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    • CommentAuthordavid
    • CommentTimeSep 13th 2009
    I have been working on a .700 inch bore vertical. The other day I took one of my other verticals apart and deep scratches in the side of the piston in the direction of travel. The cylinder is cast iron 1.001 ID, the piston is 6061 aluminum 0.998 OD, 2 cast iron compression rings and wider oil scraper. The marks are on all sides but heavier on the sides with mo wrist hole. The crank is a wet sump and this engine has a plastic end and the oil fly’s. I found no grit or metal in the oil.
    I am just looking for ideas and corrections to make the next one better.
    • CommentAuthorjay rich
    • CommentTimeSep 13th 2009
    try 7075 or 20 24 alumium for pistions its harder then 6061 and it will be a little liter ....jay
    Check the tolerance between piston and cylinder, depending in Aluminium alloy, you need to allow room for expansion.
    I had made some very tight pistons in one engine, after some time running, pistons showed clear abrasion as well as the liners.
    I rebuilt the engine using 7075 Pistons ( I know some people will criticise this, since 7075 is not recommended for pistons but works great for expansion), running against the same Cast Iron liners ( honed to the next size), allowing more clearance between pistons and liners, engine ran great but a bit of smoke in one piston, so I opened the engine to change rings, and the wear in the pistons has been almost none, they are almost as shiny as they were when first installed.
    My engine is (was)0.75 bore diameter ( after the rebuilt bore is 0.80 in ) I allowed 0.003 clearance in the top portion of the piston ( from the upper ring to the top) and about 0.002 for the rest of the piston skirt. There are so many opinions in the clearance of pistons and liners. I have found this tolerances work great and they were given to me by two folks who have been building engines for a long time.

    • CommentAuthordavid
    • CommentTimeSep 15th 2009
    So you guys are telling me my 6061 is soft and may expand as the engine heats and touch the cylinder ID.
    The harder 7075 not only is stronger, less likely to scratch or gall, but also expands less.

    What about other cylinder materials than cast iron? My goal is to go smaller and smaller till it won’t run.
    Any thoughts on drill rod or 12L14 for a cylinder?
    What about a jacket of cylinder material over an aluminum piston?

    • CommentAuthorjay rich
    • CommentTimeSep 15th 2009 edited
    ive never tried cast but know it works great as liners or cylinders or 303 stainless and i fully agree on the 3tenths or 32 yenths clearence on my 5/8the engines i got to 0.6255 bore and the run and seat fine no oil blow by if i do get blow by i know i just snaped my wrist pin and well i have dusted alot of them and the cylinders well they are realy done 2 large galls in the side walls from the pistion and the shatered wrist pin ....jay
    For smaller sizes, you may consider using cast iron pistons with no rings riding against cast iron liners, this was the old practice in air plane engines, the ability of cast iron to retain some decent levels of lubrication aided by some oil in the petrol should help to reduce friction.
    I have not used 12l14 liners in water cooled engines, they are one commonly used option for air cooled engines, like radials, some builders who have used them in water cooled engines claim it takes too long to seat rings and liners.

    • CommentAuthordavid
    • CommentTimeSep 16th 2009
    I had thought about just cast iron sleeve and piston but had never heard of it other than in a hit and miss.
    I have used drill rod on a number of my small air powered engines.
    I am wondering about 12L14 (lower cost and I have some7/8 dia) for sleeve and piston.
    If I lap for a great fit do I really need rings?
    It is probably time for some metal cutting and testing. Make a sample cylinder and piston see what it does hot and cold.
    Talking about air powered engines OK, here is something I did in some smaller engines ( 0.500 bore), tried graphite for ease of manufacturing, no friction etc, it was running against some aluminum cylinder and it failed ( wear was too much), then I did some bearing bronze pistons, no rings, just grooves for oil control and a very nice honed finished in the liner, nice fit in the cylinder and voila engine runs with air at about 10 psi.
    Try Bronze, brass or something different than the liner, 12l14 machines great, it is cheap and can be honed to a very nice finish, I do three "V " shape grooves in the piston and then lap it very gently to remove any burs.

    Drill rod is more difficult to achieve a nice finish than bronze or brass, however it does not gall like brass, so add some lubrication in your air or in the top end of the engine every now and then if you decide to use either one.


    Watch your coefficient of linear expansion. Alcoa lists both 6061 and 7075 as having coefficients of linear expansion of 13.1X10 minus 6 per inch/per degree F in the range of 68-212 deg f.

    Grey cast Iron is listed as 6.0.

    In other words Aluminum will expand over twice as fast per degree of temperature rise as Cast Iron. A cold engine is a different size than one at operating temperature.

    Following is a link to coefficients of expansion of common materials.


    With the tight tolerances of our model engines this may a real factor. Also keep in mind it is not only the radial expansion but ring clearance as well. Heat moves everything!

    • CommentAuthordavid
    • CommentTimeSep 17th 2009
    I had never taken the time to figure out the different expansion rates.
    When I run the engine and monitor temperature I throttle back before 300F. When it just run it I notice some oil on the surface smoking. So I would expect 300F rise. with the expansion coefficients that would be 0.0018 reduction in the original 0.003 piston cylinder gap. But what if the piston is hotter and expanded more? Looks like a 300 F cylinder and 450F piston would touch. I can only measure the body temperature the cylinder is pressed into.
    I know I have 12L14 and I will see if I have enough cast iron. I will make, a cylinder and piston from each, lap all parts for good room temperature operation, then heat it to 300F and see if it still moves. If its all the same alloy it should work.
    I have made a cylinder liner with 12L14 like steel(it contains some part of Pb) in Silver Angel's cylinder liner.
    It worn out after several running hours, and I had to make it by cast iron.
    The running sound is clearly different between cast iron and 12L14 and after 10 hours of running time, it has no ploblems.
    I think it caused by the different of friction coefficient.

    The clearance between piston and cylinder liner should be over the calculated values, I think.
    In some engineering text, I found that the temparature of the aluminum piston head is about 300 degree centigrade.I decided it 250 degree because my engine runs in low speed.
    • CommentAuthordavid
    • CommentTimeSep 27th 2009
    What bore and what cold clearance did you use? Did you use rings?
    Hello David
    I'm sorry for late to reply, because I had to check my old note (data) books.

    My Silver Angel has 19.02mm bore and 19.00mm alminum(2024) piston.
    After high speed running, it will stop. But Silver Angel runs always slow, so it caused no ploblem so far.
    My engine has two piston rings made by cast iron.
    Heat treatment was done according to Bob's method.
    The outer diameter of piston ring berore cutting is 19.03mm.