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Piston Knurling

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  • Piston Knurling

    Anybody out there ever done any knurling of pistons to enlarge them to fit rebored cylinders in gasoline engines? I have a really tired Techumse engine for an edger that I want to do a low buck rebuild on. I have the capability to knurl and have heard of this technique being used years ago. If anyone has experience with this I was wondering how the job turned out and what the longevity was. It seems like knurling could be a plus by keeping lubrication around the piston skirt and reducing wear. I also read that Rotax engines come from the factory with knurling on the pistons. Any thoughts? Thanks, Steve

  • #2
    Steve, I used to knurl pistons many years ago when serving my apprenticship. The system was done on a dedicated machine. We were rebuilding standard automotive stuff, 3" and better. only the thrust faces were knurled in the form of a patch. Thrust faces are that area 90* to the wrist pin. There was an arbour with a plain roller that went inside the piston skirt to support the knurling tool which was forced down on the skirt to produce the knurl. This was a hand operation as it only did a patch, about 30* wide. Automotive pistons were and I guess still are cam ground and are max on the thrust side and min at the pin bosses.
    If you can duplicate that system, I guess it would work. I doubt you could get much more than .010 thou. Hope that helps!


    • #3
      You could resleeve the cylinder. Lots of work. You may be able to pick up another motor for cheap. HSM or MW had an article (that many bitched about) on making an edger - check it out - that one should last forever...


      • #4
        Knurling pistons really isn't worth the effort unless it is a totally last resort,
        same with valve guides.
        Non, je ne regrette rien.


        • #5
          As Jim said you have to do this on a dedicated piston knurling machine. If you do this on a lathe it will collapse the piston rather than expand it. I worked in an automotive machineshop and knurled alot of pistons over the years some for small engines such as yours to do low dollar rebuilds since most of these engines are not wirth reboring. Good luck Gary


          • #6

            You might find this funny ~ I have an old automotive engineering book dated 1908 which details a method of upsizing a piston. Basically the piston is heated on a gas ring until "spit boils profusely", transfer the piston to a fly press and then proceed to 'whack'it. You may then need to re work the piston boss as it will, no doubt, have gone oval!! The idea is that the barrelling applied to the piston originally just gets increased in proportion to the reduction in piston height.
            No gaurantee's are implied by this method, if you can find an old useless piston try it first to see how hot to get it, how much to whack it, and finally to judge whether it works.
            These old boys knew how to get their engines working!



            • #7

              The edger project was a 5 part series ending in october of '98.....just in case if someone wanted to research it...



              • #8
                Use new oversize pistons with rebored engines. We used to knurl pistons that had collapsed skirts when the owner didn't want to rebore the block and buy new pistons. Pistons could be knurled in a regular lathe but the skirt must be supported internally while knurling. Knurling NEW pistons is great way to add life to a high heat situation, however trying to take the cheap way out on a Rotax rebuild, ie an ultralite could put you in a casket.


                • #9
                  Thanks for letting him know that - I was too lazy to look. I thought it was a nifty project though (screw weed eater).



                  • #10
                    Hi All,
                    Thanks for the input you have given. Just to set the record straight, the rebuild is for an edger engine. The reference to Rotax was just in passing to advise that I know some engines have knurled pistons from the factory.
                    A friend of mine made another suggestion: Bore the cylinder and sleeve it with seemless steel tubing. Seems he did this on Chevy Vega's with aluminum cylinder walls when they wore out. Supposedly this worked out well. Usually sleeves are cast iron, but perhaps steel would work as well. Probably the best thing to do is find a used edger engine for $30 and use it. The knurling or sleeving would be just to have machine work project to tinker with.