Big Bore 250s

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!

APPLICATION CHART

FAQs on 295 Kits

Q. Does the power hit really hard in the mid-range and flatten-out quick?
A. 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. 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 by 1-3 sizes. Some guys like to switch to a #8 throttle slide. That makes the 1/3 throttle really responsive. 

Q. Does the crankcase need to be grinded to accept the bigger piston?
A. No, there is plenty of space in the cases so you don’t need to send the whole engine for case mods, just send us the top end assembly for a 295 kit.

Q. Will I need to buy expensive aftermarket parts to make the kit work?
A. Our kits work with all stock and aftermarket pipes and some aftermarket pipes compliment the porting powerband that you choose. For example for more low to mid range the FMF Knarley is the best choice with a Shorty or spark arrestor type silencer. FMF Fatty pipes work great for the wider powerband, and DEP SPORT makes a great pipe for the mid range to top end powerband. A Boyesen RAD valve or V-Force works great for all-around use. If you have a 1999-2001 YZ250 or have removed the stock reed spacer on your later model YZ, you’ll need to at least use Boyesen reeds because the reed stop will contact the piston. 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 taller 3rd, 4th, and 5th gear from the 2001 Yam WR426.

Q. Will a 295 pull a 450F?
A. 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 it’s easier to ride.

Q. Will the 295cc piston make my bike vibrate? Does the crank need to be re-balanced?
A. Our custom Wiseco Pro-Lite forged piston kit is close to 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. 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. 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.

FAQs on 2-Stroke Cylinder Repairs

Q. If the piston breaks can I just powerwash the crankcase and change the oil to get out the broken pieces?
A. The crankcase on 2-stroke engines is sealed so draining the transmission oil won’t help. Most likely the metal particles will contaminate the bottom end to the extreme that you’ll need to rebuild the bottom end with a new crankshaft, bearings, and seals. We offer that service and it averages $550 parts and labor. Additionally you’ll need to powerwash the exhaust pipe (not silencer) because metal may be trapped in there and re-enter the cylinder once you restart the freshly rebuilt engine.

Q. If I send my old piston with the cylinder can you tell me what the original failure was caused from and recommend repairs to keep it from happening again?
A. Yes, we recommend that you send any and all damaged parts, especially if you’ve had repeated failures. Also you should check out the tech article (PDF format) from Eric’s book titled Piston Diagnostic Guide. We’ll diagnose your mechanical problem and recommend action solutions.

Q. My bike has an aftermarket sleeve and now the coolant blows out the vent hose from the radiator. Why does this happen and what can be done to fix it?
A. Cast-iron sleeves are installed with an interference thermal fitting method. The sleeve is chilled in a freezer and the cylinder is heated in an oven and the sleeve in quickly installed in the cylinder where it expands and locks into place. Good sleeving companies install the cylinder in a press and hold the sleeve down until it cools to room temperature and the machine the top of the cylinder flat. On your cylinder the sleeve is squeezing upwards out of the bore, which allows combustion gas pressure to escape into the cylinder’s water-jackets. The excess pressure blows-out past the radiator cap. The only was to fix it is to machine the top of the cylinder and that machine-op is included in the $50 package for cylinder boring/honing.

Q. I just had my cylinder plated for a 2mm oversize and now my bike is slower than before and pings like crazy in the upper mid range. It makes a strange screeching noise at idle too. What’s wrong with my bike and how can I fix it?
A. Plating companies are not engine builders! They don’t have people with the knowledge to do the job right. When a cylinder is over-bored there are many other factors to consider like head and crankcase clearance to the piston, compression ratio, and port heights. Just boring the cylinder 2mm oversize and installing the stock head and powervalves effectively raises the compression ratio to dangerous levels which limits the top end power and makes the powerband flatten-out lower in the rpm range and causes detonation. Additionally the powervalves may contact the piston and that’s probably the screeching noise that you hear when the powervalve is closed because that’s when its closest to the piston face. We can usually fix a top end like that for about $125 plus parts.