Honda 440EX Big Bore Kit - Part 1

Tudor Performance

Apr. 01, 2003 By Dean Waters

The reliable Honda 400EX is still very much a contender in racing circuits, but NOT with a stock motor under the plastic. The stock Honda 400EX just does not have the power output that the newer Z400 / KFX 400, Yamaha Raptor, or Cannondale do. That said, we believe the 400EX is currently one of the best buys on the used ATV market. A new sport quad is going to run you more than $6K out the door in stock form. Now consider a nice 400EX on the used market for $3,500 to $4,000. Chances are that for the same amount of total investment, your used 400EX may OUT-PERFORM that brand new showroom quad.

To make sure our Project 400EX can keep up with the new showroom equipment, we visited Tudor's Performance in Corona, CA. You may remember Tudor's from several years ago when they helped us with Project Quadzilla. While the Tudor's Performance shop was literally a sea of yellow Suzukis, Jeff admits to having built more Hondas over the years than he has Suzukis. Along with others, in the shop was a sweet Z400 with a Tudor's 470 stroker kit and a Yamaha Raptor getting a 730cc kit and custom intake for a single carb install.


While we had decided to have Tudor's assemble our new 440, to minimize downtime, we pulled the cylinder and top-end off our 400EX and shipped it to them ahead of time, so the cylinder could be bored, the head ported and the valve job performed. Yes, this disassembly can be done at home by most novice mechanics. With the help of a Clymer service manual and a couple mechanical buddies to oversee our work, we had the motor apart and ready to be shipped in under two hours.


Here is the intake on our 440EX head. The Tudor's porting involves a number of things. The casting flashes are knocked down, the transition between the port and valve seat blended and radiused, and the intake is matched to the intake manifold. The exhaust is ported to have a 26.5mm exit diameter which matches the exhaust pipe and is polished more than the intake side. The intake is left with a slightly rougher texture to help atomize the fuel.
With the porting done, the head is ready to be assembled. This Motion Pro tool makes it simple. Tudor's perfomed a 3-angle valve job and reground and lapped the valves. Since we would be using a Stage 1 drop-in cam from Hot Cams, the stock springs were used.

The cylinder has had the stock sleeve removed, been bored, and sleeved. The sleeve was then finish-honed and the cylinder decked. The sleeve was prepared for a 88.5 mm piston and can be bored at a later date for a 89 mm piston on the next rebuild. Officially our Honda will be about 435cc but we will call it 440. Prior to installation the cylinder was cleaned using ATF fluid. The ATF fluid is high detergent and does an amazing job of cleaning the cylinder while adding a little oil at the same time.

The piston is a custom JE piston built to Tudor's Performance specs. It is an 88.5 mm piston designed for 11:1 compression. Yes, we could have gone higher, but we were looking for a good reliable mid-range motor that would run on 92 octane pump gas. Prior to assembly the ring gap needs to be checked. We did find two of our rings needed to be filed to achieve proper gap. The edges of the rings were also de-burred to make sure they move freely. The piston is used to square the rings in the cylinder when checking ring gap.

We start the installation with the cylinder. We preloaded the piston using a ring compressor. The next step is a two person job - one to set the cylinder down on the bottom end while the other installs the piston pin attaching it to the rod. Make sure you have a wire on the timing chain so it does not fall down into the motor. Now torque the cylinder down and install the head studs. You will need to double nut the studs for installation.

When the cylinder is done you can install the head. You will need to thread the wire with timing chain up through the head again. Install the head bolts and torque them down, then you are ready for the new camshaft. No core needed.

The Hotcam 400EX camshaft does not use the OEM automatic de-compression system. You will need a press to remove the sprocket holder from the stock cam and then to install it on the Hotcam. Do NOT press it on using the ears but rather the inner diameter. Align the dot on the end of the camshaft with the open spline on the holder. Line up the timing mark on the left side of the case. Your cam lobes should be facing down and both marks horizontal to the head and one mark straight up and down.

Next Tudors installed the head cover. At this point you can adjust the valves to the specs recommended by Hotcam. Set cold the intake should be at .13mm and the exhaust at .15mm. With the motor buttoned up we are getting close now. Don't forget that the camshaft will need to be broken-in at initial startup. The engine should run above 3000RPM for 20 minutes with very light load.

Now we can install our Edelbrock carb. You may remember the original install on Project 400EX. We have had such good luck with the Edelbrock carb that we wanted to stick with it on the 440. Prior to the install we sent our carb down to Edelbrock so they could install the 440 retro-fit kit. The carb was rebuilt and the 440 kit installed. We bolted the carb on and connected the throttle cables.

Next up was the Tudors Performance high rev ignition box. Unlike some of the XR boxes that many sell, you will not have starting problems with this 10,000 RPM ignition box. Simply unplug the stock CDI and plug this one instead.

Last but not least was installation of the new stainless 400EX system from Bills Pipes. The system bolted up perfectly with no fit problems and looks AWESOME.
Summary Our new 440EX fired right up after we primed it. We ran it for 20 minutes in the Tudors Performance shop with a large fan blowing on it to keep it cool. We can't wait to ride it. Next month in Part 2 we will tell you how well the package works and bring you a full dyno report. We should have a couple Hare Scrambles complete so we will know just how well the kit works on the track. We expect to have a competitive, reliable machine, as happy in the wooded trails of WORCS events as it is playing in the dunes on the off-weekends, without breaking the budget.
Tudor's Performance designed the TRX440 Midrange Power Big Bore Kit to be reliable and is priced at only $1050.00.

Kit includes the Big Bore Sleeve (installed), JE Forged Tudor's Performance 11.0:1 Piston kit, Midrange Head Porting and Valve Job, Drop-In Midrange Camshaft, Hi-Rev + 1000 RPM Ignition Box and our No-Blow Cylinder & Head Gasket.
  • Contact Information

Tudor's Performance
Dept: ORC
1727 Commerce St Unit B
Corona, CA 92880
909-549-0771 Newsletter
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