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  #1  
Old 10-14-2010, 02:14 PM
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Default 83 YZ490 - Why did my Piston break?

I acquired an 83 YZ490 a short time ago. When I got it, it was low on compression, therefore I pulled the head/cylinder to find a badly beaten piston with two cracked rings. The cylinder bore was in fair shape so I honed and installed new wiseco piston/ring set (87.5mm, 0.5 mm over) and gaskets.

The bike ran fine for about 4 rides (1/2 hr ride each). On the fifth ride, all of a sudden there was a pinging sound then the bike immediately died.

After pulling the head/cylinder again, I was shocked to discover one of the piston skirts completely broken off and severe cracks in the piston. See attached pictures of the devestation. Also the rings (at the front of the piston) were ceased into ring grooves.

Can anyone shed some thoughts on what might cause this to occur?

Some additional questions/info:

1) I was using semi-synthetic 2-stoke oil mixed approx. 32-1. Was this too lean?

2) What is proper piston/cylinder bore clearance?? Could the clearance have possibly been to large/small?

Any help would be appreciated.







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Old 10-14-2010, 02:48 PM
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detonation will make the piston rock back and forth a lot. It looks like you had a combination of that and too much clearance. the piston companies usually print their recommendations for that. wiseco has a tech department that answers questions.
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Old 10-14-2010, 02:49 PM
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do you warm your bike up really good?
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Old 10-14-2010, 05:42 PM
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Actually, I don't really warm up bike before taking off, however, I dont hammer i've driven a while? What type of warm up are we talking about. 3-10 mins at start-up on idle?

How do I avoid detonation? timing?
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Old 10-14-2010, 05:53 PM
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Wiseco piston absolutly require a proper warm-up, no if, but or what about it. They expand faster than the cylinder which can cause "cold seizure".
And speaking from experience, after honing the cylinder, did you use a small file to debur the intake and exhaust port? Honing or a rebore do, most of the time, leave the port as sharp as a razor which shave the rings after a very short time, leaving a tiny bit of metal in the ring gap after a while.
Just my $0.02

---------- Post added at 04:53 PM ---------- Previous post was at 04:51 PM ----------

The warm-up I practice, is I start the bike with my boots on and I finish to dress after it is running at idle, 5-6 minutes.
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Old 10-14-2010, 06:24 PM
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From those pictures it looks to me like too much clearance. What does the top of the piston look like? Reason for asking is because I don't like the way that piston looks above the bottom ring. That looks like it was too hot / detonation. That's not cold seized.

As far as warming up a bike these days.... What we ride the most are all liquid cooled. I wait until the top of the left radiator is good and warm before I put one of them in gear. (I know that isn't helping you any.) The two air cooled motors we have (a TT-R230 and a Yamaha 400 Big Bear ATV) I just let run a minute or so. Once they are warm enough they are running well without the choke I go. I don't "hammer" on those motors anyway so I could probably get by with less time. If I were going to ride them harder I'd wait a little longer.
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Old 10-14-2010, 07:30 PM
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Please make sure that you read this post

Yz 490 tips and tricks and the head mod diagram

Then make sure that you have the proper piston clearance & that you chamfer the ports.

Plus on a Wiseco piston you should drill 2 small holes where the exhaust bride contacts the piston. also relive the exhaust bridge.
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Old 10-14-2010, 08:56 PM
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Detonation doesn't show up on the sides of the piston. It is most likely damage from all the bits of aluminum flying around in there. The collapsed ring grooves is probably from the piston rocking enough for it to snag the exhaust port. FWIW my diagnosis is WAY to much clearance. You didnt by chance throw a standard piston into an oversize cylinder? For that piston to fail that quickly....JMO Tdub
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Old 10-15-2010, 01:43 AM
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MAN, I didn't think I would get to do THAT, that fast!
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