2008 COmplete engine rebuild


I have noticed that there is a wealth of knowledge in the minds and resources of the tt community. I have been looking, researching and reading the threads here regarding my issue at hand and have had no luck finding definitive information. To explain, i have a 2008 YZ450F that started "knocking" while riding a few weeks ago. I was able to limp the bike back to the truck from my "woods" or wilderness riding without any noticeable damage. I have pulled the head, cylinder, engine, etc from the bike and have seen NO noticeable wear or defect that should cause the level of knocking i was hearing. I have since ordered NEW, OEM, Yamaha crank, (rod assy) main bearings, cylinder, piston, all gaskets/seals for complete engine rebuild. I have a manual and am mechanically inclined. My father is very mechanically inclined and even owns a bike shop...focused on Harley's (keep jokes aside) and am curious if there is any thing that i need to know that may be an unwritten or otherwise experience acquirable only things that i need to know. I have yet to split the cases, but was advised by my local Yamaha shop of which are my friends and co"racers" here in Arkansas that my problem will lie in my lower connecting rod bearing. From the topside, i can feel no noticeable play or wear, but was assured that the knock is coming from my crank. I have not been able to inspect tranny as i have been waitin on my flywheel puller to arrive. Please, I am not a novice and do not desire amatuer level responses like check the oil, etc. THis bike has been maintained meticulously had oil, coolant, etc during and previous to the Knock appearing. I am soliciting direct experience, techs, and engineer level information. I have checked spec on piston, rings, etc. The rings are at the end of spec, but not out of spec, the valves are in spec and i will be adjusting upon rebuild anyway. Please send lower end advice as i will be tearing into tomorrow am to put my $1000 worth of OEM parts in there!

Thanks to all who can contribute and i hope this information will aid another down the line.:ride:

O, cam chain and guides, etc.

Here is a quote from one of Gray's posts to a similar issue.

You might check two things:

* That the crankshaft primary drive gear bolt is tight.

* That there is not an excessive amount of rotational play in the cushion unit of the driven gear assembly (basket).

A lot of noises can come from the cam chain... they are one of the weakest links in the YZF engine. You didn't post how many hours are on the bike, but I wouldn't think you would be ready for a crank yet. Typically when the crank lets go, it will lock up before you can limp it anywhere.

usually the big end con-rod bearing does go bad but does not knock, it seizes up and stops the bike. knocking could be in order of most likely: piston slap, camchain/guides/tensioner, clutch basket, or primary drive/clutch basket gears. if its from the right side pull clutch cover and check there. if from the left check cam chain and guides. ps i am a yamaha gold cert tech with 10yrs in a major yamaha dealer. IMO

I am in the same boat with my 07 YZ450. I was out riding last week and the usualy loud engine got a little louder and started knocking. I had the valves adjusted but the knocking is still there. My local garage mech says its the piston so I am attempting a rebuild. Wish me luck.

O, cam chain and guides, etc.

Ordered these as well...

Here is a quote from one of Gray's posts to a similar issue.

You might check two things:

* That the crankshaft primary drive gear bolt is tight.

* That there is not an excessive amount of rotational play in the cushion unit of the driven gear assembly (basket).

The CS primary gear bolt was tight

The clutch unit looked good and had no play upon removal.

usually the big end con-rod bearing does go bad but does not knock, it seizes up and stops the bike. knocking could be in order of most likely: piston slap, camchain/guides/tensioner, clutch basket, or primary drive/clutch basket gears. if its from the right side pull clutch cover and check there. if from the left check cam chain and guides. ps i am a yamaha gold cert tech with 10yrs in a major yamaha dealer. IMO

Thanks man,

I split the cases over the weekend and have not been able to post back. I found some new stuff that is bad! I found that all of the tranny bearings are "bad" as they are rough and feel they need replacing. They have not given out yet, but are on order to be replaced. I did not suspect that the bike needed a crank either, but was advised by most reliable source that it would need replacing. I got a new one for $286 so i'm not terribly concerned if i have an extra laying around now. As for the piston slap/cam chain, i am going to go with this as the culprit after having disassembled every last piece of the bike. I will post an update when my tranny bearings arrive. They are supposed to be here wednesday and i am trying to assemble so i can race the AHSCS opener in Texarkana this weekend. Thanks again for the information.


Just curious. How many hours on the bike and what type of oil were you running? Oil change intervals? I am not questioning your maintenance schedule by any means. Just wondering if mine is sufficient.

I have logged just over 100 hours since i put my hour meter on, plus about that prior. At the most it has 200hrs on it. I run the best Motul you can buy (double ester) i like it cause it's green! haha! i change oil every race, so once a month or so. I practice all month and then change before race and then practice all month and change before race...so goes the cycle. Roughly every 5-6 hours. I run a PROFLow stainless screen filter and clean air filter same as oil depending the conditions...here there are primarily rocks, so no dirt/dust really accumulates.

Proflo is an oil filter. I didnt know if that sounded clear or not...

Check your stator screws, one of those assembly workers running the torque gun must have been dosing off when he assembled mine...

I had 20 hours on my '08 when I noticed a clicking sound while I was riding. It took a while to diagnose but what had happened was one of the screws holding the stator inside the left side cover had backed out and was chewed up by the flywheel and spit into the bottom end. It took a bunch of the beige colored epoxy on the stator with it. I split the cases and luckily there was no damage to my bottom end. It only ended up costing me a set of gaskets and a 2 weeks off my bike.


Yeah, I have all of that apart now and will torque and locktite them upon assembly. I found out today that my tranny bearings will be next tuesday getting here. I will have to settle for assy the frame and components. I am sending suspension off to get revalved by Roy at PRO Action in FortSmith so that will be out as well....good greif.

Alright, thanks guys. I have been hit and miss on my rebuild due to the level of teardown i did on the entire bike. I've been waiting on things. I sent the suspension off, waited there, split the cases and decided to replace all of the tranny bearings...waited there, etc.

I finally reassembled the cases and new crank this weekend. I was an easy process short of remembering how things went and consulting the owners/service manual. There were parts where i actually had to stop and think, but then again, i dont have a PDF memory! haha! The tranny felt very good going together, the cases are sealed well, all seals and gaskets replaced and inspected. I will try to post pics, but up until this point, it would have been pics of pieces of dirtbikes.

Any updates???


As of saturday night at 11:35pm i completed the build of the bike. The list is as follows..

1. Replaced all engine bearings, mains, counter balancer, water pump, etc and seals coinciding.

2. replaced all tranny bearings except for clutch pull lever on case

3. Replaced crank/CRod assy

4. Replaced piston, rings, wristpin, and cylinder

5. replaced all gaskets, all seals, etc.

6. Checked valves for clearances all within spec

7. Pro Action woods valving suspension front and rear,

8. new 6.6 rear eibach spring/seals bushings fluid in forks

9. all new plastic

10. New GYTR oring chain

11. New sprockets 14/50

12. New Bridgestone M403-404's on excel 18" rear

13. new graphics

14. New Ohlins SD2 stabilizer

15. New fluidyne radiators with MSR aluminum guards,

16. MSR kickstand

17. MSR rear disc guard

18. Motul brake fluid swap

19. Motul 300v double ester oil (after yamalube breakin)

20. New chain rub blocks and sliders MSR

21. Fastway bark busters probend w/ bar clamp mounts

22. All new swingarm/swingarm linkage and steering stem bearings

As far as the motor is concerned, I basically replaced wear parts. The knock that originally initiated this teardown was diagnosed to be piston slap due to wear on piston and cylinder. The knock is gone. everything shifts, runs, pulls, twists, etc as good as when new. With the addition of the suspension and the steering stabilizer, she is very stable and holds lines better than any bike i've ridden to date. I am very happy with my setup. I want to post pics and will try later tonight

did you wreck or are those blisters??? ouch

Blistered the hell outta my hands first race back!

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