2 stroke crank stroking ?

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Thread: 2 stroke crank stroking ?

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    Default 2 stroke crank stroking ?

    OK, I have been building 2 stroke race engines for years . And have allways sent our cranks out to be stroked , even though I will rebuild them , I have never stroked one myself. My question is about locating the 2 holes on center -- the best way . Have any of you ever done it ?

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    Last edited by AirChunk; 11-28-2008 at 09:25 PM.


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    Quote Originally Posted by AirChunk View Post
    OK, I have been building 2 stroke race engines for years . And have allways sent our cranks out to be stroked , even though I will rebuild them , I have never stroked one myself. My question is about locating the 2 holes on center -- the best way . Have any of you ever done it ?

    Never done it myself. I would imagine there would be a specialized grinder that would offset the crank center the distance of the stroke. I would like to see the process myself.



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    Yeah , I have never seen it myself . I just get to see the end product .
    A single cylinder 2 stroke crank is unlike a car crank , it is held together by the bottom rod pin . see the attached photo .

    I was thinking of making a jig that would keep the centerline of the crankcenter and the bottom rod hole centered .

    The bottom of the hole has to be tig welded up and then rebored to fit the fin in the new offset position .

    I was thinking that in order to make both crank halves the same , they should be put in the jig together and bored at the same time ????

    Attached Thumbnails Attached Thumbnails 2 stroke crank stroking ?-cr85crank-gif  


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    Has anybody ever seen this done ? Any Idea's ?



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    AirChunk,
    Like you,I also raced 2-strokes. In my case, in boats and snowmobiles.

    I did modify many of them, some to the point of destruction. LOL

    I never stroked any of them. Too troublesome.

    I'll ASSume you are increasing the stroke. If you intend to use the same connecting rod, the cylinder will have to be raised the same as the offset of the crank throw. Be sure to measure your squish band clearance before and after (must stay the same).

    Measure (with a degree wheel) port timing before you start any mods. When you finish your work check again. If you want to modify port timing, this is the time to verify.

    Be sure there is enough clearance for the rod end swing after the motor heats up.

    But you new that, LOL

    Dick Z

    DZASTR


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    Richard,
    I hear ya on all the measuring . Yes there are many things to consider . Rod angle , comp ratio, bore to stroke ratio, port timing etc.

    I am really after how to make a jig for the machine work on the crank .


    Must have been real fun racing boats . I always wanted to do that but never got the chance.



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    Get tossed out of a boat into water vs. get tossed off a snowmobile onto frozen water. It seems a simple choice. but hitting unfrozen water at high speed hurts too. LOL

    I would think with care on an accurate machine you should be able to bore the crank halves separately. You could make a bushed jig to pilot the boring tool.

    Good Luck.

    Dick Z

    DZASTR


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    Richard,
    Thanks man , I have played around the water some , water skiing anf jet skiing , and you are right the human body can skip like a rock on the water !

    I talked to a guy today that does this for a living (crank stroking , 2 strokes ) I did not pry very hard , I know some people do not want to give out there knowledge that they make a living with and I understand why.

    He did tell me this much.He said fist he will take a small ind mill , and with the crank fully together and squared up in his machine , he will make a small witness line across the 2 crank halves to help in lining the 2 back up . (I do the same thing when rebuilding them , but I use a machinist square and a scribe . ) He said then he has a jig , this is the part I have 2 figure out .
    and will indicate the holes ( to set the machine up for the final stroke ) He then inserts and welds in a 4130 bushing ( think cresent moon shape) into the lower half of the rod hole on the crank . The each half is put back into the machine and the final hole is bored in with the new offset , each crank half is done seperately , one at a time .


    I guess my hang up is how to indicate the two centers , actual rotating center of the crank This one is easy , then indicate the crank throw hole (rod pin hole ) completely square to each other ???



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    Maybe we're over-complicating this???? I ASSume the crank will have to be balanced again after mods. are done. Since we're dealing with circles, the crank throw radius is probably the most critically accurate dimension. Also the crank throw pin bore must be geometrically correct (round and parallel to a line through the main bearings).

    Yes, witness finishes are handy for reassembly, but an assembly jig locating from main and throw features would accomplish the same thing.

    The problem is, by the time all the jigs and fixtures are built and paid for, it would be less expensive to have a pro do it. Unless you plan on getting into the crank mod business. If so, go for it!!!

    Dick Z

    DZASTR


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    Quote Originally Posted by RICHARD ZASTROW View Post

    The problem is, by the time all the jigs and fixtures are built and paid for, it would be less expensive to have a pro do it.

    I thought you were going to say, it would be less expensive to build a new crank from scratch - that is what I would suggest. This way you still have a working crank if you fail to make a better one...

    Red to red and black to black, or it's ashes to ashes and dust to dust.


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    Well , building 2 strokes is part of what i do fpr a living .......
    I like to keep things in house also .
    Of coarse my engine will be the guinea pig , I have 3 stock stroke cranks for it .



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    Default 2cycle stroker

    I do a lot of crankshaft grinding and make repairs and stroker's but the two stroke is a different deal the easiest way I would think would be to make an oversied pin if the bearing's will alow it and offset bore the holes in the wheels you may not have the room you also have to deal with the comprression hight and port alinment not to mention counterwt clearance . If it has an insert bearing and not needels it could be welded or chromed . Try marine crankshaft in Calaforina Good luck Kevin



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    Kevin ,
    Do you mean try marine crankshaft in CA for some advise ??? That is what I am looking for .



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    I think the most important thing would be to be able to locate the crank halfs in the jig so that the crank pin bores are in the same position but offset radially 1/2 the desired stroke distance. You could machine a jig with a circle slightly large than the crank web diameter and then use the tapped holes that the counterweights ar ebolted two (most two-stroke cranks I have seen have bolt on counterweights) to locate in the fixture. If it is something the size of the CR85 crank shown you could probably hold it in an indexing spacer so you could press the crank halves apart, find the center of the oem crank pin location on one axis, then index the spacer to 180 degrees and machine a reference point. Remove the crank half, weld in your material, chuck up the crank half, rotate the crank half until you find your reference point, index 180 degrees move the index spacer the approprate distance and you should be pretty good.



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    RP Designs,
    It sounds like you are familiar with the 2 stroke cranks . I think that sounds like a good idea , I didn't think about a reference point . This is how you would be able to find the center of the hole you just welded/pluged up to do the offset . Hmmmmm -my gears are turning ......



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    Quote Originally Posted by AirChunk View Post
    RP Designs,
    It sounds like you are familiar with the 2 stroke cranks . I think that sounds like a good idea , I didn't think about a reference point . This is how you would be able to find the center of the hole you just welded/pluged up to do the offset . Hmmmmm -my gears are turning ......
    Thanks !



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    No problem, let me know how it turns out.



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    Just an update . I am still working a a design for a fixture . It turns out , to be able to cover the strokes that I want to increase upon (40mm , 54mm and 72mm ) it is a little more involved than I had initialy planned . But I think it will work out in the end .



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    Why not approach it in a simpler way,To offset the crank throw you could simply bore oversize the original crankpin hole in the flywheels,then you make up an offset bushing that is pressed into the flywheels having the required centre hole to suit the original crankpin.
    This way you could manufacture the required bushing in one piece then part off the two halves guarenteeing both are dimemsionally accurate for the required stroke change.

    Then because I am assuming you are increasing not decreasing the capacity a simple use of either a different conrod length or a simple cylinder spacer plate will work for increased capacity or maching both top and bottom faces of the cylinder for a destroked engine to take into account port timing not becoming an issue.

    Doing it this way will also allow rotation of the offset bush to give whatever variation of stroke you may require to for instance fit into a specific class structure.
    This was something I was involved in many years ago by destroking Kawasaki KX 60 engines to use in 50cc class mini bike racing.

    It is a much simpler process than welding (And distorting Flywheels) and it allows the use of CNC for the bushes to achieve small cost effective runs of modified crankshafts,CNC for making cylinder correcting plates to compensate for the stroke change if increasing capacity etc.

    After all many a worthy engineering project has foundered on cost effectiveness or long term reliability issues,approaching the problem in this fashion allows maximun use of standardised components aiding cost control and downstream servicing costs keeping TCO within the realms of reality and hence making something a viable commercial project if the market is limited



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    I thought I would add to my Original post on this subject, Depending on the design of the Crankshaft halves and in particular the Web designs when modifying some original Cranks to either stroke or Destroke Two Stroke built up Crankshafts the main thing I always try and do with any modification is to modify the part that is non consumable so replacement service parts are standard off the shelf items when possible.

    The ideal way of changing the stroke is by offset boring the crank halves oversize at the original crank pin location by the stroke change you desire and then making a pair of matching bushings that are a very tight press fit into the crank halves and then press fitting the factory crank pin into it's new offset location within the new bushings you have just manufactured and fitted to the crank halves.

    That way the bush itself does not need to be offset bored and fitting is a no brainer as well as manufacturing two identical bushes is simple being both made at the same time and then parted off they will be the same spec.
    However not all crankshaft web designs of the flywheels allow for such a method and sometimes you simply cannot overbore the original crank pin hole the required diameter.
    It also gets a bit trickier when doing the set up as two separate crank halves need to be bored oversize and offset equally not only for the stroke but as well to retain the radial location equal to each other.
    Easy if you have the equipment, If you do not then simply boring the crank halves oversize after clocking off the original crankpin hole and not offsetting the new oversize hole then means you need to place the required offset in the oversize bush itself, That can be easier to manufacture as all boring and outside grinding operations are done before parting off and finishing the sides which are not critical aspects to the finished piece.

    That however will require either a very skilled crankshaft builder or a special Jig to assemble the bushes to make sure the indexing is perfect alignment wise between both halves of the modified crank or it will be almost impossible to achieve ideal runout of less than one tenth of a thou in any plane.



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2 stroke crank stroking ?

2 stroke crank stroking ?