rebuilding topend questions.

11 replies to this topic
  • Dennish87

Posted May 23, 2013 - 12:03 PM


Ok, so since my yz is all tore apart because 5th gear blew, I'm going to rebuild the valves. I don't think I need replace the piston. The previous owner said it was done about 10 hours before I bought the bike and it still looks good. I want to keep the cost down because I'm laid off right now so don't have alot of money coming in. Sorry if the questions are a little noobish, but this is my first 4 stroke.

My questions are:

1. What parts do I need to replace to rebuild the head?

2. The valve clearance was just below minimum when I checked it when it was together. Can I get away with keeping the same valves and putting in smaller shims?

3. How do you get the valve seats out of the head? I've got one valve out but the stem and seat is stuck in the head.

  • Dennish87

Posted May 23, 2013 - 12:12 PM


Sorry I meant to say "how do I get the valve seal out of the head?" I shouldn't need to remove the guide correct?

  • Dennish87

Posted May 23, 2013 - 12:30 PM


Also, my intake valve spring measures 1.60. The service limit is 1.48 and the new valve should measure 1.55. What does this mean to me? Is my spring too far stretched or ok?

  • grayracer513

Posted May 23, 2013 - 12:31 PM


The seats have to be cut out, then a replacement pressed in. Only in very extreme cases are they ever replaced. The valve stem is the long, thin shaft on the valve itself. Maybe you mean the guide, the part the valve stem runs up into in the head. Valve guides are rarely replaced in a YZF, because the design all but eliminates lateral loading on the stem, so there's very little wear on these parts.

There's no reason to automatically assume you need valves just because you had less than specified clearance. In fact, if yours is a 2000 model, it's very unlikely that you do. If you remove the valves, be absolutely sure that you keep them in some kind of order so you'll be able to get them back in their same exact spots when you reassemble the head. The stainless steel valves in the 2000 model can be lapped for a better seal if they are in good condition. The Ti valves in the '01 and '02's CANNOT be lapped. Have a good local mechanic take a look and see what he thinks of the condition of yours.

If you decide you need to replace them, you have the option of using the steel valves from a 2000 instead of the much more expensive titanium ones from the later models, but you have to be sure you use springs from the 2000 as well to counter the extra weight.

Normal parts for a head rebuild are valve springs, and valve guide seals. Valves if necessary. Guides and seats only as needed, and only by a machinist.

  • grayracer513

Posted May 23, 2013 - 12:33 PM


Valve seals are simply pulled off of the guide with a pair of pliers.

  • Dennish87

Posted May 23, 2013 - 01:11 PM


The seal is the rubber part, but what about the copper looking piece. That stays in right? It'll be more obvious when I get the new ones. The only seals I could find for my bike are own which will work but wasn't sure if they make aftermarket. Bike is a yz426 2001


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  • grayracer513

Posted May 23, 2013 - 01:36 PM


If the "copper looking part" is the tension ring around the seal, no, it's part of the new seal. The seals are 4SV-12119-00-00 (intakes) and 33M-12119-00-00 (exhaust), same as the YZ450.

Note that there is a steel seat shim under the valve spring that can fall out once the seals are removed. Don't loose them. Replace them if you do.

  • Dennish87

Posted May 23, 2013 - 02:18 PM


Ok, I see what you mean. Just took them out.

  • Dennish87

Posted May 23, 2013 - 02:23 PM


Is it essential to replace the timing chain at this point?

  • grayracer513

Posted May 23, 2013 - 03:01 PM


I don't know what your chain looks like or when it was last replaced, but it would be sillier than hell not to. A potentially $3000 engine against a $30 chain?

  • hillclimb426

Posted May 23, 2013 - 03:03 PM


Might as well you have it apart and they are cheap

  • Dennish87

Posted May 23, 2013 - 03:57 PM


well, a bit of a potential problem. When I was taking the seals off my pliers got into the inside of the guides a little bit. It was just enough to scratch the corner on the inside of the guide. Is this a catastrophic problem or will they be just fine? I put the valve inside the guide and felt no resistance or dragging and couldn't in any way get it to catch even when the recess for the retainer (on top of the valve) passed through.

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