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    Hello all, I am working on getting a two stroke spark ignition engine running. It has a .7122 piston and a .7127 bore. Cylinder and piston are CI, no rings. Would the 5 tenths of space most likely keep the engine from running? Stroke is .690 from the top of the exhaust port. If I move the flywheel very slowly it will not bounce back but if I flip the flywheel by hand it does . It will pop often but irregularly, when motored over but won't run on its own. I can make a new piston but if that is not the issue there would be no need and I would go searching elsewhere. I am using the Tim6 ignition and the spark is absolutely regular and timed 4 degrees after TDC. Any thoughts are appreciated.
    • CommentAuthorgbritnell
    • CommentTimeMay 25th 2009
    No I do not believe the half thou clearance is the problem. I would try changing the spark timing to a couple of degrees before TDC. and as you are motoring it over keep adjusting the fuel mixture toward the rich side.
    • CommentAuthordavid
    • CommentTimeMay 26th 2009
    When I am worried about too much piston blow by I add a few drops of 40 Weight oil to improve the seal for a few seconds. On a 2 cycle, a small leak is more significant. On the up stoke the fuels air mixture should get pulled in to the crankcase. If the last charge being compressed leaks back past the piston it prevents a full charge from being pulled in. On the power stroke blow by will dilute the next charge with exhaust.
    The other concern I would have is the valve between the carburetor and crankcase. If it needs too much vacuum to open a full charge may not get pulled in. If it leaks the new charge may be lost before it get injected into the cylinder.
    Let us know what you find.
    Thanks for the insight. I am working on the problem and will let you know when it gets worked out. I changed the timing to a couple degrees before tdc and am getting louder more consistent pops but the engine still will not sustain itself. With the plug out it will rotate 3 or 4 turns just by flipping the flywheel and doesn't seem to have any bind. I will next check the intake valving. Thanks again. John K.
    .7122 piston and a .7127 bore are too coarse fit, it must be smaller to keep compression. The sleeve need tapered bore and will keep compression when the engine are cold and make engine starting easier. Lets take a look at a typical lapping job - that of producing a fine finished bore and piston for an IC engine. In fact, piston and bore are both lapped in separate operations (NOT both together). All of these operations will be carried out in the lathe (and I need hardly mention the importance of keeping lapping compounds off the machine, particularly the chuck and slideways). For the bore an expanding lap is ideal, and this should be some 3-4 times the total length of the bore. The first grade of abrasive would be mixed with light machine oil (10W or lighter) and liberally coated on the inside of the workpiece. Similarly, the slurry would be added to the outside (and inside assuming it is of the ventilated type) of the lap. The lathe would be started at about 300rpm (for a nominal 1" bore) and the lap passed rapidly through the bore, keeping it moving back and forth without it coming out the bore. How to hold the lap? well, perhaps the best way is with a 'floating' tailstock holder, and failing this holding with the hand is a method as good as any. Be careful when holding the lap by hand as it's possible it may jam, hold it lightly and expect the unexpected. Remember also that unless the lap is maintained dead parallel with the bore (an almost impossible task) it will tend to bell-mouth the bore a little - hence the reason for making the work a little longer than finished size and trimming to length later. When the inside of the bore has achieved an all-over grey appearance, with the fine scratches appearing even and criss-crossing both ways, and with no evidence of any deeper scratches (as might be left by the reamer) it's time to move onto the next finer grade. The work will have to be removed from the chuck to clean it properly, and this should be done with clean paraffin oil followed by hot soapy water. The same procedure applies to the lap and all traces of the abrasive must be removed. The process continues until you reach the 'flour' grade of abrasive by which time the finish on the workpiece should be very fine indeed. A final polished finish, should this be deemed necessary, can be achieved using metal polish (diluted Autosol, or some liquid chrome cleaner). The lap should be a separate 'finishing' lap so there is no chance of contamination with the coarser grades of abrasive which might be embedded in the main lap. The piston is treated in a similar way except of course the lap is female. Work will continue with the coarse abrasive until (using the un-trimmed bore as a gauge) the piston will not *quite* enter the bore. At this stage finer grade abrasives are used and work continues until the piston will just enter the bore tightly. At this stage, it is usual to finish mating the two parts by using metal polish and briefly using the piston to lap the bore directly. Great care needs be taken but this method ensures that the fit is good for the entire length of the bore.

    I am new in this forum, i am model engine builder from Norway. :)
    Regards Jens Eirik