Building Superminis

KTMOB
 

We can build your mini to meet legal limits for the classes of: mx, off-road, dtx, hill-climb, and motorcycle road-racing as well as atv and shifter-karts.

There are two ways to build a mini into a supermini, big bore cylinder and or stroker crankshaft to increase the displacement and change the powerband to suit different types of racing.

One way is budget and involves just the top end, and the more comprehensive way is a big bore and stroker, which involves a complete engine rebuild and more expense. We also offer stock bottom end rebuild with big bore top end, which is less expensive than a big bore stroker. We use Hot Rod crankshafts, bearings, and seals for stock and stroker engine rebuilds.

Regarding engine maintenance issues, boosting the displacement of an 80 to 100cc plus will boost the average “maintenance cost per hour” of riding time. That dollar per hour number varies based on the “state of tune” of the engine. For example an RM85 built to 100cc and low to mid powerband for pump gas is the same cost to operate as a stock bike, approximately $20 per hour. On the far side of the maintenance spectrum is a RM112 built for a national amateur level racer runs about $60 per hour to run.

WHO RIDES SUPERMINIS?

  1. National expert level racers competing for contingency prizes
  2. Fast-developing larger kids competing in age-group mx & off-road racing
  3. Adults of an average stature and ability, riding single-track and off-road

WHO SHOULDN’T RIDE SUPERS?

  1. Cheaters racing 85 stock class.
  2. People with no mechanical capability to maintain their bike properly.
  3. Parents who think their kid’s contingency prizes will make them rich.

BIG BORE CYLINDER KITS $475

Wiseco and Vertex are piston manufacturers that make pistons in big bore sizes. We can modify your top end parts to fit the Wiseco pistons and take your mini over 100cc for $475 complete. We build these kits for youth racers of all ability from novice to national level.

In recent years big wheel 80s and 100s have been a popular choice for adults riding off-road. The lightweight, narrow big wheel minis are even easier to ride with especially more low end power. These types of big bore kits are built for durability and easy starting, plus they run on inexpensive pump gas.

What’s Included?

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

What’s Special?

A complete package custom tuned to your wishes, with a mix of parts and labor and technical support through specialty books and free sample PDFs.

What’s Not Included?

  1. A lifetime warranty on your used dirt bike.
  2. Unlimited free 24hr. technical advice on a toll-free 800 telephone number by a team of race mechanics.
  3. Payment by stolen credit cards or forged cashier’s checks.
  4. Extortion money if you threaten to start a defamatory thread on a popular social networking forum about our products and services.


What’s Not Special?

All of the piston and gasket kits used in our kits are easy to buy and available from any motorcycle shop or on-line accessory mail-order company.


Building Superminis from $995 Parts and Labor

Our all-inclusive big bore stroker supermini performance engine rebuild for the YZ and KX85s boosts these engines to 112cc displacement to conform to AMA and NMA supermini class rules.

This performance build includes a HOT RODS stroker crankshaft and bottom end rebuild kit with bearings, seals, and gaskets. Your engine will be disassembled, cleaned, inspected, clutch and shifter parts polished, case porting and assembly.

For the top end we’ll overbore, port, and re-plate the cylinder. You’ll get the choice of 3 different porting packages and fuel types of pump and race gas. Our techs will modify the head and install the powervalves and check the squish clearance. You get the choice of Wiseco, Wossner, or Vertex piston kits and we’ll give you 20% on future spare top end kits.

Available for Kawasaki KX80, 85, 100 1990-2014 and Yamaha YZ80, 85 1993-2014.


Powerbands and Fuel Types

The 2 powerband choices are:

  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-supermoto, beginner mx, 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, road-racing, beach racing, and dtx.

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 offer free updates for 1 year on porting changes and head mods.


APPLICATION CHART

HONDA

CR80/85 1986-2014 $475

We offer 92 and 100cc. The 92cc meets 2mm/80-over rules and the 100 is legal for supermini.
Special recommendations: Magnum or Mishimoto oversized radiator, Keihin 28mm PWK carb.

KAWASAKI

KX80/85/100 1989-2014 $475

We offer sizes of 92, 99, and 107cc. The 92cc meets 2mm/80-over rules, the 99 is legal for supermini and the 107cc is the maximum oversize and combined with a Hot Rod stroker crank the displacement is 112cc. We use the Vertex 54.5mm race-forged piston in our 107cc kit, all other kits come standard with Wiseco pistons.
Special recommendations: Boyesen waterpump Pro-Circuit Kips valve, JMS head.

SUZUKI

RM85 2000-2014 $475

We offer 4 different sizes for the RM85. The displacement range is 92, 100, 103, and 105cc. The 92 covers 2mm/80-over classes, the 100cc is our standard super. The 103 is a Wiseco Yamaha piston with a shorter timing height that is ideal for 3mm stroker cranks. The 105 is a Wiseco Suzuki piston but makes the cylinder thinner at the top o-ring groove thereby reducing reliability.
(Casemods required, send complete engine or bare crankcases)
Special recommendations: Boyesen waterpump, Wiseco clutch, FMF Fatty, Keihin 28mm PWK carb.

YAMAHA

YZ80/85 1993-2014 $475

We offer 3 kits for the YZ. The 92cc size meets 2mm/80-over rules and we have 2 super configurations. The standard super is 103.5cc using the Wiseco GP piston and we mod your stock Yamaha parts. The 112cc option is an all-inclusive kit with no exchange needed. The kit is available from Hot Rod as a stroker bottom end rebuild kit and the top end is made by Cylinder Works and includes a cylinder, head, and Vertex piston. The crankcase needs special mods and we offer that for $100. This kit sells for $240 for the bottom end kit and $650 for the top end kit. You’ll still need a mechanic to assemble it.
(Casemods required, send complete engine or bare crankcases)
Special recommendations: FMF Fatty, Hinson clutch kit.

KTM

KTM $475

KTM85 and 105SX 2003-2014

The 85 cannot be bored becuase the cylinder is too thin above the exhaust. However it can be resleeved and fitted with a 52mm piston kit to make it a 100. The kit for the 105 to 112cc is $550 and that includes replating not resleeving.


FAQ’s on Superminis

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

A. Our kits work with all stock and aftermarket parts. FMF Fatty pipes work great along with long silencers. Short silencers give better low end but flatten-out on top end. A 1-tooth larger countershaft sprocket makes a big difference and compliments the greater torque of a big bore. CV4 makes high pressure radiator caps and Fluidyne oversized radiators that help keep the coolant from running too hot. Boyesen waterpump impellors and covers work great too.

Q. Will a Supermini pull a 125 and be competitive in the MX LITES class?
A. Heck yeah! A 112cc Supermini makes 30hp and the bike weighs 140lb whereas a 125 makes 34hp and weighs 200lb. The difference is suspension and handling, and most important the racer!

Q. Does the crankcase need to be grinded to accept the bigger piston?
A. Most models don’t need case mods with the exception of Yamaha YZ, Suzuki RMs. We offer free R&R of the top end with crank inspection and case mods on applicable models. That is built into the price of $475

Q. What about the carb jetting and pre-mix oil and ratio?
A. Technically you need to adjust the carb jetting when the air density changes but most supers can use standard 85 jetting. Jetting has most to do with where you ride and the average air temperature. Pump gas changes jetting too because of the chemicals used to make 4-stroke cars clean burning. On certain models, we supply specific jet needles and main jets with our big bore kits. Regarding pre-mix oil and ratios, we recommend Maxima Castrol 927 at 32:1 because it has additives to blend with ethanol pump gas and offers better lubricity for supers.

Q. What about the gearing?
A. Always gear for more speed to compliment the increased torque. Typically you should increase the countershaft sprocket by 1 tooth.

Q. Will the Super piston make my bike vibrate? Does the crank need to be re-balanced?
A. Wiseco Pro-Lite forged piston kits designed for superminis are very close to the same weight as a stock cast piston, so there are no vibration problems caused by the bigger piston.

Q. Will the engine wear out faster or be less reliable?
A. We build-in reliability with thicker cylinder plating, a coated forged piston, and polished port edges. Our Supers are more reliable than a stock 85. However a 112 big bore stroker requires frequent changes of the piston ring, every 5hrs.

Q. Is the price of the Super piston really expensive or hard to get?
A. Wiseco, Vertex and Wossner piston companies make catalog pistons for supers. We sell them for a 20% discount.

Replacement cost averages $99 for the piston kit, $25 for a gasket kit, and $15 for a top end bearing or about $150

The average service life is 5-15hrs for ring and 10-30hrs for piston changes for average riders and half that time for experts.


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.

 

CALL OR TEXT ERIC AT 1-(630) 825-5645 FOR A QUICK PRICE QUOTE ON CUSTOM PERFORMANCE ENGINE REBUILDS FOR YOUR BIKE WITH VIRTUALLY ANY AFTERMARKET ACCESSORY! Email mxericgorr@gmail.com