YZ426F Teardown & Rebuild

[basanch73]

Well, it's officially hot here in Yuma AZ and I figured I'd get a jump on tearing the eng down to check the crank, piston and cyl, and cyl head out. Anyway, after 6 yrs on this '01 426, I was amazed at the condition of an engine this old in its original condition from the factory in July of '01. With the help of Eng. micrometer tools at the base auto hobby shop, I was able to measure everything from the crank to the valve guides. Of course this was done after a 2 day evolution of cleaning the approx 1/8" of carbon build up on the piston and cyl head. I think the most amazing parts were finding original valve shim pads under the lifters and then finding no leak of solvent out any of the valves. The exhaust valves where pretty clean, but I'm here to say that once the intake valves were pulled there was mounds of built up junk. However, 5 cans of carb cleaner later and everything looks nice and fairly new again. Here's some pics.


Note my homemade valve spring compressor tool. Without the original tool from Yamy, I was able to cut and drill a cheapo 14mm socket that would compress the spring and allow the locking collars to be pulled out while a rag on the bench kept the valves from moving. The magnet was used to keep them from flying off and hiding on me. All in all, it worked perfectly. I made sure to debur the socket well after modifying it to ensure I wouldn't scratch the lifter surface.









After all the measurements were taken, the only items out of spec were the rings and only by a few thousands of an inch. With 10/40W oil of Yamalube or Maxima w/ steel filter, Engine Ice and 91 octane pump gas, I'd say things held up pretty well. Thus, for now I'm going to order a new ring set, piston pin, valve seals and new top end gasket kit. I'll be sure to take some more pics when I get things going back together. It'll be a week before I place my order, so any thoughts or tips are always appreciated.

Later,

Brian

[DRGSin393]

Wow, thats pretty cool! Nice to see things have held up so well, you do ride it right? LOL Just curious, any reason as to why youre not buying a new piston and springs also, now thats its torn down, even though theyre within spec? DO take pics as you rebuild it!

[yz250guy]

Originally Posted by DRGSin393

any reason as to why youre not buying a new piston and springs also, now thats its torn down, even though theyre within spec?

I second that question.

BTW-Very nice job!

[basanch73]

As far as the springs go, I didn't think I could find any better than the stock ones which still are at the exact height specified in the manual and at the required lbs of pressure as tested. It was only ridden on weekends at Glamis dunes during the cool months for its first 5 years by the original owner, but I've put it thru some abuse on the track and desert over the last year.

On the piston, I read and read different ideas on piston replacement and clearances to the cylinder and have been leary on aftermarket. Maybe, I've got bad gouge, but it seems that you need a larger clearance on ProX and Wiseco due to their metal and heat expansion properties (around .0045"). My current clearance is right at .0020" with the stock one, which falls right between the .0016-.0026" called for in the manual. Having said that, since the piston still looks new, I don't want buy a new piston at the same compression ratio and have to get the cylinder bored to size on top of it. The cost really not that much different with the kit and gaskets until you get into having the cylinder machined to clearance.

If there's something I'm missing on valve and piston thing, please let me know. I'm definately no expert on the matter, just relying on the few common cents I have left.

Thanks for the replies and hope to here more input. It's greatly appreciated!

Brian

Originally Posted by DRGSin393

Wow, thats pretty cool! Nice to see things have held up so well, you do ride it right? LOL Just curious, any reason as to why youre not buying a new piston and springs also, now thats its torn down, even though theyre within spec? DO take pics as you rebuild it!

[basanch73]

Thanks. I'll definately post some more with pics on reassembly. I know you've been around these 426's way longer than myself, so I'm glad to hear this gouge from your past experiences on rebuilds. Once, I get some real time after my rebuild, I'll try to post another step by step approach guide like my cam chain one to help anyone else here at ATM. I'll be doing the same to Larrys bike after mine is complete anyway, so I can get the dirty teardown pic's in there as well. I just cring everytime I hear how much others pay to have shops do things that they could do themselves for outragous prices.

Oh, by the way. Anyone going to the US Open in October in Vegas. I'll be there and it would be cool to see some faces to the ATM names?

later,

Brian

Originally Posted by Florida 393

Brian,
Great write up on the tear down and nice picture documentation. I agree with sticking with the stock valves, springs, piston and rings. I had the same findings on my 02 426 internals on tear down and in subsequent inspections. The reports I have read over the last 3 years were that the stock Yamaha valves, springs and piston were as good as needed or better in many cases.

Bill

[basanch73]

No problem Bill, just glad to help where I can.

Semper Fi back atcha',

Brian

Originally Posted by Florida 393

Brian,
When you get a chance to do the Step-by-Step with pictures I want to move it to the Knowledge Base forum as I did before. Your careful analysis and descriptions are a tremendous help. I print them out and put them in my notebook of technical articles on the YZ 426F. Thanks for the hard work and sharing your skills with us. Semper Fi, Gunnner!!!

Bill

[Malcolmv8]

That cylinder head picture is awesome. I've never seen a cylinder head completely dominated by valves. 5 valves in there Just amazes me that the whole surface area of the head is valves. Great stuff.

Malcolm

[Malcolmv8]

I'm impressed just with what I see in the picture. Now if I can just figure out how to bolt them up to my Mustang

[Malcolmv8]

I know exactly which motor you are talking about. It looked HUGE from on top with all those intake runners. Great running motor too.

[DCox]

Originally Posted by Florida 393

You have to hold one and look at them up close to really appreciate the Genesis head design. I cleaned mine in 2003 when I pulled things down and replaced the piston/rings/wrist pin and it amazed me. The 250F head is even more amazing to see with everything worked into such a small area. If the YZ150F is a Genesis head it will be something to see. And they are stone reliable.

Bill

I am wondering if there is a time limit or anything on the patent for the five-valve design. From everything I have read, Yamaha really hit the nail on the head with regard to reliability and I wouldn't be surprised if the other OEMs would like to move in that direction in the future.

[basanch73]

Well, guys. I'm about a wk out from receiving my parts. I ended up ordering the following:

- 5 new stock valve springs (one exhaust was out of degree angle spec, but cheap to replace all.)
- 5 new valve stem seals
- 1 HotCams Refill Valve Shim Pad set (180-205mm: 5each)
- 1 new stock complete piston ring set w/new wrist pin and lock rings.
- 1 Top end gasket kit.
- 1 Clutch and side cover gasket (for my clutch job to follow) and spare ball bearing for good insurance against losing the thing.

Grand total w/shipping for all came to $145.00 from [Only registered and activated users can see links. ]. Of course you can throw in the $35 for new Engine Ice, Oil and Carb Cleaner. Either way, I've got my rebuild cost to bring the motor to almost original condition under $200 by doing my own labor.

Until Parts Come In!

Later,

Brian

[The Baron]

If I were you I'd put a new Hi Compression piston in while you are at it. A JE 13.5:1 will give a little power increase and is much stronger than the stocker. The stock piston may look fine and be within tolerances but they really aren't meant to be in that long. Pistons are wear items and it is my personal opinion that if you are going to tear the top end apart, you might as well upgrade to a good aftermarket piston like either a JE or CP. JE's are pretty reasonably priced, CP's are a little more expensive. Both are rock solid pistons built to very close tolerances, unlike the Wisecos (at least from what I have heard). If you were to compare a JE or CP next to the stocker, you would have no issues with putting one in your motor, they are truly works of art. It's almost a shame that they are inside the motor.

[basanch73]

Well here you go. I finally got my rebuild done. However, that was the easy part. Writing this guide was much more time consuming, but I hope it helps!

Steps for Top End Engine Rebuild on my '01 YZ426F

Initial Tips: Ensure you have a clean area or table to place out all the parts. An old white sheet on a rollaway worked fine. Most importantly, do not tighten anything on the motor without a ft & in lb torque wrench. You definitely don’t want to ruin the whole process by stripping out the aluminum threads within the motor. Have a friend available if possible and that might want to learn a thing or two in the process.

Special tools needed: Bike manual; Piston ring compressor (2-5”); Straight, small snap ring pliers that compress, not expand; (10-150Ft lb) and (40-200In lb) torque wrenches; 12mm "crows-foot" socket (like open end of wrench) to fit on in lb torque wrench; Valve spring compressor tool (I used homemade, with instruction below); 1/4" socket driver; scribe; small telescopic magnet & Lots of Patience!


* I cut a 14mm cheapo 1/4" drive, 6 point socket with a hacksaw in two places and then had the cut ends snapped of with some pliers. I then deburred the outer area to keep it from possibly scratching the head's surface that the lifter rides up and down on. You may need to use a different size depending on the socket's wall thickness, but it doesn't matter as long as it has the same size outer diameter to match the diameter of the valve spring's upper retaining collar.


Consumables needed: Note pad and pencil; safety wire; rags, lots of carb cleaner; tooth brush; new oil and coolant.

Parts Needed (Based on My Motor’s Condition): Stock Ring Set w/new Piston Pin and lock rings; Stock Valve Seals and Springs; Stock Side Cover gasket w/O-rings for plugs (could be reused depending on condition); Top End Gasket kit from Cometic; New exhaust gasket (Cometic's kit doesn't provide it) and a HotCams valve shim refill pack (5ea of 180-205)

MOTOR DISASSEMBLY

1. Remove seat, tank, shrouds, sub-frame, carb (drain fuel bowl), exhaust (plus it's gasket in the head), coil, to include disconnecting all the cables and wires in the way.
2. Drain oil and coolant (remove bottom bolt to waterpump and oil filter cover and cycle motor to completely drain for both).
3. Clean all loose debris around frame area above the valve cover and blow out any debris in plug area with compressed air.
4. You don’t have to remove the radiators, but I did for convenience of working room and to inspect hoses. Either way, you must remove the hoses at the bottom of each to ensure no trapped coolant leaks into motor upon disassembly. Additionally, I removed the coolant tube on the front of the cylinder head.
5. Remove Spark Plug and Breather Tubes.
6. Carefully remove valve cover and seal.
7. Remove the Three-way oil line assembly that bolts to the head and to the case and put in a plastic bag. Be sure not to loose the crush washers and keep the bolts in the part of the line that they came from since one has a different torque value.

8. Remove your timing and crank plugs on flywheel cover.
9. With the bike in Neutral, turn the motor over by ratchet and socket on the crank nut until the "I" mark lines up with the top and bottom notch on the timing plug hole. You want to get the bike to Top Dead Center (TDC) on the compression stroke to relieve all spring tension to all the valves by having them all closed at the same time. At TDC Compression, the "I" (not "H" behind it) mark should line up and the exhaust and intake cam lobes will all be facing outward from each other. Additionally, there will be three dots on each of the cam gears that should be at 9, 12 & 3 O'clock with the 3 o'clock dot on the exhaust cam and the 9 o'clock dot on the intake cam pointing at each other when the crank ("I" mark) is positioned at TDC Compression. (As an added note of faith, I inserted a long #1 flathead screwdriver in the plug hole to notice the flat point where it neither goes up or down, which verifies true TDC of the piston.)
10. At this point mark the caps directly behind the cam gears with a scribe or other sharp object to the 12 o'clock dot positions for additional reference. Do the same for the crank if your going to change the cam chain during your teardown. (I quickly found out that pencil and marker quickly rubs off with oily hands) It is highly recommended to take pictures for reference later. I also wrote down that there were 7 links between the 12 o’clock dot and tooth on each cam as reference.

11. At this point, check your valve clearance and write it down on paper. I just drew circles to represent the lifters in respective positions that I numbered on paper and then wrote the clearance to the side of each. Later, I wrote the pad side down in the middle of each circle.

***( If you will be changing out your cam chain during this process, please use my Cam Chain Replacement Guide post). I had done this already towards the end of the season due to noise from its slack, so this wasn’t done at this time. Please note that getting the chain on and off with the head and cylinder removed will allow you to remove the chain guide facing the front of the bike, which makes removing and installing the chain to the crank gear a breeze.***

12. At this point, run safety wire around the chain and secure it to the frame somewhere to keep it from falling in the motor upon cam gear disassembly. Using safety wire allows you to quickly remove and reattach it easier for further disassembly.
13. Loosen cam tensioner 10mm cap bolt w/crush washer and then remove the tensioner from the motor. This will loosen the chain tension for cam removal.
14. (PUT RAG OVER OPEN CASE AREA ON CAM GEARS TO KEEP HALF MOON SHAPED CAM BEARING RING FROM FALLING INTO CASE) Carefully remove cam bolts, caps and cams w/retaining ring separately and put into separate sandwich bags to keep clean and together for exhaust and intake. You don’t want to mix up those bolts, so keep them in their place in the caps.

15. With the cams removed, you can now remove one lifter and shim pad at a time and put them in their own numbered plastic bag. Write the pad size down on your not pad for that number exhaust or intake valves position. Keeping everything for each valve separate and numbered in its own bag is a must. Each valve and its parts must be put back in the exact same area of the head it came out of.
16. Now, remove the decompression mechanism; the 4 large head bolts; 2 (12mm) nuts on front and rear on cylinder and the 2 (l0mm) longer nuts on the side.
17. With the head no longer bolted to the cylinder, CAREFULLY tap it loose and pull/wiggle it off the cylinder while paying close attention not to bang the top of the head on the frame. You’ll need a second person to fish the safety wired cam chain down thru the head as you pull it off and then re-secure it from falling in. (That is if you’re not chaining the cam chain during this process)
18. With the head off, remove the chain guide facing the front of the bike. It just pulls out of the notch in the cylinder. (If you're not changing the chain, this is where you have to have it pulled tight. Not doing so can allow an extra link or so get bunched up down there upon reassembly, which will leave you without enough chain to get the cams back on.) Yes, it did happen to me!
19. Next, remove the one 8mm bolt to the cylinder that attaches it to the case.
20. Slowly slide the cylinder off, but be sure to have a clean rag handy to place it around the connecting rod and hole in the case to keep debris out and to carefully rest the rod and piston on. Again with help of a friend, have him or her fish the safety wired chain thru it and then re-secure it tightly. (That is if you’re not chaining the cam chain during this process)
21. Next, use the snap-ring pliers to remove the lock ring that retains the piston pin.
22. Before pushing the pin out thru the piston and connecting rod, you’ll most likely have to debur the edges of where the ring sits in its place.
23. Lastly, remove the piston pin and take the piston off. Now you’re ready to start the inspection process per your manual.

CRANK, PISTON, AND PRE-HEAD INSPECTION

For the inspection process, I used a feeler gage, straight edge, adjustable dial micrometer (like one used to balance rims), digital inside/outside micrometer (like one used to check rotor thickness) and, of course, the manual for tolerances. I used the feeler gage to check the crank at the bottom and attached the adjustable dial micrometer to check the side to side play of the connecting rod. All inspections for inside and outside diameter were done with my digital micrometer. Although probably not needed, I had a special piston cylinder diameter gage to check top, middle and bottom diameters, which I was able to use from our base auto hobby shop. The digital micrometer could have been used at the top and bottom, but if you have an engine shop full of tools to use, why not use them. I also used the feeler gage to check the ring end gaps. (Note: Before you check your cylinder diameters and ring clearance, rent a cylinder wall honing tool from AutoZone FREE. Then hone the cylinder wall surface just enough to remove the glazing/shiny look.) The straight edge was used to check all flat surfaces.

Of course to check the head assembly, I had to tear it down separately. However before you do so, hold it above a drip pan and pour some carb cleaner or solvent into one intake or exhaust orifice at a time with the head tilted to allow it to remain in that valves area. If the valve surface and seat are good, you shouldn't see any solvent leaking out of it while the spring's tension holds it closed.

Now is a good time to clean all the old gasket stuff off all of the cylinder and head assy. surfaces and check the condition of the O-ring on the case's coolant orifice and the O-ring that's on the metal coolant tube removed from the front of the head earlier.

HEAD & VALVE DISASSEMBLY, INSPECTION, AND REASSEMBLY

1. If you do not have an actual valve spring compressor tool called for in the manual, place a clean thick rag between your work area and your head assembly to keep the valves from moving down under spring pressure.
2. With my modified socket attached to my 1/4" driver, I carefully put pressure on the spring assy. At the same time I had my telescopic magnet inside the socket area to catch the split locking collars as they came loose.
3. Now just remove top collar and spring. (Magnet works great for all this)
4. Pick up the head and carefully remove the valve.
5. Remove the oil seal and then the lower spring collar.
6. Before tearing down the next valve, be sure to put these parts in their own separately marked bag for that number valve and its position within the head.
7. With everything but the valve seats and guides removed, clean the head thoroughly with your many cans of carb cleaner and a tooth brush (or like scrubbing item that won't mar the surface).
8. Inspect all areas called for in the manual.

* The valve stem seals must be replaced, but putting new springs in was an easy choice with over 6yrs of service and the fact that it was only $16 for all 5 new factory springs. If you're not at the minimum valve shim pad yet and you didn't have any leaks when doing the valve seat check, you shouldn't have to worry about the valves and their seats. (When I mention the pad, I mean that as the valve wears on its seat; its stem will move upward requiring a smaller pad to obtain the proper clearance between the lifter and the cam. Thus, once the valve seat has worn to the point of needing the smallest number pad available for proper clearance, the valve seat would have to be replaced.) The only thing left to check would be the proper valve stem guide clearance. Per the manual, mine was spot on spec after 6 yrs.*

9. Reassemble with your new parts while coating each liberally with oil, minus the valve face and seat. Remember that the painted spring ends face up when installing them and that the blue are intake and the greenish/yellow are exhaust. The book will tell you that the end of the spring with the wider spaced windings is the top end, but they must of painted them this way to make it easier for people like myself. Also, remember to put the lower spring collar over the valve stem guide first, before installing the valve stem seal. (Note: If you install the valve first, the collar and seal will go down in place. I used a plastic spray tube from the can of carb cleaner to do this before realizing it.)
10. To get the locking collars in place within the top collar on the spring, I simply placed them down in the collar (small end coned end down) and compressed the spring. They fell in place for the most part, but I did have to use a scribe or small screwdriver to push one or the other in its seat at times. You'll know quickly if their seated properly as you release pressure slowly. If not, try again.
11. Be sure to install any different size shim pads at this time if you need to based of your previous valve clearance measurements and what the manual's chart calls for.
12. Secure the cams down with the caps in their TDC compression position while the head is still removed.
13. Now, re-verify your clearances again. If all are correct, remove the cams and their retaining caps.
14. Here's a few pictures of reassembly to help.













MOTOR REASSEMBLY

1. Install your new base gasket on its cleaned up mating surface. Again fish the chain thru and re-secure.
2. After the cylinder wall has been honed and cleaned thoroughly with carb cleaner, lightly coat it with some oil. Be sure to do the same on the piston skirt, rings, and piston pin hole area.
3. With new rings properly installed on your stock piston or new one per your manuals order and position (MUST BE CORRECT) and one lock ring installed and the piston skirt, install the piston from the bottom of the cylinder with a piston ring compressor tool, which can be rented FREE as well from AutoZone. Just compress the tool onto the piston and rings slowly, to keep the ring's positions from shifting during the process. Also be sure to have the piston facing the right position when installing it into the cylinder. (Exhaust front/Intake rear)
4. You'll need a friend on this step. Have your friend hold the cylinder with the piston in the cylinder at the bottom and just enough to still allow the piston pin to enter thru and connecting rod. You'll have to fish the cam chain thru its slot in the cylinder first and reattach securely. Once that's done, carefully push the piston pin thru and secure it with the new lock ring (holes facing up).
5. Keeping the crank from moving, have your friend carefully slide the cylinder down onto the base gasket and motor. The piston should be at TDC with the cylinder attached to the motor with the one 8mm bolt installed.
6. Wipe off any excess oil from the top of the piston that was pushed up.
7. Install the chain guide back into the notch within the cylinder.
8. With the cylinder and head mating surfaces clean of all oil and old gasket material, place your new head gasket on. You'll know if you have the correct side facing up when the holes align. They won't if you have it upside down. (Note: If you have a friend hold the head assy., you can fish the chain thru it and the gasket at the same time and then secure it tightly for the last time.)
9. Install the Head assy. back on by only securing it with the 4 main large head bolts. Don't torque them down per the manual at this point. Be sure to install the two shorter length bolts on the clutch side and the longer ones on the cam chain side of the head. (Note: I had a bolt get hung up on the base gasket and wouldn't thread in fully the first time. If you notice more than normal thread resistance at first, STOP! I solved this by releasing the one 8mm cylinder bolt and 4 main head bolts and raised the cyl and head assy. up together about a half inch. I then put the head bolts in place while I watch them pass thru the base gasket.)
10. Carefully place the cams, without their caps and half retaining rings, on the head and see if the chain has enough slack to go back over them. If you can't, the cam chain was not fully seated on the crank gear before putting the chain guide back on. Thus, you'll have to remove the head assembly and guide to get the chain fully onto the crank gear with no slack. With the guide installed and someone holding tension on the chain with their finger, you'll know if you have slack if the chain binds when trying to rotate the crank back and forth. If it does bind, just remove the guide and rotate the chain around until it seats fully around the crank gear.
11. With the chain fully seated on the crank gear and secured at the top, go ahead and torque all the Head bolts in proper torque sequence and value per the manual. Here's the pic for the one that needs the crowsfoot.

12. Put some fresh oil on the cam journals and lifter surfaces.
13. Reinstall the decompression mechanism. Yes, I still have one.
14. Reinstall the cams while carefully running the chain back over them. This must be done slowly to ensure that the chain goes on with the crank and each cam in the proper TDC Compression position (you can use my cam chain replacement post as additional reference on this). Additionally, the caps and the half retaining ring must seat properly in place to allow their bolts to properly secure in place. Just take your time!
15. Torque the 8mm cam cap bolts (86in lbs) in a "criss-cross" fashion once the chain is on with the cams and crank at their TDC Compression positions. (Note: The torque is crucial in keeping the cam caps and journals from warping and you must have a 1/4" drive inch/pound torque wrench to torque the center intake cam cap bolts with enough clearance between them and the frame to do so.)
16. Install the cam chain tensioner and then release the tension with a small, skinny flat head screwdriver. You'll notice the chain become tight instantly. Now, just secure the tensioner's cap bolt w/crush washer. (Note: If you've only tightened these 8mm bolts on the tensioner as with those on the side and clutch covers by hand and haven't had a leak or broken bolt, just do what you've been doing with caution. I finally torqued one of these to the proper 86 in/lbs and snapped the head off one. I blame this on myself by probably over torque'ing these bolts to the point of slowly weakening/stretching them. Fortunately with only the head broke off, I was able to back it out with some pliers and replace the bolt. Immediately, I bought my own 1/4" drive 40-200 in/lb torque wrench and now torque everything per the manual.)
17. Install your valve cover, timing and crank plugs on the side cover.
18. Install your Three-way oil line to the motor and head with the torques listed in the manual.
19. Install your coolant lines and radiators (if removed).
20. With a clean steal or new paper oil filter installed, fill the bike to the proper oil level. Also, refill the bikes coolant to its proper level.
21. Cycle the engine a few times by hand with the kick starter to move the oil and coolant around and then check for any leaks.
22. Install your new spark plug and clean air filter.
23. Reinstall all removed hardware listed in Step 1 of the Motor Disassembly.
24. Finally, starter' up.

FINAL NOTES

Aside from the head bolt getting caught up and the cam chain getting bunched up at the crank the first go around, the rebuild went pretty smoothly. Drawing and writing each part position and measurements down on my scratch paper as I went along was a huge help.

As far as after the rebuild goes, I noticed some light noise once it gone running for the first time, but it soon disappeared after the first run. I probably wouldn't have noticed this if I hadn't rebuilt the Head assembly, but I expected at least a few minutes of running for the new valve springs and such to fully settle in. Additionally with the added compression, I noticed a little more popping from the exhaust on decelerations from 1/4 to closed on the throttle than I did before the rebuild. It sounds exactly like most of the supercross bikes coming off their jumps at the races.

Anyway, I know many recent magazines on the shelf have good related articles with pictures on different newer models but hopefully this helps out those trying to keep the older Yamaha Thumpers alive and well.

Well, Good Luck and don't hesitate to post or PM me with any questions!

Brian

[Keetoman393]

Excellent work! Thank you for taking the time to document this with pictures! I moved this to the knowledge base so it will be easier to find. Thanks again!

[Octane]

Originally Posted by DCox

I am wondering if there is a time limit or anything on the patent for the five-valve design. From everything I have read, Yamaha really hit the nail on the head with regard to reliability and I wouldn't be surprised if the other OEMs would like to move in that direction in the future.

Dont count on it. Yamaha is actually moving away from 5-valve heads because 4-valve heads actually have better flow charactaristics. Dont quote me on this, but I think I read that 4-valve heads have more of a vortex effect and atomize the air/fuel charge better in the mid and high end of the rpm range.
The new YZF R1s have 4-valve heads and I wouldnt be surprised to see Yamaha phase-out 5-valve heads in the near future.