Thumper Racing Big-Bore Kit - Abused

Thumper Racing Rhino Big-Bore Kit
The goal for this project was to give the Yamaha Rhino motor better grunt off the bottom and a strong mid without causing overheating or hurting the reliability. And since the fundamental parts of any Thumper engine are performance, reliability and price, owner Gary Hazel wanted to keep the engine modifications to a minimum.

Fortunately, this is possible thanks to a stock Rhino 660 cylinder that has enough room for an increase in the bore without the hassle of installing a sleeve. That means Thumper was able to basically bolt its 690 bore-in kit right into the Yamaha. The kit includes everything-piston, gaskets, jetting kit and an oil filter. The $339 Thumper 690 kit uses a Wiseco high-compression 102mm forged piston, which permits a very tight cylinder-to-piston clearance. This in turn provides better performance and a quieter motor. The increased compression of 11:1 means the engine now only sips 93-octane fuel, a slight sacrifice in the name of power.

Although Hazel is testing a new cam for the Rhino, he chose to keep things simple on this build and stick with the capable stock cam. It works well with this kit and helped hold down the cost. The only other mods done to the motor were adding Thumper's $119 serviceable oil filter (buy one, then clean and reuse it) and filling the engine with Klotz (800/242-0489; www.klotzlube.com) MX4 15w50 synthetic oil. The kit design is intended to be user-friendly; a capable home mechanic can pull off the installation without difficulty.

To complement the basic big-bore parts, Hazel got the boys at Scary Fast (310/668-9050; www.scaryfast.net) to send a prototype Rhino Juice box. The spark box should retail for $265 and has an ignition curve that gives the motor another 1000 rpm. Based on prior testing, these additional revs make Thumper's big-bore motors even better performers. Letting the big Rhino exhale easier was an FMF (310/631-4363; www.fmfracing.com) PowerLine muffler. Besides keeping the bark muted, the $299.99 exhaust is much lighter than the stock boat anchor and, most important, looks cool. Getting all of these newfound ponies translated into forward motion was tackled by ITP's (909/390-1905; www.itptires.com) new TerraCross R/T tires and C-7 machined wheels. These meats made a huge difference in the handling of the Yamaha. The 26x9R-14 front and 26x11R-14 rear tires worked great whether in rocks, mud or hardpack. At $221.95 and $233.95 per wheel, respectively, these babies are a must have for any side-by-side machine. Just make sure the air pressure is up before pitching it sideways for the camera

Based on past experiences, Hazel and the gang figured they would probably find some trouble. That translates into getting stuck, and the best tool to solve that problem is a winch. Hazel chose a Warn (800/543-9276; www.warn.com) 3.0s winch with synthetic rope. With the $670 insurance policy and Warn's mounting system that permits a back or front winch positioning, it was time to seek some trouble.

After a week of testing other machines, we finally got a chance to roll out the Thumper Rhino. Bedecked in its smooth camouflage, shod with the aggressive ITP radials and sporting a security-blanket Warn winch, the side-by-side whined with an energetic plea to hit the trail. We didn't disappoint the eager machine, and a stomp on the gas pedal was answered with a throaty bark and an exhilarating surge from the 700cc motor. Mind you, it was still a Rhino and no sport quad, but it gobbled the trail like a starving dog devouring a steak.

But it wasn't all about high-speed hijinks. As we were sampling this creation at Barnwell Recreation Area, site of two GNCCs and a WORCS race, we couldn't resist trying the Thumper on one of the infamous steep and rocky downhills. The UTV didn't let us down; the TerraCross treads clawed up and down the tricky jeep trails as Hazel and coconspirator Dick Burleson showed off the Rhino's agility for Thad Josey's and my cameras. Satisfied there, we set off for more open environments and more modeling work. It turned out that in spite of the apparent hill work, the simple powerslide around the trees proved to be the trouble we were seeking. With Hazel at the helm, we were making one more corner for Josey's camera when disaster struck-the tires gripped a little too well and punted us over on our side. It turned out that a good first-aid kit was a lot handier than the Warn. The Rhino gained a few scratches to its tough hide while I got to tour a small eastern Texas emergency room and we gave ATV advice to the doc as he sewed up my arm. Now that's fun!

Accident aside, the Thumper Rhino 690 kit paired with the rest of the accessories certainly answered the call for muscles in the affable Yamaha.

Thumper Racing: 903/938-0570; www.thumperracingusa.com