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jstout

05-11 YZ 250 Stroker and Big Bore

49 posts in this topic

I know there are a few big bore kits out there but I'm curious if there is anyone out there running a BB and stroker. If so, what kind of displacement are we looking at? Think about it, a 2 stroke with a couple extra pounds but comparable power to a 450.

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I know about the BB kits. Let me rephrase the question. Does anyone make a stroker rod or kit?

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There is no room to stroke a newer 250. Trust me id have it done already if you could. I have just about every mod done to my 295 race engine you can do.

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I think you could have a big spacer at the bottom of your cylinder thus creating some space but what would be the benefit of a longer stroke?

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I think you could have a big spacer at the bottom of your cylinder thus creating some space but what would be the benefit of a longer stroke?

???????

Contact Harris Performance for stroker infomation. I believe the 340 kit he is working on is both a big bore and a stroker.

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I know about the BB kits. Let me rephrase the question. Does anyone make a stroker rod or kit?

Just to clarify, a different rod does nothing for stroke, you need a new crank, or an offset crank pin, or the crank cheeks welded and a new hole machined for the lower rod pin further off center.

I run a long rod, displacement is 100% stock.

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There is no room to stroke a newer 250. Trust me id have it done already if you could. I have just about every mod done to my 295 race engine you can do.

You mean there is no room w/o trenching & welding the cases.

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dunno about bikes but a big bored motor, and a stroked motor are the same thing, eg 327 chev stroked to 355 would require rods, pistons, machining of bores, crankshaft, etc, to "bore out" a motor is just doing the same thing?

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dunno about bikes but a big bored motor, and a stroked motor are the same thing, eg 327 chev stroked to 355 would require rods, pistons, machining of bores, crankshaft, etc, to "bore out" a motor is just doing the same thing?

A big bore is machining out the cylinder for a larger diameter piston.

A stroked motor is a new crank that gives a longer stroke, or up & down movement of the piston.

Either can be done on their own and give more displacement. Or combine them for even greater displacement increase.

Examples-

Stock YZ250 has a 66.4mm bore and 72 mm stroke - 249 cc

Going to a 72 mm big bore - 293 cc

Move crankpin 2mm for a 4mm stroke increase, with stoke bore - 263 cc

Combine that big bore and stroker - 309 cc

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the power characteristics would be slightly different though wouldnt they: big bore v stroker?

At least in my head its makes sense of how the power differences would work between the two.

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Problem with increasing the stroke is that the piston now moves up farther in the bore. Using Adam728 example, 4mm stroke increase means that the piston will come up 2mm out of the bore. There are four ways to deal with this issue:

1) machine the cylinder head to allow room for the piston (poor choice)

2) replace the rod with one that is 2mm shorter. (poor choice - rod angularity)

3) find a piston with a 2mm shorter "pin height" (could be an expensive custom piston)

4) raise the cylinder with a 2mm spacer (as Arnego said, the most practical choice)

The ideal setup would be to go with a 2mm longer rod and a 4mm spacer under the cylinder. This would give you roughly OEM rod/stroke ratio. As you decrease the rod/stroke ratio, you increase the side loading (skirt load) on the piston and cylinder at a given RPM. You also increase the G-Forces on the connecting rod, piston, and wrist pin at a given RPM. Basically you NEED a longer rod if you want your top end to survive. Also, raising up the cylinder by 4mm would give you approximately the same secondary compression ratio which is important on a two stroke.

Adam728, do I recall correctly that you have a KTM rod assembly in your YZ? I know you've posted the details before, but can you jog my memory and elaborate on that setup? Do you have a cylinder spacer or a different piston? It seems like I remember the rod being 3mm longer or something like that?

Of course with any stroker and/or big bore you need to re do the cylinder porting. It seems that you could keep roughly the same port durations and timing and just use a degree wheel to apply them to the new setup.

It certainly seems like it can be done... I've thought about experimenting with something like this myself. I've decided to try it with a YZ125 first since parts (cranks, pistons, cylinders, heads) are a little less expensive. Hence I'm in the market for a 2006+ YZ125 right now.

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Im not an expert but ive been told by gorr it wouldnt be possible with a stock case. The pins as far out as it can be on a stock crank. It would take a major revamp of the cases and crank. Like posted above you would have to build a cyliner to follow. My 295 make more then enoufgh power for me and im already facing reliabilty issues so id never build a stroker.

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Im not an expert but ive been told by gorr it wouldnt be possible with a stock case. The pins as far out as it can be on a stock crank. It would take a major revamp of the cases and crank.

I agree as far as the stock rod goes.

The only real option is to weld up the holes in the crank halves and go to a smaller crank pin and a smaller diameter rod & bearing to clear the cases. It would be extremely labor intensive to do the stroker work on the crank, not to mention you would have to find an off the shelf rod (from another application) with the right dimensions to make it anywhere near affordable. Furthermore, the beam on the rod may still hit the top of the cases, requiring you to machine material out of the top-center of each case half.

The amount of labor involved pretty much makes it un-profitable for a shop to do this kind of work. That's why MAX RPM charges so much for their YZ167 stroker builds. You aren't getting $2000 worth of parts, it's the labor and know-how involved to make it all work out and last.

In my mind, this is the kind of project for a guy who owns a machine/fabrication shop, has plenty of time, knows a lot about two stroke engine geometry and porting and enjoys a unique project just for the sake of doing it. For the money, you're better off getting a KTM 380, CR500R, or KX500 engine and doing a frame swap. Those engines have proven reliability and plenty of power.

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Nevertheless it is a nice thread, it clears a lot of questions people could get when going bigger on bore.

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