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By: Charles Salyers

Piston and ringsWith today's pursuit of gaining a more powerful engine, most companies have charged into building the monster motor.  But most people cannot afford a high cost engine buildup so I looked into a low cost performance kit.  I choose to use a TRX 400EX since it has become a very popular quad on the race track, trails and not to mention the dunes and found a few that stand out.  My choice was Powroll since they have been doing 4-strokes just about longer than anyone.  I contacted Powroll and spoke with Pam.  She informed me of several options to go with to make it a low cost buildup. I went with the 415cc Big bore kit.  This kit cost $401.90 and comes with a 87mm JE piston with rings, clips and pin , CDI, vented air box lid and a new head gasket.  It gives a 20% power increase.

400 EX bottom endTo start I tore down and inspected the engine (using the factory repair manual )  I took the cylinder, piston and rings to a local machine shop with the clearance specs. provided by Powroll to have it bored and honed.  When I received the cylinder back from machining it was ready to be reassembled.  I started by replacing the base gasket with a new Honda gasket. Next, I installed the Powroll piston into the freshly bored cylinder and set it into place.

Powroll Top endI installed the piston with the arrow facing the exhaust.  Then I reinstalled the new head gasket and head torqueing to Honda specs.  Next was reinstalling the cam and valve/rocker cover.  Afterwards I checked and adjusted the valves to factory specs.   Total time for kit was about 5 hours for teardown and reinstallation.  After everything was put back together, the need for a larger jet in the carb was needed to allow a rich fuel mixture for proper break-in, also the clip on the needle was moved down one notch.  Once I was satisfied that everything was ready, the break-in was done by the guide lines given by Powroll.  I then rechecked and adjusted the valves again. After that I rechecked my jetting to make sure it was jetted correctly.

A recap of parts are as follows:

1. Pro-circuit pipe-
2. K&N filter w/outerwear- $55.00
3. K&N powerjet kit- $45.00
4. Powroll 415cc bigbore piston kit- $234.95
5. Powroll vented airbox lid- $49.95
6. Powroll high rev CDI- $117.00
7. Honda factory cylinder base gasket- $6.50
8. Honda factory oil filter- $3.25

Seeing I already had a pipe on the 400EX , I added the price seeing it was tested with it on.  So now on with the test ride!  We got a new 2001 400EX all stock to test out this new 415cc hop up, man were we surprised!

CcRiDeR flyin' high!In a 300ft. drag the 415EX pulled away every time, this puppy was hitting hard from bottom to topend.  It had the feel of a stock TRX250R not your normal 4-stroke grunt. We even switched the riders back and forth to make sure it wasn't just the rider. So next we hit the trails to see what it would do there.  The 415EX hits the top much quicker than a stock 400EX so we were flying up hills and around the trails like it was meant to do.  For the money invested into the 415cc this was a serious deal for a fast rev hard hitting power package!

For more info about what Powroll can do for you give them a call and ask for Pam or Pete.  They are very friendly people and can answer any questions you may have about building your own 415cc or larger 400EX or any 4-stroke you may have.  Just tell them that you seen this article on ATV Source Magazine, and they'd be glad to work with you on your quad projects.

P.S. Next we try a 38x40 HV Lectron Carburetor on it .

4115 SW 19th Street
P.O. Box 920   
Redmond, OR 97756
Phone: 541.923.1290 
Fax: 541.923.5637

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