05-11 YZ 250 Stroker and Big Bore


48 replies to this topic
  • jstout

Posted August 13, 2011 - 07:34 AM

#1

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.

  • arnego2

Posted August 13, 2011 - 09:07 AM

#2

read through this, happy reading.

  • jstout

Posted August 13, 2011 - 02:26 PM

#3

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

  • BRM

Posted August 13, 2011 - 04:38 PM

#4

I have not found any

  • arnego2

Posted August 14, 2011 - 02:52 AM

#5

For what reason you want or need a longer stroke?

  • s14 2510s

Posted August 14, 2011 - 03:47 AM

#6

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.

  • arnego2

Posted August 14, 2011 - 04:34 AM

#7

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?

  • Budlite

Posted August 14, 2011 - 05:27 AM

#8

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.

  • arnego2

Posted August 14, 2011 - 05:43 AM

#9

Thanks,
At the moment I'm happy to get by with 249cc maybe in the future.

  • adam728

Posted August 14, 2011 - 09:03 AM

#10

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.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • CRFThumper

Posted August 14, 2011 - 10:48 AM

#11

maxx power.com

  • jar944

Posted August 14, 2011 - 07:39 PM

#12

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.

  • yzed250

Posted August 14, 2011 - 09:05 PM

#13

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?

  • adam728

Posted August 14, 2011 - 09:23 PM

#14

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

  • tim512

Posted August 15, 2011 - 12:22 AM

#15

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.

  • cwtoyota

Posted August 15, 2011 - 06:00 AM

#16

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.

  • s14 2510s

Posted August 15, 2011 - 07:44 AM

#17

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.

  • cwtoyota

Posted August 15, 2011 - 08:17 AM

#18

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.

  • arnego2

Posted August 15, 2011 - 08:22 AM

#19

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

  • yzed250

Posted August 15, 2011 - 08:48 AM

#20

well i just learnt some new stuff, good to read this type of thing!





Related Content

Forums
Photo

different year plastics by YJB006


Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 2 replies
Forums
Photo

1995 yz125 by sven4130


Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 7 replies
Forums
Photo

2015 YZ125 "intro" by Racer X by YamaLink


Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 18 replies
Reviews

Yamaha YZ85 2015 by Bryan Bosch


Yamaha YZ85 2015
  • 0 reviews
Forums
Photo

Everything for a beginner! by danieljames


Dirt Bike   General Dirt Bike Forums   What bike should I buy?
  • 6 replies
 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.