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Thread: 2 stroke crank stroking ?

  1. #25
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    Default Re: 2 stroke crank stroking ?

    Quote Originally Posted by Coxsteve View Post
    Everything needs to be accurate as crank alignment when assembled is adjusted by the rather rough sounding methods of squeezing it it a Vice hitting it with a soft copper mallet to twist the halves in relation to one another or spreading the halves with a large radius cold chisel, The aim being to make sure the main bearing journal sections of the crank halves are as near as perfect aligned in all planes.

    All this whilst still making sure the side clearance for the con rod thrust washers stays in spec not quite as easy as it sounds.

    A jig to aid alignment when initial pressing together helps but experience a slow careful approach aided by some straight edges such as a machinist square on at least two points on both crank halves at the same time will keep things within a margin of error where the above standard practice with the copper mallet etc will suffice in final alignment.

    Some engines such as Kawasaki two strokes use a satin finish on the crank pins where they fit into the crank halves, With these get it as right as possible before final pressing as they are a ***** to fix afterwards if it is out too far as the crank halves do not like to rotate on this satin finish.

    If a multi cylinder like Yamaha TZ350 twins race model cranks it really is worth the effort to make a good jig to aid assembly, As for triples definitely make a jig it really does make it not just easy but actually possible to do the job in a reasonable timeframe
    Not everything. Yes the pin hole has to be parallel to the main bearing shafts, and the center line distance from pin hole to main bearing shafts has to be accurate. But the clocking of the crank pin does not have to be as gnats ass. The crank can be trued if the pin is parallel and the center lines are the same. If one of the new larger pin holes in the crank is advanced a degree or two and the other half is retarded, one , two, or three degrees if the center distance is the same and the pin hole is parallel the crank can be aligned.



  2. #26
    *Registered User* osheen's Avatar
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    Default Re: 2 stroke crank stroking ?

    Hello guys,

    I just joined to resurrect this thread. I too have built many 2 strokes for the last 30+ years. Also seen stroked cranks but haven't seen how they actually do it. I built a YZ125 into a 150 by boring 3mm and stroking 4mm. I had a place called Max Power stroke the crank and I think they do it the simplest way. They have a crank pin made with a 2mm offset. The OEM Yamaha rod bearing has rollers loose from the cage so it allows everything to be slipped over offset pin, assembled and then pressed together, Doing it this way is just press and play without having to mess around with crank pin holes and worrying about how to make dead nuts true when machining. The offset pin is $200 by itself. I also had them balance it. They drilled and added tungsten on opposite side of crank pin. So with their labor and pin it was about $450 total I think not including new rod and bearing.

    I thought about building a stroker crank for my son's YZ125 also. But still wonder about doing it myself (because I like to do things myself instead of paying someone else) by boring out pin hole bigger offset 2mm, then pressing in a sleeve and boring back to original diameter. You would just have to make sure the crank journal and pin hole were on same plane..... Or TIG weld up inner side of hole with filler rod then re-bore offset 2mm. But you'd have to figure out how to keep journal and pin hole on same plane after welding up hole and putting back on mill.

    AirChunk, did you ever stroke your crank yourself?



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2 stroke crank stroking ?

2 stroke crank stroking ?