WR450 -06 engine rebuild questions

Hi guys!


Currently rebuilding a friends WR450, and I have a few questions for you YZ experts. Hope it's OK I post here, otherwise please move into the WR page!


Reason for the rebuild was an irritating noise from the upper parts, kind of rattling, following RPM. No loss in performance, no signs of evil in the engine oil. Unknown hours on engine, but I'd guess A LOT!


I've found three things inside the engine upon disassemly that caught my interest:


* Left exhaust valve bucket has some marks on it from the little decompression actuator pin. Light marks on the perimeter of the bucket cam contact surface, no marks in the center of the surface where the cam lobe itself makes contact with the bucket. Normal?


* Massive, and I mean massive wear on the cylinder wall at TDC. I think this is where the noise is coming from, as there would be severe piston slap judging by the wear. Any opinions?


* Some wear in the conrod small end/piston pin. Pin can be wiggled some, feels like the conrod small end bore is slightly elongated in the up/down axis, but OK in the fore/aft axis. Opinions?


Apart from this, the engine seems very nice, never opened before and in great nick.



I am of course replacing the cylinder, piston kit, timing chain, bucket, and crankshaft. Which of these parts are interchangeable with YZ450 parts, and which years are OK in the -06 WR450 engine? 


Which aftermarket parts are recommended, and which to avoid?


Many thanks/


Lasse Svedberg

The marks on the bucket are normal, especially with the WR, as many of them have decompression pins that are squared off on the end, rather than rounded.


Wear beyond .05mm is unserviceable.  You will either need to re-plate the cylinder or replace it.  If the wear appears as a narrow "shadow" right where the rings stop at TDC, it's probably due to wear within the ring grooves of the piston, which lets the rings twist at top and bottom as the piston reverses direction.  Also, the piston may show signs of the ring lands (the area between the ring grooves) has been in contact with the bore, which is a clear indication that the piston is worn out.


No spec given in the manual for the wrist pin bore in the rod, but I would evaluate its condition by first cleaning it up with some Scotch-Brite, then measuring the bore in several places to ensure that it is round and straight within .025mm, or I'd replace it.


All of those parts you listed are the same on either engine.


Use only OEM parts, for the entire job.  The pistons  are often less expensive than aftermarket and are as strong as anything available.  OEM gaskets are just plain better.

Thank you, GR for the quick response!


Would you say the bucket needs replacement, or can it be reused? I guess the little marks from the decomp pin wouldn't be an issue once the engine is fired up?


I've noticed there are Cylinder Works cylinder/piston kits for sale on ebay really cheap, any opinions on these? Which brand of pistons are they using?


Hot Rods cranks fine for these engines?


Which years engines use the same parts (cylinder/piston/crankshaft)?



Use your own best judgment.  Look at the wear on the sides, how deep the dents are, etc.  They don't matter a wit once running, but if it's a high mileage engine already, it might be an idea to replace it just the same in order to avoid a broken bucket later.  That would be a very odd occurrence, but why sign up for it, so to speak?

You are right, sir. I'll get a new one.

No, it will not.  You need parts for the Gen 1 engine: '03-'05 YZ450F.

Thank you!

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