Building Superminis

The new rules for the 250s by the AMA and FIM give the 250 2-stroke the run of the show. If you want to race 250Fs in the 250 class it has to stay a 250 and we can port it to suit your riding style. If you want to race against 450Fs, it has to be at least 295cc and that kit is $660. If you race dtx, hare scrambles, hill-climb, or ice-racing we can build an “80-over” (2mm) 265cc for only $600 parts and labor. Our Big Bore Master Guide page has listings of all the models and big bore options. If you want keep your bike a 250 and get maximum performance along with high quality top end parts, check out our Super-Stock page.

What’s Included?

All of our kits include cylinder porting, head mods, and powervalve service. You get your choice of 3 powerbands and 2 fuel types. All the top end parts are included too like a Wiseco piston kit, top end bearing, alignment pins, and oversized gasket kit.

Powerbands and Fuel Types

The 3 powerband choices

  1. More low to mid-range with a flatter top end. By concentrating the power down in the lower revs it makes a powerband that lugs without chugging, yet is responsive enough to steer with the rear wheel. Perfect for technical mountain climbing, single track, Arenacross, ice-supremoto, short-track and TT.
  2. More mid-range to top end and a bit softer off the bottom end. This choice is best for EXPERT mx, desert, dunes, dirt drags, beach racing, and dtx.
  3. Wider smoother powerband with less mid hit in the mid-range. This choice makes the power wider like a 4-stroke 450 but without the harsh decleration. It’s great for off-road, hill-climb, and VET or intermediate mx.

The 2 fuel types

  • We machine your head flat then machine the squishband and bowl to match your powerband and fuel choices. The fuel types are unleaded pump 92 and race gas of 98-110. Pump gas is the most popular choice because you can also run race gas.

We make it easy!

We make your big bore kit easy to install because the cylinder is returned with the powervalve system cleaned and timed with new gaskets installed under the valve covers. All you have to do is install the piston to the connecting rod, coat the piston and rings with pre-mix oil, install the base and head gaskets provided, and torque the head. We set the cylinder to piston clearance and finish so you don’t need to break it in, just warm it up and go riding!



1973-1985 265cc $425

1986-2007 265cc $600


1978-2008 265cc $600

1993-2008 295cc $660


1987-2010 265cc $600

1996-2010 295cc $660

Yamaha YZ250

1976-1990 265cc $425

1991-2014 265cc $600

1999-2014 295cc $660


1995-2014 265cc $600


1998-2013 265cc $600

1998-2013 295cc $660


2000-2014 265cc $600

1996-2006 295cc $660

2007-2014 295cc Use KTM Hard Parts Kit

TM 250MX

2000-2014 265cc $600


Just CLICK & PRINT a copy of the Service Order Form and follow the directions on packaging your parts for shipping. All the options are listed for services, powerband choices, fuel types, spare parts, shipping methods, and payment options. Click here for the service order form

FAQs on 295 Kits

Q. Does the power hit really hard in the mid-range and flatten-out quick?

A. Click to reveal:
Typically big bores hit hard and flatten-out and that’s what magazine testers have complained about. The reason is that tuners cheap-out and skip a step, cylinder porting. We include porting with all of our big bore kits and YOU CHOOSE the powerband type. We have a wide variety of customers using 295s for everything from mx to beach racing at 100 mph, to technical mountain climbing, hare scrambles, and dtx.

Q. What type of carb changes will I need to make, and will the pre-mix oil ratio change?

A. Click to reveal:
Generally speaking you’ll need to lean the jetting of a big bore because the ratio between the carb and cylinder bore change. On my YZ250, the stock jetting worked best from 55-70 degrees Fahrenheit at sea-level to 1,500ft of altitude. So for hotter weather, I had to reduce the slow jet by 1 size, lean the needle 1 clip, and switch to a leaner main jet. Some guys like to switch to a #8 throttle slide. That makes the 1/3 throttle really responsive. The aftermarket carb from Carb Parts is a 39.5mm Keihin PWK and pre-jetted for 250cc and bigger to eliminate the stock solenoid pump carb which is prone to malfunctioning. The Carb Parts 39.5mm carb makes 2hp extra on top end. As far as the pre-mix ratio and oil, there’s no need to change it, just use a good brand of oil and mix it at the ratio listed on the bottle. We recommend Maxima Castrol 927.

Q. Does the crankcase need to be grinded to accept the bigger piston?

A. Click to reveal:

No, there is plenty of space in the cases.

Q. Will I need to buy expensive aftermarket parts to make the kit work?

A. Click to reveal:

Our kits work with all stock and aftermarket parts except FMF SST pipes which make big bores run hot especially on the late model RM250, but the FMF Fatty pipes work great, and DEP SPORT makes a great pipe for the YZ250 and gives big bores more top end. A Boyesen RAD valve works great for all-around use. Steahly flywheel weights from 8-11 ounces work well for single-track off-road riding. If you’re thinking of a wide ratio transmission mod on the YZ250, the 295 pulls the bigger gear with no worries, in fact most people just go for the tall 5th gear from the 2001 Yam WR426. Its best to search ThumperTalk for the details on the case mods and gearbox part numbers.

Q. Will a 295 pull a 450F?

A. Click to reveal:

Our 295 ported for mid-range to top end has more peak power than the average 450F but the four-strokes still have a bit of an acceleration advantage in hard dry conditions. The main advantage of a 295 is the bike is much lighter, there’s no brutal deceleration, and its easier to ride.

Q. Will the 295cc piston make my bike vibrate? Does the crank need to be re-balanced?

A. Click to reveal:

Our custom Wiseco Pro-Lite forged piston kit is the same weight as a stock cast piston, so there are no vibration problems caused by the 295 pistons.

Q. Will the engine wear out faster or be less reliable?

A. Click to reveal:

We build-in reliability with thicker cylinder plating, a 2-ring coated forged piston, and polished port edges. Our 295cc kits are more reliable than a stock 250.

Q. Is the price of the 295 pistons really expensive?

A. Click to reveal:

The price of the 295 piston is the same as a 250 piston, plus our custom Wiseco has anti-wear coatings, 2 rings, more surface area to transfer the heat, and a smooth intake to boost the engine’s over-rev. Replacement cost is $160 and average service life is 50-100 engine hours.

Q. How fast can I get my kit and do you have core exchanges?

A. Click to reveal:

Turnaround time is based on the month of the year and if you choose an exchange core or want to retain your original cylinder. Turnaround times are fastest in the fall months and the prime season is from January to August with a peak demand in March and a wait of as much as 8 weeks.

Q. What if I live outside the USA, in Canada, Europe, Australasia, or South America? Can I get a big bore kit and buy parts from Forward Motion?

A. Click to reveal:

Yes, we specialize in international business. Just CLICK & PRINT a copy of the service form and send it with your parts. Send your parts by airmail (Par Avion) or a popular shipping company like UPS, FEDEX, DHL, or Purolator. State on the customs documents that you are sending your engine block to America for repairs. We’ll resend your customs docs back with your order and declare the value under $200 USD so you don’t pay taxes. We’ll send your proper invoice by airmail, not email because the EU monitors private email. We’ll email the airbill tracking number so you’ll know when to expect the package. We are experts at international shipping and documentation.


Instagram IconFacebook link  YouTube link  Twitter link

Millennium Technologies Powersports Products & Services:

Millennium Technologies Revolution Performance Forward Motion logo Ballistic Performance Components logo xport cnc logo