Top end question

6 replies to this topic
  • gonzo

Posted June 09, 2004 - 12:26 AM


Well I have almost 2 years on my top end in my 450. I think it's time for a new one. Anyway how long does it take and how difficult is it? What's the hardest part?

  • Anssi

Posted June 09, 2004 - 07:06 AM


It's a piece of cake. The hardest part is getting the piston clips into place and even that is not too hard. Takes maybe 2-3 hours depending how intense you want to make it.

  • gonzo

Posted June 09, 2004 - 07:52 AM


Ok cool. Do you have to rehone the cylinder?

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  • Anssi

Posted June 09, 2004 - 09:37 AM


The only YZ450F top end I've done was putting a big bore kit on my dad's. The cylinder still had the original honing pattern visible after about 60 hours of riding, but YMMV.

  • Rider_727

Posted June 09, 2004 - 05:03 PM


I just did a tear down on my 03 yz450 and still had the factory hone pattern. Rings were still pretty dam sharp for as many hours that I have on the bike. And no scoring at all. I think it could have gone two or three more years.

  • Rich_Rohrich

Posted June 09, 2004 - 06:10 PM


Ok cool. Do you have to rehone the cylinder?

Diamond honing to a plateau finish is the best method from a ring seal standpoint but you don't have to hone it to get a good seal.
It is worthwhile to deglaze the barrel using a #7447 maroon Scotchbrite pad and some WD-40 or mineral spirits. This will help pull any old burnt oil from the cylinder wall surface and allow the new rings to seal much better. Follow this with a thorough cleaning with mild soap and hot water and you are ready to assemble. :thumbsup:

  • Vanilla_Gorilla

Posted June 10, 2004 - 04:16 AM


you should perform a top end rebuild every 2 seasons at least, of course depending on your amount of use. I use an hour meter and rebuild my top end, piston, valves, and all every 40Hrs of use. I STRONGLY recommend flex honing your cylinder with a ball type hone to remove any glazing and create a FRESH 45 degree crosshatch paternt o improve ring sealing and help the rings break in. Wash with soapy water untill there is no grit left int he cylinder. If your rings feel sharp at the edges, throw them away, they are worn. rings have slightly chamfered edges to reduce friction and aide in oil scraping. Also, if time allows, you should lap your valves, unless you have access to a valve and seat grinder, after which, valve lapping is still recommended. then set your valve clearences with the head off. this is much easier. then assemble your engine and your good to go. :thumbsup:

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