2 stroke crank stroking ?

[AirChunk]

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 ?

[stevehuckss396]

Originally Posted by AirChunk 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.

[AirChunk]

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 ????

[AirChunk]

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

[RICHARD ZASTROW]

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

[AirChunk]

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.

[RICHARD ZASTROW]

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

[AirChunk]

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 ???

[RICHARD ZASTROW]

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

[dynosor]

Originally Posted by RICHARD ZASTROW 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...

[AirChunk]

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 .

[Kevin Taylor]

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