I just received an extra 250 motor for my Polaris Trailblazer. I am looking at building a 7mm stroker motor out of it and boring it out .40. I am fairly mechanically inclined. I am an Aircraft A&P Mechanic and have built several engines. Who sells stroker cranks and rods? I've go a flow bench at work and a good bud who knows how to port two-strokes. I'll bore the jug myself.

 

az bad boyz does strokers so they probably have a crank. post this in the polaris forum to get better results.

 

flow bench aint going to do jack for you on a 2-stroke. Too many variables that make power other then what will be shown on the bench. Examples would be transfer rooves, exhaust shape/width, intake height, ect. Just because it flows air doesnt mean it'll make the power.

Why a 7mm? What rod length? If you have your mind set on that combination crankworks will build you a crank. Probably the best crank on the market for building whatever a normal joe wants.

Why are you boring it 40 thou? You loose transfer area and that is a bad thing.

 

BeechMan - Thanks for pointing me to Crank Works. I have only done minor flow testing on 4-strokes. I see what you are saying about too many variables with a 2 stroke. My buddy who does two-stroke porting said that with boring it out 40 thou he was going to raise and widen the transfer ports. He was also going to widne the exhaust ports as well. He is mapping the original ports this week and then mapping them again after the bore job. He has done some good work on some of the other guys bikes and atv's in the shop. Plus he is doing it as a favor, so it is FREE!!!!

The extra 7mm stroke will give me the needed torque I am looking for in this 250. I don't think 4mm will quite do it, and I think 12mm is too much for reliability.

 

OK I have a question here. Will the crank assembly off a 300 xpress fit the 250 case. I see the bearing's for the 250 and 300 crank assembly are the same. I noticed the Rods are the same as well. With the 300 crank it will give me 5mm's more stroke. I could machine an extra 5mm spacer between the case and jug. That would give me 264cc's with std bore, and later down the road I could put a 300 jug on it.

 

depends on how wide the crank halves are and what their diameter is.

On the 7mm thing. Boring the cylinder isnt going to help him widen his transfers or raise them one bit. If anything he is going to raise them so high he'll go into water. Now that would be funny! Anyways. If it were me i would go a 5mm. Why? Because the sideload a 7mm will put on a dinky piston may be more then it can handle. Ive never been in one of those polaris motors, but your friend doubtfully knows where water is on the machine and he just sounds shady to me. You can set all your port heights and widths with a stoke bore, anything you can do with a stocker you can do with a stroker. Hope some of this helps.

 

call up dennis at az bad boyz im sure he would help. he does 12mm strokers on the trailblazer and his popular one is a 7mm i believe