YZ450 Rebuild question


1 reply to this topic
  • Christopher McClelland

Posted March 29, 2012 - 02:15 PM

#1

I have a 03 yz450f not sure how many hours. She runs like a top. The valves were redone when I got the bike. Im just wondering how to tell when the top end needs to be rebuild and if I would need to split the case and purchase a new crank aswell. Can the cylinder be bored or do you have to buy a new one.

I know how rebuilding motors goes I think.

Pull the head bore the cylinder oversized rings slap everything back together. Will it be fine like that?

how bought a complete overhaul. The above but replacing crank bearings instead of buying a new crank.

How do you rebuild these things without spending 400 on a new crank is my question. Also can I use a compression tester to see when the top end needs to be freshened up.

If I freshen up the top end wouldnt the bottom end explode?

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  • bkoz

Posted April 01, 2012 - 11:12 AM

#2

I have a 03 yz450f not sure how many hours. She runs like a top. The valves were redone when I got the bike. Im just wondering how to tell when the top end needs to be rebuild and if I would need to split the case and purchase a new crank aswell. Can the cylinder be bored or do you have to buy a new one.


If the bike is running fine the only way to inspect the top end is to do a leak down test or by pulling the piston/rings and measuring them. When the top end is off you can also do some measurements on the con rod big end bearing to try and determine it condition.

The cylinder is nikasil coated. You cannot overbore it like old school cylinders. If the cylinder measures out fine and still has a crosshatching pattern it can be reused with no issues. If there is signs of wear you can replace the cylinder (surprisingly cheap), have it re-coated, big bored and/or sleeved and re-coated or just install an aftermarket big bore cylinder.

Pull the head bore the cylinder oversized rings slap everything back together. Will it be fine like that?


Not an option see above.

how bought a complete overhaul. The above but replacing crank bearings instead of buying a new crank.

How do you rebuild these things without spending 400 on a new crank is my question. Also can I use a compression tester to see when the top end needs to be freshened up.


You can have the crank rebuilt with a Hot Rod kit or and OEM con rod kit. This "usually" a cheaper option than buying the entire crank kit. And if your bottom end is apart budget for new trans bearings/seals and the replacement of any worn gears/shifting parts.

A compression test is worthless without disassembling the auto decompression mechanism. And even then it is no where as useful as a leak down test.

If I freshen up the top end wouldnt the bottom end explode?


Not if the bottom end is tight. It sounds like you are unsure of the condition of the engine below the head. It would be worth it to buy a piston/ring kit, seals, cam chain (very important, should be replaced with every piston change) and pull the cylinder off and have it measured/inspected. Also at this time measure the con rod bottom end bearing. If the cylinder and con rod measurements check out and the trans shifts fine and there is no signs of metal in the oil you will probably be ok for a little bit. If you want to do it right and be confident in the bike for a lot of years budget for a new piston, rings, cam chain seals, hot rod con rod kit + crank rebuild, and trans bearings. If your lucky that's all you'll need. You never know what worn gears/shafts/gear shift parts may also need replacing.

The joy of modern 4 strokes. So damn pricey to rebuild.





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