Yamaha YZ250F Big Bore

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11/16/2018 1:49 PM

Currently have an 18' and wondering what impressions the 270cc kit did for this bike? Anybody do this mod and how did you like it versus stock displacement. Thx

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"A rule is a rule and without rules, there is Chaos"....Cosmo Kramer

11/16/2018 2:01 PM

My buddy actually just did this to his 2015 and claims it he loves it. He was back and forth on just getting a new leftover bike or doing the big bore kit but decided to do the big bore. I believe he used an Athena cylinder and also fitted a new Yosh system to go along.

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2017 CRF450r

11/16/2018 6:28 PM

I had a big bore 290 Athena kit yz. It was in a 2004 chassis . Crazy bottom that signed off on top. I used stock head and cams, and leaned out the jetting. It made me lazy, I short shifted and put one tooth smaller on rear sprocket.

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11/16/2018 6:42 PM

I had a 290 and just like the other guy said good bottom end but nothing up top. Then i did a 269 cylinder and a +3mm long rod that made it to be a 285cc. That was a really good setup. In all reality though if i could take it back I would had not done either bike and just got a 450 earlier in life. Tons of money wasted on mods that made the bike unreliable. Both bikes blew up with about 15-20 hours on them. If you need reliable faster bike get a 250 smoker or a 450.

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11/16/2018 7:18 PM

Buddy of mine did it to his 18 yz250f, loved it. He's waiting patiently for a big bore to be developed for his 2019 250f.

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11/16/2018 8:53 PM
Edited Date/Time: 11/16/2018 8:54 PM

Vet57 wrote:

Currently have an 18' and wondering what impressions the 270cc kit did for this bike? Anybody do this mod and how did you like it versus stock displacement. Thx

Ive built a few

Personally suggest sticking around 270 -
With proper mapping its quite a good improvement for cheap

with quality port work, and cams its a monster

Back in 2014–5 we did one that is on a thread somewhere on thumpertalk that details the power gains back then

Fast forward to current and its even stronger. Power wise a well built one will out drag race most 450s.

I have quite a few to date - only one that failed was the first one on the junk 201-15 rod. Now they can be very reliable.

If you love the 250 f and want more go for it
If you have a 250f and want a 450 feel dont..

Just buy the 450

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11/16/2018 10:47 PM

I had a 2007 CRF 250 With in Athena 280 big bore kit on it and to top it off my buddy worked at white brothers during the time they closed up shop and gave me one of the Tom Morgan race team spec heads from some Honda team ported with xelydine valve adjustable Cam timing etc etc it was badd ass when I made it that big bore kit with that spec head and a little boy carbon fiber white brothers pipe, And replace the Cam that was in the head with a stage to hot Cam yes I know blasphemy but that bike absolutely ripped it ran every bit as strong as a 400 but was this light as a 250! And had power everywhere from bottom to top the only downside was it had more engine braking then it did stock but what an incredible machine especially with the later model carbureted Honda 250 4 strokes had that extra large carb it worked excellent with this big bore kit.

Plus it had be kit RG3 suspension on it which was absolutely incredible the only reason I ended up selling it is because every once in a while it's still suffered from the infamous Honda bog and cut out on me at the face of a big gapped peak to peak double at Paris and almost killed me it was at that time a year later after I healed that I discovered my love for 2 strokes again because I wasn't having fun on any of the 4 strokes I had be it the big 450 or the 250 And any size in between.

We already know the Yamaha 250's have absurd power in their stock configuration a big bore kit in that bike would be absolutely incredible and perfect for someone who doesn't want a 450 but needs more than a 250 delivers you just have to build it right and make sure you put top and focused cams in the head so that the power doesn't stay down low this way it will spread the power across the entire rpm range and work incredibly well.

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11/16/2018 10:51 PM

Never really understood this. Your going to spend a lot of money, spend time, and loose some reliability. Not to mentioned if not properly set up will be just as slow, if not slower, that a stock 250F. Just buy a 350 and be done.
I would much prefer to be riding my bike, getting better on it, that constantly working on it. In my opinion, if you are looking for more bottom on a 250F you need to work on your corner speed. Spend money on lessons and practice, practice, practice corners until they become natural.

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11/16/2018 11:55 PM

Derek Harris wrote:

Ive built a few

Personally suggest sticking around 270 -
With proper mapping its quite a good improvement for cheap

with quality port work, and cams its a monster

Back in 2014–5 we did one that is on a thread somewhere on thumpertalk that details the power gains back then

Fast forward to current and its even stronger. Power wise a well built one will out drag race most 450s.

I have quite a few to date - only one that failed was the first one on the junk 201-15 rod. Now they can be very reliable.

If you love the 250 f and want more go for it
If you have a 250f and want a 450 feel dont..

Just buy the 450

How does a full built Yamaha 270 compare to a stock KTM 350 on the dyno?

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11/17/2018 3:56 AM

Talisker wrote:

Never really understood this. Your going to spend a lot of money, spend time, and loose some reliability. Not to mentioned if not properly set up will be just as slow, if not slower, that a stock 250F. Just buy a 350 and be done.
I would much prefer to be riding my bike, getting better on it, that constantly working on it. In my opinion, if you are looking for more bottom on a 250F you need to work on your corner speed. Spend money on lessons and practice, practice, practice corners until they become natural.

I've had the 350..matter of fact I've had pretty much all the bikes including all the 2 Strokes. I can ride the 250F as well if not better than the rest. Not looking for bottom, corner speed, etc as I can throw it down..lol. I am just curious about the characteristics with the 270 compared to stock. I've never ridden a BB 250F but I do have a friend who did this to his KTM and liked it. I love the YZ as it as, just wondering if the kit is worth the $$ for what it produces. More curious than need. Thx

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"A rule is a rule and without rules, there is Chaos"....Cosmo Kramer

11/17/2018 5:46 AM

Vet57 wrote:

Currently have an 18' and wondering what impressions the 270cc kit did for this bike? Anybody do this mod and how did you like it versus stock displacement. Thx

Derek Harris wrote:

Ive built a few

Personally suggest sticking around 270 -
With proper mapping its quite a good improvement for cheap

with quality port work, and cams its a monster

Back in 2014–5 we did one that is on a thread somewhere on thumpertalk that details the power gains back then

Fast forward to current and its even stronger. Power wise a well built one will out drag race most 450s.

I have quite a few to date - only one that failed was the first one on the junk 201-15 rod. Now they can be very reliable.

If you love the 250 f and want more go for it
If you have a 250f and want a 450 feel dont..

Just buy the 450

Alex814 wrote:

How does a full built Yamaha 270 compare to a stock KTM 350 on the dyno?

More power than early generation 350s

Less power than current 350.
My mild mofied 250fs are now more hp than all japenese 250 2 strokes were stock
A good 270 is around the power of the early honda 450s and yz400

If you want a 350 power - buy a 350. Its one of the best built stock engines per cc on the planet minus the 250cc ktm

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11/17/2018 6:02 AM

I've got an athena 270 kit on my 18 and really like it. I ride gncc's and hare scrambles with it and it works great.
gives it more low and mid-range grunt. I went down a tooth on the rear sprocket with this so I can pull 3rd gear longer with it. Pretty damn easy to install too.

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11/17/2018 9:14 AM

I did the 272 RPM Ice Cube kit. I loved it. It gave me more torque everywhere. It allowed me to do jump sections a gear higher and thus helped set the suspension at the jumps better. It rode easier than my 450 and there just was not a wrong gear to be in almost anywhere due to the extra grunt. I did not need to rev it as much as before and thus the valve life was better.

Paw Paw

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11/17/2018 1:46 PM

Also are most of you just dropping in the kit (piston/cyl) stand alone or are you adding cams as well? Not interested in doing head mods either. Keep in mind if I go this route I also want to maintain reliability as well.

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"A rule is a rule and without rules, there is Chaos"....Cosmo Kramer

11/17/2018 3:20 PM

I love the 270 kit. Harris did the mapping on my KTM 270 kit (and he made the kit). I did no other mods, great engine. It retains all of the good 250F traits and adds a lot of rideability. I've seen no difference in reliability either. If you can go 270 (not constrained due to racing/class issues), it is the best mod as far as bang for the buck.

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Motocross....a once in a lifetime experience....every weekend.

11/19/2018 10:27 AM

I did the cylinder works 270kit to my '18 KTM sxf. I like it a lot, I've put 10hrs on it with no issues at all.

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11/19/2018 11:41 AM

I've got an RPM Ice Cube 276 Cylinder to put on my 09 YZ 250f, but I can't seem to get enough time to wear out the Vertex piston I put in it when I got the bike. One day I will pull the trigger.

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11/19/2018 12:00 PM

Talisker wrote:

Never really understood this. Your going to spend a lot of money, spend time, and loose some reliability. Not to mentioned if not properly set up will be just as slow, if not slower, that a stock 250F. Just buy a 350 and be done.
I would much prefer to be riding my bike, getting better on it, that constantly working on it. In my opinion, if you are looking for more bottom on a 250F you need to work on your corner speed. Spend money on lessons and practice, practice, practice corners until they become natural.

Vet57 wrote:

I've had the 350..matter of fact I've had pretty much all the bikes including all the 2 Strokes. I can ride the 250F as well if not better than the rest. Not looking for bottom, corner speed, etc as I can throw it down..lol. I am just curious about the characteristics with the 270 compared to stock. I've never ridden a BB 250F but I do have a friend who did this to his KTM and liked it. I love the YZ as it as, just wondering if the kit is worth the $$ for what it produces. More curious than need. Thx

Posts like that one always amuse. If you don't understand them why not just STFU and not post?

I just put a Cylinder Works kit on a '17 YZ250f for one of my son's buddies. He is a decent rider but not blazing fast. I would've preferred to have maybe had some minor head work done for better flow etc to go along but he is a kid and had his mind made up so I did the work to make sure all done right.
He really likes it. Says noticeable difference on bottom and didn't seem to lose much top but then again he isn't really a revver. He raced over weekend and was pulling better starts for sure. Bike has a slip-on but stock otherwise.......

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11/19/2018 12:07 PM

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11/19/2018 5:31 PM

Vet57 wrote:

Also are most of you just dropping in the kit (piston/cyl) stand alone or are you adding cams as well? Not interested in doing head mods either. Keep in mind if I go this route I also want to maintain reliability as well.

From a reliability stand point - good head work more reliable than any cam set that delivers more power..OEM level testing on cams for durability is far more extensive than any aftermarket testing (practically zero durability testing vs an OEM)

I haven't seen a big bore any ANY current 250f loose top either - often the power is just so improved down low it makes the top feel less good relatively speaking.

Properly mapped most big bores make about the same percent gain bottom to top. Just flat out stronger...

I'd say the worst stocker is the KXF for a big bore - it's port isn't the best.
But the yzf, ktm, crf, and even rmz respond well to more CC

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5/9/2019 6:56 PM

Any info on when a big bore kit will be available for the '19 YZ250f? I've called Cylinder Works and there doesn't seem to be any definitive date. Other companies?

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8/19/2019 5:29 PM

sparker245 wrote:

Any info on when a big bore kit will be available for the '19 YZ250f? I've called Cylinder Works and there doesn't seem to be any definitive date. Other companies?

Bump.. any info on the ‘19 250F big bore 270 kit??

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8/20/2019 9:59 PM

I've been keeping in contact with Cylinder Works about the '19 YZ250F Big Bore Kit, and the last e-mail I received a week ago said they're hoping to have them available in October. Sure hope so, I loved the 270 kit I had on my last YZ250F.

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8/21/2019 4:53 AM

I think I might just have to go this route with my 19 when they become available.

Love my 19 YZ250F, a 450 these days is a little to much, think a BB kit could just make this great bike just a little better again.

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8/21/2019 6:12 AM

I have a Derek Harris built KTM Sxf 270 with head work, RnD throttle body and Vortex. Hands down best bike I have ever owned. Peak HP is just shy of 53 on pump gas. Have about 40 hours on it and getting ready to do a top end, so I see no additional service requirements versus a normal 250f.

You guys should definitely do it, the 270 imo is the perfect machine. The only place where I have wanted more power was trying to climb some 300 meter tall dunes.

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8/21/2019 6:53 AM

Have you checked with Max-RPMs or Millennium? If they don't have a "kit" you can send your cylinder in to get bored out and replated and just source out a piston or have customs made. I'm thinking about going back from a 450 to a 315cc bored and stroked 250 myself.

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recmx.com

REC MX, JE Pistons, Wiseco, Rusk Racing, Yoshimura, Enzo Racing, Dubya Wheels, Asterisk, Rocket Performance, CCR Sport, & MMR

9/9/2019 12:41 PM

Cylinder works is now listing big bore gasket kit for 19 yz250f. Must be getting close to production of the kit.

http://www.cylinder-works.com/ProductInfo.aspx?item_id=11980

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9/9/2019 3:13 PM

I build 2 CRF250's (an 05 and 08) with a IceCube 270 kit and ron hamp head. They worked very well... Carb setup was key. The new FI models with the correct remaping and must be quite good.

Currently I'm on a 16 KTM 350 XCF and the 350 lugs a bit better but there was nothing wrong with the BB 250F power wise. Heck, I'd be fun to have KTM 250XCF with a BB kit on it setup right. That would be a fun bike!

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9/11/2019 1:47 PM

Talisker wrote:

Never really understood this. Your going to spend a lot of money, spend time, and loose some reliability. Not to mentioned if not properly set up will be just as slow, if not slower, that a stock 250F. Just buy a 350 and be done.
I would much prefer to be riding my bike, getting better on it, that constantly working on it. In my opinion, if you are looking for more bottom on a 250F you need to work on your corner speed. Spend money on lessons and practice, practice, practice corners until they become natural.

What you are forgetting is that the rotating mass and gyroscopic behavior of that 270 is going to be a lot less. That translates into a lighter feel. Flicking the bike into and out of corners. Like a 125 that can smoke a 250 or 500 back in the day (think O'mara at des nations).

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Ironman Texas, May 17th, 2014, I became an IRONMAN.

9/11/2019 2:27 PM

Heard too many horror stories. 350 is just so much easier.

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