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Another Big Bore Question

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I see lots of threads about big bores but a lot of them are old.  I think I'm set on a Big bore kit and a Stage 3 hot cam for my 2007 250x but which kit should I get?  The 270 or the 280?  It sounds like the 270 is more common but why?  Does it rev better?  Sounds like the 280 could be a little hard starting and slower reving.  I race harescrambles BTW and need more wide open power but want to maintain or increase bottom end/lugging power. 

 

Also, the 280 kit I see on ebay for high $400s is a wiseco piston but a magnum cylinder - which I never heard of and have a hard time finding info on.  Anyone know?  Are they good or should I go with the more pricey Athena?

Edited by maxpower_454

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I recently fitted the 280 athena .Its a good bit of kit and fitted very easily ! I have been steadily going through the mods as and when either the bike needed parts or when items came up for sale at a good price .The 280 was the last thing i have done previous is the cam to an R vortex ignition ,jetting c.c.c mods ,and probably some other bits .the bike is a little better .With the big bore kit specifically no issues starting easy enough i would say i have lost a little low end snap only noticeable on the slower /crawling sections but easy enough to get used to . The extra power works if your able to keep it in the right area its a little deceptive as it goes really well using lower revs but feels slower but its not ..I am still using most of the rev range though i hoped to ride it a little more with less revs but it still likes being revved.. The gains are minimal but noticeable i have said elses where on this forum i would not bother if i had to do it again but i would rather keep the crf as a trainning bike and keep a newer up to date bike for racing .Thats not to say the crf aint fun i am still grinning when i ride it but it has its limitations just like me ...

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I'm Just going to give you some food for thought (not lashing out). If you race harescrambles, going to a 270 or 280 will bump you up into the open class with the 450 and 300 2ts. However if you go through twigsters race prep thread, the end really, he talks about the 280 kit. Most recent one I've seen for our bikes

 

Edit - Guess I took to long to write this out here at work. 

Edited by hudsen8

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I'm a vet so I don't have to worry about CC's. 

 

Thanks for the feedback twigster.  Anyone run the magnum kit?  Anyone compare a 270 to a 280cc?

 

I read a dirt rider magazine article regarding the cylinder works kit and they really liked it.  Said it rev'ed to the moon and didn't have a "chuggy" feel to it like an xr400 does and supposedly some of the other big bore kits they tested do.     

Edited by maxpower_454

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My only reason to BB is for the chug factor. If I could get the exact type power of my XR4 I would be deliriously happy. The smaller chassis, modern susp and better overall handling with E-start would be perfect for me.

Edited by YHGEORGE

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The 280cc Magnum kit has an aluminuim sleeve vs the Athena with a steel sleeve. The Magnum sleeve is weakest where the skirt section is, has to be thin to be a drop in on the older CRF's. Most of the piston thrust is above the skirt. I have one in my bike now and will measure it for wear after I get about 70 hrs on it. You will notice a healthy increase in low end torque. The CW 269cc BB kit is nice, but requires you to open up the case on older CRF's for the thicker skirt, it uses a Vertex piston that is lighter than the Wiesco piston, smaller piston skirt.

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The 280cc Magnum kit has an aluminuim sleeve vs the Athena with a steel sleeve. The Magnum sleeve is weakest where the skirt section is, has to be thin to be a drop in on the older CRF's. Most of the piston thrust is above the skirt. I have one in my bike now and will measure it for wear after I get about 70 hrs on it. You will notice a healthy increase in low end torque. The CW 269cc BB kit is nice, but requires you to open up the case on older CRF's for the thicker skirt, it uses a Vertex piston that is lighter than the Wiesco piston, smaller piston skirt.

 

Thanks for the great info 418.  Just to be clear, you have a Magnum kit in your crf 250?  I don't have a problem doing the simple modification for the CW kit and they sure have a great track record and a nice Italian piston but I sure would like an extra 10cc while I'm at it!  I don't want a slow reving engine either though.  An update eon how yours is wearing at 70hrs would be great.  I just worry about the durability of the magnum since it's a lesser known brand that I can't find much info on. 

 

George, I see what you're saying but I don't think the ability to lug the engine is what they were referring to.  I think they meant a slower rev, higher vibration engine. 

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Thanks for the great info 418.  Just to be clear, you have a Magnum kit in your crf 250?  I don't have a problem doing the simple modification for the CW kit and they sure have a great track record and a nice Italian piston but I sure would like an extra 10cc while I'm at it!  I don't want a slow reving engine either though.  An update eon how yours is wearing at 70hrs would be great.  I just worry about the durability of the magnum since it's a lesser known brand that I can't find much info on. 

 

George, I see what you're saying but I don't think the ability to lug the engine is what they were referring to.  I think they meant a slower rev, higher vibration engine.

I currently have the Magnum kit in my bike, before I had the CW kit, the piston and bore looked great and checked out good when measured at 71 hrs. When I put the CW kit in I put in a 1.5 mm longer stroke crank. I ride trails and race hare scrambles, I never hit the rev limiter. With the increased power I had to drop 2 teeth off the rear sprocket.

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I currently have the Magnum kit in my bike, before I had the CW kit, the piston and bore looked great and checked out good when measured at 71 hrs. When I put the CW kit in I put in a 1.5 mm longer stroke crank. I ride trails and race hare scrambles, I never hit the rev limiter. With the increased power I had to drop 2 teeth off the rear sprocket.

 

Sounds like you have the exact experience I was looking for.  I do the same type of riding.  Is the engine slower reving or worse in any way with the magnum kit vs the CW kit?  I'll be really interested to see how the magnum cylinder holds up.  I think the CW kit is aluminum nickel plated just like the magnum. 

 

Did you change cams with the big bore?  I was thinking a stage 2 or 3 hot cam

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The CW kit revs faster, not enough to matter for me. I also have a MRD exhaust, tried a Hot Cams GS stage 2, but it did not have enough clearance from the intake spring retainer to oil seal during mock up. My head will need shorter intake valve guides, will replace them on next piston refresh, and install GS cam.

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The CW kit revs faster, not enough to matter for me. I also have a MRD exhaust, tried a Hot Cams GS stage 2, but it did not have enough clearance from the intake spring retainer to oil seal during mock up. My head will need shorter intake valve guides, will replace them on next piston refresh, and install GS cam.

 

Does the 280 have noticeably more power than the 270?  So you're running a stock 250x cam?

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Does the 280 have noticeably more power than the 270?  So you're running a stock 250x cam?

The difference is about 7cc, the 280 is 276.7cc if my memory is correct, power wise they felt about the same. I checked the compression on each kit and the CW was about 1/4 point higher as assembled, neither kit delivered the advertised compression ratio, both were lower than advertised. To get the advertised ratio, the piston would need a zero deck height. My bike is an 07 R with the stock cam.

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Interesting. I figured the difference would be noticeable. It's still another 7cc and that's the big thing swinging me toward the magnum kit.

Why did you take cw kit out?

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Interesting. I figured the difference would be noticeable. It's still another 7cc and that's the big thing swinging me toward the magnum kit.

Why did you take cw kit out?

Was time for a piston change, so I changed it out and I wanted to see if the 1mm larger piston made a difference. The CW kits extra compression and lighter piston helps it out. I have 18 hrs on the Magnum BB and have no issues. Next piston change I will order a thinner base gasket to up the compression some.

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I run a 270 Cylinder Works big bore with a stage 2 hot cam, opened the air box, increased the jetting (150 main jet etc), new FMF exhaust with Powerbomb header pipe. Bike works like a well (like a beast). I think larger than that will be too much complications for cooling, jetting, exhaust etc. Plus you may get oil spilling from the transmission breather hose.

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I run a 270 Cylinder Works big bore with a stage 2 hot cam, opened the air box, increased the jetting (150 main jet etc), new FMF exhaust with Powerbomb header pipe. Bike works like a well (like a beast). I think larger than that will be too much complications for cooling, jetting, exhaust etc. Plus you may get oil spilling from the transmission breather hose.

My 284cc set up has none of the negative issues you described and I ride woods to include single track.

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I run a 270 Cylinder Works big bore with a stage 2 hot cam, opened the air box, increased the jetting (150 main jet etc), new FMF exhaust with Powerbomb header pipe. Bike works like a well (like a beast). I think larger than that will be too much complications for cooling, jetting, exhaust etc. Plus you may get oil spilling from the transmission breather hose.

 

What cam did you have in it before the stage 2 hot cam?  Also, is it the stage 2 hot cam for 04-07 or the 08-09 version?  I picked up a 08 cam at the recommendation of crfsonly and planned on running that with the big bore but I was thinking about the stage 2 as well.  They recommended going with the 08-09 version of the stage 2 hot cam with 07R springs. 

 

My 284cc set up has none of the negative issues you described and I ride woods to include single track.

 

Just curious, is your bike bored and stroked?  I think the Athena kit is actually 275.7cc and the magnum should be the same bore, right?  I think the CW kit is 269cc. 

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What cam did you have in it before the stage 2 hot cam?  Also, is it the stage 2 hot cam for 04-07 or the 08-09 version?  I picked up a 08 cam at the recommendation of crfsonly and planned on running that with the big bore but I was thinking about the stage 2 as well.  They recommended going with the 08-09 version of the stage 2 hot cam with 07R springs. 

 

 

Just curious, is your bike bored and stroked?  I think the Athena kit is actually 275.7cc and the magnum should be the same bore, right?  I think the CW kit is 269cc.

My engine combo is 82mm piston and +1.5mm crank.

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