2008 Rebuild - Crank/Advice?

[Wiz636]

2008 YZ450, runs great and I maintain it very well. OIl changed at 10 hours max, usually at eight hours, fresh airfilter every ride. Bike has 452 hours on the meter. I put a new piston & rings in it about 200 hours ago just for the heck of it. I don't moto the bike but I ride/race offroad pretty hard. About the only issue with the motor is that occasionally when kick starting it will not build pressure when trying to find the compression stroke with the kick starter...so maybe a sticky valve? Valves clearances are all in spec.

 

So my questions are 1) should I plan on replacing the valves, and 2) should I start thinking about rebuilding the bottom end just as preventive maintenance?

 

Can anybody comment on any of the aftermarket crank assemblies (Hot Rod, Wiseco, etc) or should I just stick with OEM?

 

I've read through this thread https://www.thumpertalk.com/topic/1087950-2008-yz450-locked-her-up/?hl= crank  shaft and it looks like it contains a pretty good parts list.

 

Also, can somebody recommend a good source for OEM parts?

Edited December 8, 2014 by Wiz636

[Jim813]

My 08 did the same thing with the compression lacking sporadically. I don't recall when it started but I went through a full rebuild with the same issue, although it didn't seem to effect me much.

How long do you plan to keep the bike? With a full rebuild your looking at a sizable investment. I'd make sure it is worth the cost to rebuild, and unfortunately you won't get most/any of that money back if you try to sell after rebuilding.

I had 375 hours on my 08. I went through four pistons, one complete head, and one crank rebuild. I rebuild the stock crank with a rod kit and the help of a friend, but it can be a pain. I had good luck with OEM parts and got a good deal on them, and didn't see any reason to go with anything else. I've had good luck with NCY Motorsports, motosport, and cheap cycle parts when ordering OEM parts.

[grayracer513]

My choice on the crank would be 1) new OEM, 2) rebuild with OEM parts, and 3) Hot Rods.  And no, not the other one.

 

If the piston is the original, it may be the source of the loss of cranking compression.  The ring grooves loosen up and let the rings twist.  When you are evaluating the cylinder, run a flex hone through it quickly first, and look for "shadows" at the top of the stroke that didn't clean up.  These are from the rings biting and twisting as the piston reverses direction.   if they don't go away in under 15 seconds with a silicone carbide brush hone, you probably need to replace or replate it.  Have it measured in either case.

 

Your valves are in spec, so the only question is, how much have they ever been adjusted?  If never, or only once, and not more than one shim size, they're probably alright.  Remove them and clean the carbon from the stems and the back of the valve head, but stay off the sealing face.  Look for visible wear on the faces and replace any that have a continuous bright wear stripe around the face, or any even barely visible ridge on the edge.  In the picture below, the near valve is OK, the far one is well beyond usable.  I replaced only the center intake on mine when I did the top.

 

[Wiz636]

Thanks for the feedback. I'm on the second piston/rings and they have about 200 hours on them. The loss of cranking compression is intermittent...I only experience it about 30% of the time. Could this still indicate rings?  Also, the valves have never been adjusted which is pretty amazing to me.

 

I love the bike and plan on keeping it for the foreseeable future so I think I will just go through everything.

 

Heading to Baja with it in February and I don't want any major mechanical issues to happen while I'm there!

[grayracer513]

Thanks for the feedback. I'm on the second piston/rings and they have about 200 hours on them. The loss of cranking compression is intermittent...I only experience it about 30% of the time. Could this still indicate rings?  Also, the valves have never been adjusted which is pretty amazing to me.

 

 

The loss of compression that you describe is more often just carbon on the valve stems, or sometimes crowded up against the valve face so that the valve doesn't quite come all the way shut under spring pressure alone at low speed.  Cleaning them up is usually all that's needed.

[sckrocodile]

The easiest way to check your rings is to pull compression on the cylinder.. Get the bike warmed up and then put a compression tester on the cylinder and give it a few kicks.. then pull the tester out, pour in about a tablespoons worth of oil, put the tester back on.  If you see a very large increase in pressure then you know your rings are in need of replacing.  Iv used this method for years.  With that many hours id probably start gathering parts for a lower rebuild though.  I am in the process of doing a 450 and have gotten a wiseco upper and lower kit with a jug. I havnt gotten it all together but everything that came with it has been perfectly in spec and looks great Ill have it in the bike in about a week.

[grayracer513]

Oiling the cylinder will always show an increase in compression (especially if you use too much), and conventional compression testing is almost totally invalid on high compression singles.  A leak down test is the only accurate way to check.

[n16ht5]

I got a set of hot rods crank bearings but when they came in the package said "made in tawain..." needless to say I am never using anything hot rods. I used OEM crank bearings + crank, wiseco 13.5:1 piston. 

[grayracer513]

You would find, if you researched it, that a number of the components of your Japanese bike were made by vendors in Taiwan. Most Taiwanese manufacturers hold quality at Japanese levels, and those supplying to Japanese manufacturers are required to.

China is another thing.

[n16ht5]

Yes, but not crank bearings!!! I'll stick to Japanese bearings thank you

[camman]

I got a set of hot rods crank bearings but when they came in the package said "made in tawain..." needless to say I am never using anything hot rods. I used OEM crank bearings + crank, wiseco 13.5:1 piston.

Those bearings are made at NTN Japan taiwan facility. Best factory I've ever seen. You need to recognize that most of your bike is manufactured outside Japan. That's why it's called a world economy.