Where have the big bores gone?

stone881
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stone881
A few years back all the magazines seemed to be testing big bore 250f's and in the last couple of years I don't remember many, if any.

I am really interested in a KTM/Husky or Yamaha 270-290 cc four stroke. Have been riding 250 smokers the last couple of years, but am bored and want to go back to a 4 stroke, just not really into the mass of a 350/450.

Anybody know what's up with the absence of big bore 250f's?
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BobPA
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BobPA 4/19/2017 8:24pm
4/19/2017 8:24pm
350's feel very close to a 250f with the benefit of a shit ton more hp
JWACK
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JWACK 4/19/2017 8:54pm
4/19/2017 8:54pm
BobPA wrote:
350's feel very close to a 250f with the benefit of a shit ton more hp
Agreed.
Bruce372
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Bruce372 4/19/2017 10:00pm
4/19/2017 10:00pm
I think Derrick Harris built a sxf250 big bore that made 55hp but it needed custom gears.
colintrax
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colintrax 4/19/2017 10:01pm
4/19/2017 10:01pm
Bikes make so much power and lost so much weight who needs a big bore? A 250F isn't enough, look at a 350 or 450.

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stone881
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stone881 4/20/2017 5:38am
4/20/2017 5:38am
BobPA wrote:
350's feel very close to a 250f with the benefit of a shit ton more hp
Not to me, I have ridden a 2016 350 with AER and it was a nice bike, but still a lot of mass. Plus you have to ring its neck to get to the power.
Motofinne
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Motofinne 4/20/2017 6:07am
4/20/2017 6:07am
A modern 250 is fast enough for most weekend warriors and the 350 is also a really good choice for those who like to use high rpms and keep up with 450s.

I don't see why anyone would put a big bore on a 250 unless the stock 250 has had a major engine failure and you go with a Athena big bore kit instead of OEM 250 parts.
stone881
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stone881 4/20/2017 9:37am
4/20/2017 9:37am
Motofinne wrote:
A modern 250 is fast enough for most weekend warriors and the 350 is also a really good choice for those who like to use high...
A modern 250 is fast enough for most weekend warriors and the 350 is also a really good choice for those who like to use high rpms and keep up with 450s.

I don't see why anyone would put a big bore on a 250 unless the stock 250 has had a major engine failure and you go with a Athena big bore kit instead of OEM 250 parts.
The reason I was thinking of doing it is because it is more fun to ride a 250 vs. 350/450 on a track. However, I also ride badlands and dunes and a 250f isn't always enough. Of course the perfect weapon is probably a smoker, but I have been riding them the last 8 years and want another thumper.

A 270-290 kit would bring the rpm's range down and thus make the bike more reliable. Plus there are enough gains to be had to make it work in the dunes.

At least that's what I am thinking.
Motofinne
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Motofinne 4/20/2017 9:40am
4/20/2017 9:40am
Motofinne wrote:
A modern 250 is fast enough for most weekend warriors and the 350 is also a really good choice for those who like to use high...
A modern 250 is fast enough for most weekend warriors and the 350 is also a really good choice for those who like to use high rpms and keep up with 450s.

I don't see why anyone would put a big bore on a 250 unless the stock 250 has had a major engine failure and you go with a Athena big bore kit instead of OEM 250 parts.
stone881 wrote:
The reason I was thinking of doing it is because it is more fun to ride a 250 vs. 350/450 on a track. However, I also...
The reason I was thinking of doing it is because it is more fun to ride a 250 vs. 350/450 on a track. However, I also ride badlands and dunes and a 250f isn't always enough. Of course the perfect weapon is probably a smoker, but I have been riding them the last 8 years and want another thumper.

A 270-290 kit would bring the rpm's range down and thus make the bike more reliable. Plus there are enough gains to be had to make it work in the dunes.

At least that's what I am thinking.
I see your point, maybe a big bore is the solution for you then?

But you are right in your first post, big bores are rare these days.
SeaClassExpert 4/20/2017 10:18am
4/20/2017 10:18am
Motofinne wrote:
A modern 250 is fast enough for most weekend warriors and the 350 is also a really good choice for those who like to use high...
A modern 250 is fast enough for most weekend warriors and the 350 is also a really good choice for those who like to use high rpms and keep up with 450s.

I don't see why anyone would put a big bore on a 250 unless the stock 250 has had a major engine failure and you go with a Athena big bore kit instead of OEM 250 parts.
stone881 wrote:
The reason I was thinking of doing it is because it is more fun to ride a 250 vs. 350/450 on a track. However, I also...
The reason I was thinking of doing it is because it is more fun to ride a 250 vs. 350/450 on a track. However, I also ride badlands and dunes and a 250f isn't always enough. Of course the perfect weapon is probably a smoker, but I have been riding them the last 8 years and want another thumper.

A 270-290 kit would bring the rpm's range down and thus make the bike more reliable. Plus there are enough gains to be had to make it work in the dunes.

At least that's what I am thinking.
"A 270-290 kit would bring the RPM's range down and thus make the bike more reliable"


There is literally nothing to prove this.
TJMX947
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TJMX947 4/20/2017 11:19am
4/20/2017 11:19am
I've always heard those 290 kits cause the bore/stroke to be over square and it actually hurts performance and creates reliability problems. Although you're running lower RPMs the stock rod isn't designed for that heavier piston. Seems like a 269/270 kit was the set up back in the day and from everything I read you don't lose that free reving feel of the 250F.
JWACK
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JWACK 4/20/2017 11:23am
4/20/2017 11:23am
BobPA wrote:
350's feel very close to a 250f with the benefit of a shit ton more hp
stone881 wrote:
Not to me, I have ridden a 2016 350 with AER and it was a nice bike, but still a lot of mass. Plus you have...
Not to me, I have ridden a 2016 350 with AER and it was a nice bike, but still a lot of mass. Plus you have to ring its neck to get to the power.
Just buy a CR 500!! Light weight twostroke. Big bore. Don't have to ring it's neck. Holeshots galore. Rooster tailes longer than the eye can see. Bend handlebars just hanging on. Yes you know you really want one now. Buuuuurrrrrrraaaahhhhhpppppp..
stone881
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stone881 4/20/2017 8:29pm
4/20/2017 8:29pm
I talked for 20 minutes to Derek ( I think that was his name) at HP Race Development in Texas. Am convinced that the 270 kit on the KTM is the way to go. He said about 4 hp with the larger jug and some mapping.

And for a little bit more, mod the head to flow better and a change to the throttle body butterfly. That would put it right around 50 peak hp and maintain the same power curve all for around 1500 bucks.

The link below is pretty eye opening when given the full package.

http://www.hpracedevelopment.com/news/hp-releases-20155-fe-2016-and-201…

I found a low hour 17 that I can hopefully get for the right price and then go with the bigger jug and head mods. Will let everyone know how awesome it is!
kzizok
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kzizok 4/20/2017 8:42pm
4/20/2017 8:42pm
They're still around. They're called 450's.
Bruneval
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Bruneval 4/21/2017 7:59am
4/21/2017 7:59am
Bruce372 wrote:
I think Derrick Harris built a sxf250 big bore that made 55hp but it needed custom gears.
Interesting about the gearing. I have a genuine ex-PC supercross bike ('09 KX250F). When I bought it a lot of the good stuff like A-Kit and Titanium was taken off, but the head was ported quite heavily and it has the copper valve seats. The gears appear to have had some kind of treatment too, so they may be custom but I really don't know. When rebuilding the engine for peace of mind (as it was clearly a mixture of used race parts and new OEM), I had the opportunity to buy an Athena 290cc kit so went with it. For sure the motor rips (PC pipe, cams, carb mods, lightened flywheel etc) but I had a nightmare getting it geared properly as a big bore - it seemed fine before as a 250F.

I ended up going with a much larger front sprocket and going down 3/4 teeth on the rear and even then it sill gets through the gears a bit too quick. Bike is quite hard to ride really, and I ride faster on a semi-standard CRF, so most of the PC mods and big bore is no good for me. Still, it's an interesting bike and I will never sell it. Other interesting thing it did was fry one of the original Rekluse clutches a few years back (the one with the ball bearings). The bike clearly has way more power than the clutch was designed for and even on the most aggressive engagement settings, it was eaten up and good for the bin in no time.
Millennium Technologies 4/24/2017 1:13pm
4/24/2017 1:13pm
We builds tons of big bore 4-stroke and 2-strokes. Compression has more effect on reliability than bore diameter.

I personally have a 270cc YZ250 and I love it. It makes 46hp, relatively low compression, and the last rebuild was at 60(!) hours and it only really needed valve train components.
stone881
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stone881 4/25/2017 6:41am
4/25/2017 6:41am
Millennium Technologies wrote:
We builds tons of big bore 4-stroke and 2-strokes. Compression has more effect on reliability than bore diameter. I personally have a 270cc YZ250 and I...
We builds tons of big bore 4-stroke and 2-strokes. Compression has more effect on reliability than bore diameter.

I personally have a 270cc YZ250 and I love it. It makes 46hp, relatively low compression, and the last rebuild was at 60(!) hours and it only really needed valve train components.
That pretty much confirms my understanding. Typically a big bore will make the bike more reliable because you don't need to rely on higher compression to make power??
Millennium Technologies 4/25/2017 7:29am
4/25/2017 7:29am
Millennium Technologies wrote:
We builds tons of big bore 4-stroke and 2-strokes. Compression has more effect on reliability than bore diameter. I personally have a 270cc YZ250 and I...
We builds tons of big bore 4-stroke and 2-strokes. Compression has more effect on reliability than bore diameter.

I personally have a 270cc YZ250 and I love it. It makes 46hp, relatively low compression, and the last rebuild was at 60(!) hours and it only really needed valve train components.
stone881 wrote:
That pretty much confirms my understanding. Typically a big bore will make the bike more reliable because you don't need to rely on higher compression to...
That pretty much confirms my understanding. Typically a big bore will make the bike more reliable because you don't need to rely on higher compression to make power??
Yes and NO.

The flaw in your statement is "will make the bike more reliable" when it should say "CAN make the make the bike more reliable that a high compression standard bore engine build, because the power increase is made with displacement, not compression and/or RPM."

Make sense?

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