Engine Performance

Hop up your weak stock engine for higher performance. From big bore kits to stroker cranks, to head porting to camshafts the possibilities are endless.

Big Bore kits - Stock bottom end

Hc piston

Wiseco High Compression Piston. 10.25:1 Compression. Available from STD to +2mm

STD - 282cc

+ 0.50 - 286cc

+ 1.00 - 289cc

+ 1.50 - 293cc

+ 2.00 - 297cc

317 Big Bore Kit

350 Rancher sleeve installed in 300 cylinder, 350 piston, rings, wrist pin. Custom gaskets. Must use worn timing chain. Available from STD to +2mm

BORE 78.5mm STROKE 65.5mm (stock)

STD - 317cc

+0.50 - 321cc

+1.00 - 325cc

+1.50 - 329cc

+2.00 - 333cc

355 Big Bore kit

NLA - Was made by High Lifter. JE Piston, custom sleeve, Cometic gaskets. 354cc

BORE 83mm STROKE 65.5mm (Stock)

380 Big Bore Kit

Thumper Racing. 376cc, Piston, Sleeve, Gaskets.

BORE 85.5mm STROKE 65.5mm (Stock)

TRX350D top end and crankshaft

You may be thinking, 350D top end, wait what? Here's the deal. You can swap in a 350D crankshaft and entire top end but it takes some modifications to make it work. First you need to have the flywheel end of the crankshaft machined to match the taper on a 300's crankshaft, so that it will accept the 300's flywheel. Next you need to find a "Reverse Grind" camshaft, these are available both from Megacycle and Webcam. Reverse grind cams are essentially ground 180 degrees opposite of OEM. You are using the 350D top end so you need the 350D's camshaft reverse ground. The reason for this special camshaft is that the 350D engine is designed to spin in the opposite direction as the 300's engine. It is recommended that with ANY aftermarket cam you should run hardened rockers and stiffer valve springs. Now, the rest of the modifications are fairly minor, the guide for the cam chain tensioner needs to switch sides, this is easy as there is already a spot for it on the 300's case. A top motor mount will need to be fabricated to accept the 350 cylinder. Next is the exhaust, the 350D uses DUAL exhaust ports and the exhaust exits to the left side of the engine, opposite of the 300. You can modify the head pipe for a 350X or 400EX but neither is bolt on. The rest of the exhaust will be a custom build as well. But the good news is, that's about all it takes to make the swap. Your machine will now be 350.4cc.

BORE 81mm

STROKE 68mm

HC piston

Wiseco sells 10.25:1 and 12:1 compression pistons for the TRX350 and ATC350X, these will fit a 300 running the 350D top end. Sizes ranging from STD to +1.00mm overbore.

STD - 350cc

+0.50 - 355cc

+1.00 - 359cc

XR500 Piston

XR500 piston and sleeve installed in the 350D cylinder. Piston 3mm taller than factory 350D piston, will require either +3mm spacer under cylinder, or machine material off top of piston. High compression aftermarket XR500 pistons may require more material to be removed. CHECK CLEARANCES. Available from STD to +1.93mm. These pistons ALL require case machining to fit the sleeve into the bottom end.

BORE 89mm STROKE 68mm

STD - 423cc

+0.50 - 428cc

+1.00 - 433cc

+1.50 - 437cc

+1.93 - 442cc

+6.00 - 482cc*

Note* This is a +6mm big bore kit intended for the XR500 cylinder. The sleeve and piston is available but it is not known if they will fit in 300 cases even with machining. If they DO fit the machining would be quite extensive.

ATC350X/TRX350 BBK

There were MANY big bore kits specifically designed for the 350 engines over the years of varying sizes. These kits used purpose built pistons made to work perfectly with the 350's combustion chamber. IF you can find one of these big bore kits I would STRONGLY recommend them over the previously mentioned XR500 "kits"

300EX TOP END BASED KITS

The good news is that the 300EX top end is a little bit more of a simple swap than the 350 top end. The top end will more or less bolt on, there is a couple extra holes in the 300EX head that need to be plugged off and there is also an extra vent tube sticking out of it that will need to be addressed. You will still need the reverse ground camshaft, available from Megacycle or Webcam. This swap will open the door to all of the 300EX big bore kits which are readily available.

**This HOW TO is UNDER CONSTRUCTION awaiting further revisions. If you have anything to add to this or corrections to make please contact me.

Other Engine Modifications

Notice the holes drilled into the drum of the clutch to lighten it, from the factory. Fits directly onto 300's primary gear.

1985 250 Big red centrifugal clutch

Factory clutch drum is drilled out to make it lighter. Adding this to a TRX300 will increase throttle response but reduce torque

400/450 Foreman Centrifugal Clutch

Swap in a 400/450 Foreman centrifugal clutch for better holding power. This clutch is a must for large tires and large GR's that push the limits of stock components. Requires clearance modifications to the crankcase cover, such as machining off the support ribs inside where the clutch will go. May also need to use stacked cover gaskets.

Lightened flywheel

Still looking for more throttle response and don't care too much about torque? Lighten up the factory flywheel on a lathe. How much? How little? Who knows?

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