Our 400EX Modifications and Secrets

(Refer to the motor mods feature for all motor buildup tips)

If you disagree with our advice, great. Find out what works for you, then come to the track and beat us. Do that, and we will copy you and then "We'll Be Back".

Many savvy racers have freely shared their advice with us keeping our mistakes to a minimum and expenses down. Out of respect for those individuals we are publishing all we have learned. The following advice is for MX racing only. Different approaches are needed for hare scrambles, cross country, flat track, and trail riding. If you really want to MX race a 400EX and are starting from scratch as we did, expect to spend $10-15,000 (including the initial new bike) and a lot of mechanical time. Over two years we will replace almost every stock part but the motor, hubs, spindles, throttle, and calipers.

Front Plastic -

Click for pic of Meier Racing Front Plastic
Click for pic of Tank Cover Notches
We removed the stock front plastic immediately when new and stored it for use at resale time. We installed Meier Racing front plastic. The plastic is much stronger and has not cracked. There were no mounting holes drilled and mold trim had to be removed. Color did not match original or the rear plastic. The side mounting bolts constantly loosen. The front brackets keep breaking and have been repaired or remanufactured several times. The headlights are removed with this setup and the key switch and neutral lamp must be remounted. We drilled holes and mounted them as near to stock location as possible. The stock gas tank cover was retained and slotted holes were made to accommodate it in the new front plastic. We eventually replaced the front brackets that kept bending and breaking with 1 inch pieces of heavy aluminum angle. Far more durable. This plastic already has the fenders reduced in size, a modification to keep the wind from taking the front of the bike when on a big jump. This plastic also removes the headlights which you don't need for racing and will only get broken anyway.

Rear Plastic -

The stock rear plastic held up great until the sub-frame started to bend. That caused the plastic to split on both sides. We pop riveted in from the back, heavy plastic left over from our sled dog racing sled runners. This repair has been repeated multiple times. We will replace the rear plastic this winter with Meier based on the heavier plastic.

Handle Bars -

Click for pic of TAG Handlebars

The stock handle bars were immediately replaced by Renthal. These lasted through two crashes that bent the steering stem both times but the bars held. Finally the crash in the Motorama did in the stem and bars. The latest bars are the TAG Metals 1" tapered bars. These are 17% stronger than Pro Taper. So far they have held in a crash that snapped the new chrome molly steering stem in half. We used a 50% off coupon from the door prize at the MSC award dinner to get two sets of these bars. One trick to reduce vibration is to fill the handle bars with foam from a can of expanding house insulation.

Grips -

Click for pic of Grips and End Caps

We installed new grips, any kind made for ATVs will do, but we recommend looking for ones marked hard. There is no need to safety wire them. We sprayed paint into the grip before installation. That keeps them from twisting. We installed grip locks that are aluminum end caps with a split bar that slides into the end of the handlebar and tightens against the inside wall. This will keep the grips from coming off the end of the bar if the paint loosens.

Clutch Lever -

Click for pic of PEP EZ Pull Clutch Lever

We installed the PEP ez pull lever. 30% easier pull but more travel is required. They simply re-drill the cable mounting point closer to the lever mount point. Simple leverage principle. Keep a spare lever and perch in the truck. These are the easiest items to break in a crash.

Kill Switch -

Click for pic of Kill Switch
Click for pic of Kill Switch Wire Splice Point

We installed a barrel type kill switch with fork style pull release. The first one failed opened and would not close. The replacement has been fine and saved Sean in several crashes. The switch is mounted near the center of the bars on the inside facing the rider protect it from crash damage. We kept the stock kill switch/starter button/ headlight switch because it has a better starter switch than after market. Be careful to look for a kill switch that has a Y or pig tail connector on the hot lead and a bolt connector on the other. Near the coil (left side near your knee just under the plastic) will be a bullet connector in the black wire with the stripe. Pull the bullet apart and insert the kill switch wire with the Y pig tail into the socket and then put the original wire bullet in the other end. Put the ground under the bolt holding the coil to the frame. Simple, fast, waterproof, no cutting wires.

Steering Stem -

Click for pic of Steering Stem Upper Mount

The stock stem was bent in an early crash with the handle bars pushed down onto the gas tank. The stem was straightened with great difficulty without kinking the tube structure. A 3/4" solid steel bar was cut to length and it fits snugly into the center of the stem shaft. This added weight but was strong enough (Click for pic of Steering Stem Lower Mount) to last through several crashes until the BIG ONE at Motorama. After a lot of money spent on aftermarket stems I recommend the stock stem with the steel bar added. If you bend it without kinking then just put it in a shop press or under the wheel of your truck to straighten it. Our first after market stem was a Bougard. Availability and price forced the selection. No recommendation here, can't say it is any better than any other brand. We bent this one once and used a shop press to make it straight again. You will keep bending stems. If it is a simple bend just put it in a press and put back into operation so long as the tube is not kinked and the bar mount is level. The stock Honda part is cheaper than after market and most likely just as good. Keep the stem mount at the top greased. Keep checking the seals for the bottom bearing and the bearings themselves. Any of these can add friction to the turning and tire the rider quickly. Update** In a mid air collision in Evansville on a 40ft double the Bougard was snapped in half (also needed a lot of welding to the frame). We installed a Laegers (against my above recommendation but we got a good price), with vibration damping clamps. 00 Buck welded a new chrome molly tube into the Bougard so we have a spare. Maintanence items here; periodically grease the top mounting gromet, and take the stem out of the mount on the bottom and clean and re-grease the upper and lower seals, replace seals and bearings if needed.

Skid Plate -

Do not remove the stock rear skid plate. The brake rotor and chain sprocket will get bent costing big bucks. (Had racers say that without the plate if you get stuck on a berm it will get you out but damage the sprocket and rotor, not worth it to me, make your own choice) Do add a QUALITY full frame skid plate. The DG Baha model is thicker than most others. You will bottom this bike repeatedly and without the full skid plate the frame tubes on the bottom will be flattened and structurally weakened and then break. A-arm skid plates are un-necessary for MX and will only add weight.

Chain -

The stock chain is an o-ring design and is strong enough. An o-ring chain cannot be lubricated other than to keep outside surface from rusting. Do not waste money on fancy lubricants. Frequent cleaning and spraying with WD40 is all that is necessary. When the chain stretches or does not operate smoothly, replace it. Keep the chain properly adjusted with about a 1 and 3/8" sag at mid point. Remember to check the chain at both ends of the suspension travel it will tighten and snap (or worse, break the swingarm) if the resting sag is not proper. Replacement need not be an o-ring design. Regular chain will accept lubrication and properly maintained may last longer than o-ring. The stock chain has no master link. This can be a problem during certain repairs and maintenance. A master link can be ordered from HONDA and installed which will allow faster track-side repairs of some major items. A chain breaker is a handy item and works fast without damaging the chain. We replaced the chain after one season when we needed to go longer with the longer swingarm. DID makes a special 520 ATV O-ring that is tough. When installing the master link, the plate does not just slide on the studs, it must be press fit just enough to install the clip in the grooves. Before assembly coat the studs and sides of the chain with white brake grease, add the O-rings and slide on the side plate. DO NOT use vice grips to press the plate on the studs. Find a hex nut that will just fit over the stud end. Use a c clamp to alternately press the plate on the studs a little at a time alternately until the clip fits into the grooves in the studs. Too much and the plate will crush the O-rings. The open part of the clip should face away from the direction of chain travel when the bike is going forward.

Bearing Carrier -

We frequently disassemble the rear axle/carrier in order to assure the seals are still working and the bearings are not dead. When we installed the wider axle we found the bearings were very rough. We opted to order an after market carrier with tapered bearings along with the axle. The tapered bearing uses a car front wheel bearing that is designed for 18,000# loading. far more than the stock single row roller bearing. The stock single row bearing will not hold up for MX. There is an aftermarket carrier that mounts a double row of roller bearings. The double row will take a straight downware load but not a side load like MX will generate. The tapered bearing divides the load carrying between horizontal and vertical loads. After several races an inspection revealed the sprocket side seal had failed and ruined that bearing. We got the NAPA bearing and seal numbers from a knowledgeable racer. The bearing is an exact match and the seals are far more heavy duty than the aftermarket part. These secret parts will only fit the carrier made for tapered bearings. Email me for the secret part numbers only if you need to replace the tapered versions. We ordered the carrier from RPM, there does not seem to be any difference in the many sources for the tapered carrier. RPM warranties theirs and stands behind what they sell.

Axle -

To race you need a wider axle in the back to get better cornering speeds without tipping over. THERE IS NO CHOICE HERE!!!! There are many warranted axles available. They are made of very hard material. From my engineering background and classes in materials science, I know that hard means brittle. All of the hard axles break eventually in MX. EVERY ONE that I have seen break on the 400s break at the retaining ring groove. Buy the RPM Dominator or new Dominator II. It will bend a little but it will not break. You can finish a race day with a slight bend, if it breaks your out of the moto and need a backup to finish the day. A warranty doesn't seem so handy when they break in a race. (The warranty is only for the first replacement anyway, you might get a second but no more than that). RPM will straighten or replace their axle and you can do it when you choose. We bent the axle on both sides, maybe a quarter inch, and raced another season without ill effect. (Jeremiah Jones and the Kangaroo Kid who jumps on Ripley's TV show use the Dominator for good reason) (Secret, the Dominator II uses space age alloy, won't bend or break).

Wheels -

Click for pic of Douglas Front Ultamate Billet Beadlock Rim with ITP HOLESHOT 19" Tire

Stock Honda front rims will hold up for a while but will eventually be egg shaped in MX. The rears need to be changed so you can change the tire diameter, stock is too tall for racing. Use bead locks for ease of tire removal and mounting, even by hand at the track. The outer bead just unbolts. You might have to stand on the inner bead or run the truck wheel over the inside bead to get it off, but you can do it at the track without a tire machine. We chose the Douglas Ultimate Billet Beadlock. It has a rolled edge rather than welded. The billet center has the same tapered lug holes as stock Honda. All others are flat surface and require a washer under the lug nut so as to not dig into the aluminum. The tapered holes and tapered lugs have a larger surface area and no washer so they will not loosen in a race. When we stripped a beadlock bolt, we called Douglas. They sent a full replacement set of bolts and inner plates for free. The only down side when we bought was the lack of choice of offset. All that Douglas had then for the front was 3x2 offset. These fronts are set to too wide a stance (when added to the +2 a-arms) and leave too much of the caliper showing for possible damage. Our total width is just over the 50" allowed. Douglas just announced a 4x1.25 offset rim for wide a-arms. We will get these for use at the Nationals. You can also get the champion beadlocks, they have a choice of offsets but have the flat lug surface. Note that the tapered lug holes on the Douglas are a lot deeper than on the Honda stock wheel. The stock lug nuts have the right taper but are not deep enough and will have almost no extension for the lug wrench or socket to grab. If you choose the Douglas you will need their deep nuts as well. Their nuts are full coverage so the lugs are protected from dirt and damage. Nothing more maddening than damaging the end of a lug that has an open nut and having the lug strip out the hub when you try to get it off. Hubs are expensive and you have to cut the lug off.

Tires -

We find that the ITP holeshots are good for all of our racing surfaces. Some guys have different tires for different conditions. Staying with one tire allows you to have a single set of backups and rims. We use the 19" fronts, some like the newer 20".

Frame -

Click for pic of Welded Frame at A-arm showing grease fittings and cutouts
Click for pic of Foot Peg Frame Reinforcement
CLICK for pic of Front Shock Mount Gussets

The Honda stock frame is crap! Great geometry but made from soft steel. MX racing twists this frame causing stress fractures around every weld. Repeat, EVERY! All stock frames that are raced need constant welding. The front shock top mount needs a triangular gusset added at least at the top. The frame tabs for the bolt at the top of the engine will crack on both sides. No amount of welding or reinforcing has solved this on our bike. It actually cracked the top of the engine needing tig repair. It appears that the frame twists in a manner that the bolt pushes upward cracking the tabs. One racer suggests that the bottom of the frame needs a welded reinforcement to keep the motor from pushing upward on the bolt. We have not analyzed this to agree or not. The frame will always eventually crack just to the rear of the foot peg mounts where the frame bends upward. The remedy is to weld a 1/4" curved plate underneath the frame for several inches around this bend to reinforce it. Eventually you will flip and push the top of the steering stem mount back. Bend the frame and mount back into place and re-weld as stock. Resist the temptation to reinforce this area. This should remain a sacrificial point so as not to bend a more critical non repairable point on the frame. Many of the stock frame welds are poorly filled especially the foot peg attachment points, and need to be filled in for strength. After a year and a half of twisting, welding, and we fear rusting on the inside, we are buying a new frame. The stock to aftermarket price ratio is almost 5 to one. We will not go through 5 more frames before moving on to a different bike so we are opting to get a stock frame. We will weld and gusset it and re-powdercoat it before installing.

Sub-Frame -

Click for pic of Right Side Subframe Mod
Click for pic of Left Side Subframe Mod

This is another weak point in the stock Honda bike that you can count/bet on breaking. The left side diagonal member is curved to allow the mounting of the battery. This curve causes a weak point with the weight of the battery adding to the problem. The diagonal member will bend, causing the top horizontal member to bend downward, in turn causing the plastic to break. Aftermarket sub-frames are expensive, are made of harder metal but have the same weak design. One other expensive solution is to pay over $400 for an aluminum air box that mounts the battery in the center as well. Still leaves the curved and weak diagonal support. Solution is to fix it before it breaks. Duh! Get some thick walled fairly hard tubing. Weld it from the diagonal member close to the point it mounts to the frame, to a point on the top horizontal member just under the point the rider sits. Take a piece of flat stock 1/2" wide and weld it to the under surface of the diagonal member creating an I beam effect. This transfers the rider weight to the frame at the foot peg point and strengthens the diagonal member from bending. (look elsewhere on the stock subframe to see the I beam approach used in the form of an added tent to the straight members). On the other side of the subframe do almost the same thing by adding a new tube from the diagonal to the top horizontal members. However the exhaust header will be in the way. Pick a point on the diagonal that has a straight shot to the rider seating point of the top horizontal, clears the exhaust, but is as close to the frame mounting as possible. Our modification has not moved or twisted in over a season of racing and is cheap.

Swingarm -

Click for pic of Typical Broken Stock Swingarm
Click for shop manual pic of Bearing Area
Click for pic of RPM Swingarm

We were lucky, the stock Honda aluminum casting held for over a season. It is not unheard of to break these in one race. Eventually you will need an aftermarket steel replacement as the cheaper long term solution. Everyone makes one and the price can vary $200. We opted for RPM. We love their axle and service. They just came out with a new in-house manufactured unit. We opted to order the +1.5". With the increased power of our motor we had two problems. Balancing the wheelie and wheel spin in the hole-shot, and the wheelie of power-on out of a turn taking out the ability to steer. We need to put more weight bias to the front. The down side to a longer swingarm is slower turning due to the increased overall bike length. A good rider will overcome this with the throttle and rear tire spin. The suspension will need to be re-dialed significantly. A very bad crash resulted from not doing this methodically. Shocks already near the end of adjustment may need a rebuild. We found that the front tended to compress more on take off and therefore rebounded more giving extreme vertical acceleration to the front end. Front compression and rebound both need to be stiffened. The added leverage on the rear also causes it to compress more and therefore rebound more, kicking off the lip of some jumps. Rear compression and rebound need to be damped more as well. It took a lot of test time to get the bike back in balance and rider confidence to return after the crash. Don't do this mid season like we did. This is the most significant change you can make aside from front a-arms and shocks, but is by far the most difficult to dial in. Maintenance item; Periodically you need to remove the swingarm and service the pivot bearings and seals. Too often this is overlooked. The result is the pivot bolt rusting and preventing disassembly. Cutting this apart and rebuilding is quite a chore. The small amount of labor to grease this area is well worth it.

A-Arms -

Click for pic of Upper Ball Joint
Click for pic of Grease Fitting Mod
Click for pic of Roll Design/Custom Axis Front End

The front needs to get wider for tipping stability, and increased travel. Everyone makes a-arms for this bike. The far out best solution is the one engineered by Doug Roll to repeatedly win the Baja 1000 and 2000. He offers a totally engineered solution for a-arms and front shocks that is tuned for MX and the rider weight, expertise, and style. This setup will not break even in the severest of crashes (the frame will break instead). The top ball joints are camber adjustable. The bottom ball joints which take the bulk of the load, are installed in tubular housings that are part of the lower a-arm. These arms are made of strong alloy and are the only ones heat treated after full assembly. We chose +2 +1. Plus 4 total inches wider and one inch forward. This puts us at or over the 50" total width limit when the Douglas front wheels are installed. Going to the nationals this is an issue, and is pretty wide to get used to for the rider. This width is too much for arenacross. The added length is magnified by the longer swingarm and further demands good rider throttle and rear spin technique to overcome the resistance to turning. However this creates a very stable platform along with the wider rear axle and allows much faster cornering. Maintance item; the top and bottom ball joints have a fiber inner shell that needs to be lubricated with teflon. Teflon is hard to find, go to a gun or sporting good store. We had to replace the pins and sleeves at all a-arm pivot points. Most replacement arms do not have grease fittings for the 400ex. We had 00 Buck make us a new set of pins and sleeves of a better design than roll. We added grease fittings to each pivot point using short 14-28 grease fittings. The front upper and lower mounts need to be notched enough to clear the fittings at full suspension down travel.

Front Shocks -

The stock Honda shocks are crap with only preload adjustment. Good to get started in the Novice class but have to go quickly to the junk heap. One rider we know has gotten through Novice and Amateur on a stock front suspension. He just ripped the shock off the lower a-arm two weeks ago. He tires easily from the pounding and we beat him by waiting for the last lap and shooting by the weary sole. The race community is very consistent in belief that PEP is the best shock. No argument here. However they are also consistently 6-8 months in delivering. Inconvenient and downright obnoxious in my opinion. The only other accepted brand for big air is Custom Axis. Axis just happens to be the brand selected and engineered into the roll design front end kit. There are too many choices of dual rate, triple rate, compression only, or double adjustable. We specified the rider ability, weight, and MX, and selected the triple rate springs with both compression and rebound damping. This front end is the most significant and dramatic change that can be made to the bike. This is like riding a deep cushioned couch around the track and is totally adjustable. Maintance item; the top and bottom shock pivot points need to be greased periodically. UPDATE; We just had 00 Buck rebuild both front shocks, he is an Axis authorized service shop. We left the springs and shims alone but had him add disks to the valves of both compression and rebound. We were too close to the adjustment limits on both.

Back Shock -

The stock 400ex rear shock is fine for most applications. It is rebuildable, and has both compression and rebound adjustment. We had extra disks added to both compression and rebound to keep in the middle of the adjustments. We just added an Ibock spring, 5 points heavier than stock. This shock can be serviced locally by Jay Rapp, call me for his number. Maintenance item;

Both ends of the shock and the three joints of the shock pivot arms, all have bearings and seals that need to be greased and inspected regularly to keep them from rusting. Sean has crushed two sets so far.


The CDI ignition box needs to be replaced. A White Brothers or other brand will add 1000 RPM to the rev limiter AND will give a spark advance curve that adds to lower end torque. This is especially important for higher reving modified motors.

Air Box -

The stock air box and snorkel are too restrictive especially for the air flow of modified motors. Some racers remove the top. We recommend leaving the lid on and drilling one inch holes to the rear so they are not over the filter. A wet filter does not flow well. Parts Unlimited sells inserts but they are not worth it.

All stock air boxes will break. The top tabs that fit over the subframe will break off. That is not sweat. The rear mounting tab will break off. We ground that off and took inch and a half wide heavy aluminum flat stock and bent it to follow the profile of the air box from mid bottom, up the back, and then over the sub frame. Pop riveting this into place supports the box from the bottom. This uses the stock mounting bolt and is as easy to remove as the stock. If done right the top fits secure as well.

Air Filter -

The stock foam air filter will not flow enough air when properly oiled. Some racers prefer the foam UNI replacement. Personally I do not like to oil foam filters. Just as popular is the K&N paper filter. It needs to be oiled to properly protect the engine from dust. We bought a case of spray K&N "red" (thicker) oil. Buy some commercial foam filter solvent (gallon or larger). Put it into a pail and make some sort of porous spacer for the bottom so the dirt can settle. This stuff is the same solvent as used in parts washers. You can get a 5 gallon can from NAPA for the price of one gallon of filter solvent. Just wash the filter out in this and then use Palmolive dish washing liquid soap to rinse it out. Dry fully before oiling again. We use two filters so we can change between motos. We also use the Outerwears Pre-filter cover on the K&N. This collects the larger dirt and keeps the filter from clogging. The slight loss of air flow from this cover on a clean filter is made up by keeping the filter from clogging.

Carburetor -

From the factory the stock carburator is set to run too lean with a 148 main jet. If it is not bumped to a 150 you will run hot (you will see the exhaust headers turn blue for half their length). This will cook the exhaust valve seals. You will know this when puffs of blue smoke are produced on backing off the throttle coming into a corner in a race. (blue smoke on acceleration is a sign of worn rings) If you add an after market silencer and K&N filter to a stock bike you will need to go to a 152 main. You should also go up one size on the pilot jet. This will prevent a bog when pegging the throttle from idle. If you modify the motor and port the heads you will need more flow. The stock carb with a 170 jet will work on our screaming 425 but does not flow enough at high RPM. Sparks racing will bore the stock 35.5mm to about 37mm which helps with a mild motor package like a 416. To get the full potential from our full out 425 we went to the Keihn FCR 39mm full race carb. This is the full race version of the YZ426 stock carb. The FCR is recommended by LRD and Sparks. You will need a 160 main and a 50 pilot, with the air screw 2 turns out from closed. This carb really woke up our motor. It has no choke but has a monster accelerator pump. We found that we had to reset the accelerator pump fork adjustment to rid us of an off idle stumble from too much boost. This carb fits the stock intake and air tube and clamps. It is just a little taller than the stock so some truck camper insulating foam strip across the top of the plastic layer under the gas tank will keep the tank from hitting the top of the carb. Unlike the popular Lectron carb, this carb will shut the engine off after a flip and will easily restart without flooding after you roll the bike upright again. This is also a problem with the Elderbrock replacement carb. LRD has dyno tested this carb to be 3hp better than the Lectron across the full rpm band. We use Sunoco 100 race fuel. LRD says our 425 is dyno tested with 93 octane but we don not trust it. A local dealer has the 100 at the pump for motorcycles.

Intake Tube -

The intake tube will always fall off the carb in MX. The carb may also fall off the motor intake plenum. The prevention for both problems is to keep forward pressure on the tube/carb assembly. We put two small holes in the air box on the front right side. We put tie wraps from there to the frame at the plastic mounting point. This keeps the tube and carb securely in place.

 Exhaust -

 Battery/Electric -

Brake Calipers -


Oil Strainers -


Front Wheel Bearings -

 Clutch -

 Rear Break -

 Shift lever -

The Prodesign billet aluminum replacement shifter is the most common replacement for the weak stock shifter. The stock shifter will bend inward and prevent you from shifting during a race.









Nerf Bars -

Nerfs are mandatory to race. They keep other bikes wheels from crushing your foot or breaking off your shifter. They can be used to put your foot futher out to the side in really tight turns. We tried the DG nerfs that fit over the stock foot pegs. This adds a quarter inch to the peg bolts and causes them to break or snap or causes the frame mount to break. We opted to move to the AC PROPEG with integral pegs. This helps the problem but does not eliminate it. Frequently check to see if the peg bolts are bent. If so replace them immediately as they are about to break. Make sure the weld around the frame mount is fully filled in for strength.

Tips and Tricks -

The rear hub nuts need to be tightened regularly. To make it easy do not use cotter pins. Use spring clips instead. Unfortunately these are prone to falling off in a race and allowing the nut to loosen. Just put a tie rap across the ends to solve the problem. Easy to cut so the pin can be removed quickly. Rear number plates are a must or you will not be scored. You do not want a full plate as it is too big. We cut the plates in half. We secure the plates with double tie wraps so the plate swings freely. This keeps it from being lost when you get hit from behind. Keep it loose so it does not swing up and remain unreadable.


 The stock seat gromets will fall off in a race. Just put in a side hole and tie wrap them in place.