Reliability of 250F Big bores

S.Loyer
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S.Loyer
Edited Date/Time 5/18/2021 6:09am
I'm a heavier dude, working on losing the weight but thats irrelevant. Im not a fan of 450's i hate riding them. I tried the 350 and all though better, it still wasnt my taste. I really love riding 250fs they are just the perfect bike

I have a 2020 KTM250F and it goes fast i think its perfect but being heavier and riding at a track with a lot of uphill corners kills me. So I have been thinking of trying out a big bore.

Looking for how this will change the reliability of the bike, what to look for, what kit to use, race gas, etc.

My bike just hit 25 hours.
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Paw Paw 271
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Paw Paw 271 5/7/2021 3:25pm
5/7/2021 3:25pm Edited Date/Time 5/7/2021 3:27pm
The good big bore kits do great things on the 250f bikes. On my 250f I installed a 272 kit and loved the out come of this. The bike had much more torque and pulled great all the way to the top rpms. I found that i di not have to rev it as much to get the best out of the bike. I found that the added low end power used with woops or jumps right out of turns to be areas of the track where the bike did it's best as well as starting line pull to the first turn. The bike was very reliable. Now there are good kits out there and then there are cheap units out there. This is another case of getting what you pay for. I have also ridden the same bike with a stroked and big bore kit in it that yielded 315 cc and to tell you the truth the 272 was much easier to ride for me. The power of the 315 kit was great but it did loose some of the top end rpms so it limited the effective riding rpm range of the engine.

Paw Paw
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Rider 5280
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Rider 5280 5/7/2021 8:52pm
5/7/2021 8:52pm
Paw Paw 271 wrote:
The good big bore kits do great things on the 250f bikes. On my 250f I installed a 272 kit and loved the out come of...
The good big bore kits do great things on the 250f bikes. On my 250f I installed a 272 kit and loved the out come of this. The bike had much more torque and pulled great all the way to the top rpms. I found that i di not have to rev it as much to get the best out of the bike. I found that the added low end power used with woops or jumps right out of turns to be areas of the track where the bike did it's best as well as starting line pull to the first turn. The bike was very reliable. Now there are good kits out there and then there are cheap units out there. This is another case of getting what you pay for. I have also ridden the same bike with a stroked and big bore kit in it that yielded 315 cc and to tell you the truth the 272 was much easier to ride for me. The power of the 315 kit was great but it did loose some of the top end rpms so it limited the effective riding rpm range of the engine.

Paw Paw
Which big-bore kits do you consider "good" and which ones wouldn't you run yourself?
harescrambled
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harescrambled 5/8/2021 8:57am
5/8/2021 8:57am
In my old 250f I ran a Athena 290 kit. Reliability was excellent, installation was easy, with the fit and finish being excellent. They're a bit pricier than other brands, but worth it in my opinion. Didn't have any issues with rod or main bearings either. You do have to pay attention to the kit manufacturers maintenance intervals though. Closer attention than if it was stock
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Paw Paw 271
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Paw Paw 271 5/8/2021 11:44am
5/8/2021 11:44am
To me RPM big bore kits are the best. They cost a bit more, but the results and dependability are worth the extra dollars and you can get spare parts as needed. This is an all new cylinder with bigger water jackets.

Paw Paw
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stone881
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stone881 5/8/2021 4:28pm
5/8/2021 4:28pm
S.Loyer wrote:
I'm a heavier dude, working on losing the weight but thats irrelevant. Im not a fan of 450's i hate riding them. I tried the 350...
I'm a heavier dude, working on losing the weight but thats irrelevant. Im not a fan of 450's i hate riding them. I tried the 350 and all though better, it still wasnt my taste. I really love riding 250fs they are just the perfect bike

I have a 2020 KTM250F and it goes fast i think its perfect but being heavier and riding at a track with a lot of uphill corners kills me. So I have been thinking of trying out a big bore.

Looking for how this will change the reliability of the bike, what to look for, what kit to use, race gas, etc.

My bike just hit 25 hours.
I have a 2017 250 Sxf bb (270), that now has about 75 hours on it. Put a new top end in at 55 hours, didn't need to, but that's what I am used to doing. So far no issues with reliability and love the power. Keep in mind that mine also has head work done, a Vortex and R and D throttle body.

Power wise it feels just like stock except more power. Having also owned a 350, I like this bike more mostly due to the handling vs. the 350. It doesn't make the same torque as the 350 of course, but pulls and pull and pulls. In short, go for it! Derek Harris built mine, but I guess maybe he isn't the best regarding response to client, or so others have said.



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S.Loyer
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S.Loyer 5/16/2021 3:30am
5/16/2021 3:30am
You guys have sold me, Can I get some links to your kits you use?
Im at 25 hours now, I think when I hit 50 im gonna go through the bike and id like to throw it on, should be mid-late june
Rider 5280
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Rider 5280 5/16/2021 11:33am
5/16/2021 11:33am Edited Date/Time 5/16/2021 12:02pm
Paw Paw 271 wrote:
https://maxrpms.net/

Paw Paw
What do you recommend for top-end rebuild intervals? Every 50 hours? 75?

Additionally, did you get the head ported to better match the larger bore (= larger fuel/air flow rate)?

Cam-wise, did you stick with stock cams or modify/replace those too?

Lastly, fuel mapping - what approach do you recommend here? Get a Vortex or GET? Stock OK? Did you need to get time on he dyno to tune things properly so they run right?

Sorry for all the questions, just generally wondering what is all involved in getting a bog-bore 250F to run right ... as they do sound amazing!
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stone881
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stone881 5/16/2021 12:25pm
5/16/2021 12:25pm
Paw Paw 271 wrote:
https://maxrpms.net/

Paw Paw
Rider 5280 wrote:
What do you recommend for top-end rebuild intervals? Every 50 hours? 75? Additionally, did you get the head ported to better match the larger bore (=...
What do you recommend for top-end rebuild intervals? Every 50 hours? 75?

Additionally, did you get the head ported to better match the larger bore (= larger fuel/air flow rate)?

Cam-wise, did you stick with stock cams or modify/replace those too?

Lastly, fuel mapping - what approach do you recommend here? Get a Vortex or GET? Stock OK? Did you need to get time on he dyno to tune things properly so they run right?

Sorry for all the questions, just generally wondering what is all involved in getting a bog-bore 250F to run right ... as they do sound amazing!
I did the head mods and Vortex, but was advised that the KTM cams were fine. This was all tuned on the dyno for pump gas at 5000 feet elevation by Derek Harris at HP Race Development.

I put a top end in at 55 hours, but am going to go longer on this one. After tear down it didn't really seem to need the top end that soon.
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Paw Paw 271
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Paw Paw 271 5/16/2021 3:22pm
5/16/2021 3:22pm
Paw Paw 271 wrote:
https://maxrpms.net/

Paw Paw
Rider 5280 wrote:
What do you recommend for top-end rebuild intervals? Every 50 hours? 75? Additionally, did you get the head ported to better match the larger bore (=...
What do you recommend for top-end rebuild intervals? Every 50 hours? 75?

Additionally, did you get the head ported to better match the larger bore (= larger fuel/air flow rate)?

Cam-wise, did you stick with stock cams or modify/replace those too?

Lastly, fuel mapping - what approach do you recommend here? Get a Vortex or GET? Stock OK? Did you need to get time on he dyno to tune things properly so they run right?

Sorry for all the questions, just generally wondering what is all involved in getting a bog-bore 250F to run right ... as they do sound amazing!
No head work done. No rejetting or remapping needed. About 70 hours per piston and rings.

Paw Paw
1
dbx33
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dbx33 5/18/2021 6:09am
5/18/2021 6:09am
270 kit on the 17-22' ktms is the bees knees.
ive got a brand new in box kit for sale if anyones looking
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