Big bore, why?

I've been seeing these big bore kits that you can buy for your 250f to get a little bit more power but I fail to see the point of them. If you want more power why not get a 450 or 350 instead since you won't be allowed to race the bike in the 250 class anymore. Am I missing something here?

Here in the US, the vet classes are open displacement. Smaller vets may prefer the lighter weight of a 250F but are outgunned by the 450's, so a big bore 250F may be the hot ticket for them. Same deal with smaller guys that don't race.

Here in the US, the vet classes are open displacement. Smaller vets may prefer the lighter weight of a 250F but are outgunned by the 450's, so a big bore 250F may be the hot ticket for them. Same deal with smaller guys that don't race.

+1.........:thumbsup:that why I put a big bore in my KX250F....

I've been seeing these big bore kits that you can buy for your 250f to get a little bit more power but I fail to see the point of them. If you want more power why not get a 450 or 350 instead since you won't be allowed to race the bike in the 250 class anymore. Am I missing something here?

i agree. and i dont see the point in stroker kits. its turning 13500 rpms seems like itd be hard to add any stroke

i agree. and i dont see the point in stroker kits. its turning 13500 rpms seems like itd be hard to add any stroke

Adding stroke and bore means it doesn't have to turn 13,500 to make power.

I got mine just because I would not have to wait for my cylinder to be machined & nickasilled. Rode in the morning. Big bore arrived in mail, assembled that night. Rode next day. I'm going to send in the stock cylinder soon, just to be ready for next time.

why not

i dont ave one yet but a little more displacement with the weight of my 250 would be nice,im kinda short the extra weight of a 450 makes a big diffrence,also i like to get air so i dont really want to lower a bike,im no tgetting any younger shaving the seat is not an option for my lower back...all those seem like good reasons to boost the displacement of a 250 chassis,though i am considering going 2t

Say you just spent a bucket load of money on a new bike your paying off.Resale value is poor and you have a high residual so selling and upgrading to a new bike is not an option, neither is buying an older model with more hours, older tech and more wear and tear is also not an options. Say the power bug bites you on the ass and says you could use a little more grunt good to have options.

This almost happened to me cause i was going to get a 250 thinking as a weekend rider,little experience this would be plenty but thought no i will get the 450 and learn from there and i'm glad i did cause i would have been searching for more power out of the 250 by now and i know the only way would have been a big bore kit.

I've been seeing these big bore kits that you can buy for your 250f to get a little bit more power but I fail to see the point of them. If you want more power why not get a 450 or 350 instead since you won't be allowed to race the bike in the 250 class anymore. Am I missing something here?

A stock 250f is fun..............but a ported big bore YZ269 that only weighs 205 lbs. dry and can run neck and neck with a stock 450 is priceless.

The extra torque is what I wanted the most out of my big bore........it worked....:thumbsup:

A stock 250f is fun..............but a ported big bore YZ269 that only weighs 205 lbs. dry and can run neck and neck with a stock 450 is priceless.

2t or 4t yz269? 4t I'll call BS:moon:

Which is a better or more effective way to go, a big bore kit or porting and polishing. I had my sons 2 stroke ported and polished cylinder and the head was cut down and polished. It added significant compression and HP, and kept it legal in the class he is in. I also heard that the big bore kits lessen the life of the motor.

I got mine just because I would not have to wait for my cylinder to be machined & nickasilled. Rode in the morning. Big bore arrived in mail, assembled that night. Rode next day. I'm going to send in the stock cylinder soon, just to be ready for next time.

this was my reasoning when I put a big bore on my 450. The cost of a complete kit (with cylinder) was very close to having the cylinder replated along with new piston/rings/gaskets/bearing/etc anyway, so I figured... why not? The stroker crank was a bit more expensive but with over 400hrs on the stock crank (bearings replaced once in that time) I needed to take my displacement to 511.29cc's!:confused:

If you like your bike and it is in good condition otherwise, the $500-$1000+ you will spend on a big bore/stroker could save you from spending $8000 on a new bike that will need setup ($$$) to get right for you anyway. As long as you realize that you will have to start all over with your jetting and your cam profile may no longer be ideal (but not necessary to change) I say go for it.:thumbsup:

Which is a better or more effective way to go, a big bore kit or porting and polishing. I had my sons 2 stroke ported and polished cylinder and the head was cut down and polished. It added significant compression and HP, and kept it legal in the class he is in. I also heard that the big bore kits lessen the life of the motor.

IMHO I would only do the big bore if the cylinder needed to be replated, that was the case with mine.

On a 2t sending your cylinder out for a big bore usually results in a port job anyway, but my money says a good porting is worth more hp than the minimal increase in displacment if they had each been done seperately.

On a 4t I found that the porting alone did little for power until I got a new cam to go with it, I would have a tougher time saying which mod (big bore/stroker or cam/porting) made the bigger difference, the cam/porting was definately the cheaper of the 2.

2t or 4t yz269? 4t I'll call BS:moon:

the weight is about right. my yz stock weighs 208 but it has a bunch of titanium parts so id say its down to 205 dry

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now