences the BEST lube to put on it is zippo lighter fluid. (but I didnt tell you that) . Its the only stuff that I have found that doesnt clog the paper instantly. FWIW I have never caught it on fire.

    If your RPMs are correct then you can even end up with the correct angle on the crosshatch

    Measure diameter OFTEN and let it COOL. Expansion will skew measurements.

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    • #3
      What you need is an expanding barrel lap. You can make them, but it's quite a job and they aren't terribly expesnive. See for example www.mscdirect.com or www.travers.com or www.reidtool.com . Personally, I would not use wood because of the danger of bell-mouthing the bore. That's a danger anyway, but less with a barrel lap.

      Get the bore about half a thou undersize, then put a SMALL amount of #320 lapping compound on the lap, chuck it in the tailstock, and run it in and out of the bore by sliding the tailstock back and forth while the lathe turns around 250 rpm. (I'm assuming you have the cylinder mounted in the lathe here.) Add a little very light oil if need be, and a little more lapping compound if need be, as you go. Too much lapping compound will lead to bell-mouthing the hole.
      Brownell's www.brownells.com usted to sell a kit of 6 or 8 2-oz containers of various grades of lapping compound, which for a home shop will likely be a lifetime supply. Everyplace else seems to sell it in 1-lb cans, which would be enough to lap about a million model engine cylinders. So check Brownell's, before you buy a pound of the stuff.

      ----------
      Try to make a living, not a killing. -- Utah Phillips
      Don't believe everything you know. -- Bumper sticker
      Everybody is ignorant, only on different subjects. -- Will Rogers
      There are lots of people who mistake their imagination for their memory. - Josh Billings
      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
      Don't own anything you have to feed or paint. - Hood River Blackie

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      • #4
        As I see it you are lapping the piston to fit the already lapped cylinder. I think the split external lapping tool the author refers to would be similar to "tool L" but with a screw to pinch the gap to fine tune the diameter, but I'm guessing here.
        I'm not sure but it seems I've read somewhere that the "crosshatch" patern while desirable on a ringed engine cylinder is not used on lapped engines.
        Bruce

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        • #5
          SGW is correct about "bellmouthing" it.. Measure often..

          Boomer That sounds accurate. But the crosshatching, ie scratches, are there to hold oil. Thus preventing metal to metal contact. The grit used to create these would be determined by the thickness of the oil that is going to be used I think.

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          • #6
            If you're lapping the piston...you need a "ring lap." Make one from a disk of aluminum, bored to fit the o.d. of the piston and split on one side, with a screw you can turn to close up the slit and tighten the lap on the piston. As with the bore, # 320 is probably about right. Apply a small amount to the piston , slide the lap onto the piston, hold it WEARING A HEAVY GLOVE! and run it back and forth the legnth of the piston while the piston is turning about 250 rpm. As the lap loosens up from wear, tighten the screw slightly. Again, don't overdo the amount of lapping compound, and add a little light oil if it starts to dry out.
            ----------
            Try to make a living, not a killing. -- Utah Phillips
            Don't believe everything you know. -- Bumper sticker
            Everybody is ignorant, only on different subjects. -- Will Rogers
            There are lots of people who mistake their imagination for their memory. - Josh Billings
            Law of Logical Argument - Anything is possible if you don't know what you are talking about.
            Don't own anything you have to feed or paint. - Hood River Blackie

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            • #7
              Todd, check with your local jeweler and see if he will sell/give you a small bit of Jewelers Rouge. If you have been careful with your turning and boring, you won't have to do a whole lot of lapping.

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              • #8
                Not knowing this engine, what is the bore, what material is the piston and cylinder? I assume no rings are involved?

                how sophisticated are the instruction/project?....the instruction lap the piston to the cylinder makes me wonder whether they are literally using one as a lap against the other; an un-sound mechanical process that will only increase the clearance between the two. As others are pointing out, lapping is correctly done between a lap and the workpiece, ideally with the lap being adjustable and made of brass or bronze so that it can hold the compound.

                .

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                • #9
                  The Crusader .60 is a five part construction article begining in the July/August 2005 HSM. It's a two stroke glow (or gas) engine with a bore of .937" and a stroke of .875" (.60 CID). The piston is fine grained cast iron or Mehanite and the cylinder is made of 12L14 steel.
                  The author says if the cylinder has been HONED to its final dimension the piston should be carefully machined...lap the piston to the cylinder...if desired, a split external lapping tool may be made and used to bring the piston to its final size.



                  [This message has been edited by Boomer (edited 01-05-2006).]

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                  • #10
                    Thanks for the replies all! Yes, I am lapping a piston to fit an already honed cylinder. I used a wheel cylinder hone to for the cylinder. I would guess it to be about 320 grit. I think I will make a split-ring lap, that way if I overdo it all I will ruin is the piston. I heard somewhere sometime of using toothpaste and WD-40 as a lapping mixture.
                    I wondered about the instruction to lap the piston to the cylinder. That sounds to me like you would wind up with a clearance of 2x the largest piece of grit in your lapping mixture.

                    Thanks again. Todd.

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