HOW TO DEAL WITH A CATASTROPHIC YAMAHA YZ250 BLOW-UP

September 26, 2014
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LIVINENGINE4

Engines blow up! It’s a fact. You can’t avoid it. But, you can be prepared with enough knowledge to know what to do when your bike lets you down at the least opportune time. The MXA wrecking crew has a 2007 Yamaha YZ250 that we built as a project bike several years ago. We use it to test parts, mods and aftermarket accessories. Sadly, it blew up.

We didn’t need to call in Scotland Yard to find the culprit in the explosion. The test rider admitted that he ran low-octane pump gas in the bike—although he claimed that he never heard any pinging, only a death rattle over a big jump. Yamaha YZ250s are jetted on the border of pingdom. If you add an aftermarket pipe, ride in sand or run low-octane gas, you run the risk of detonating the top of the piston crown into a piece of shrapnel. When the top of our YZ250 piston chunked, the rings broke, gouged big divots out of the cylinder and proceeded to pepper the head with shotgun-style steel ring fragments. Nothing inside the top end was reusable.

When you break a ring, you waste the piston and the cylinder head…at the very least.

We also gouged the cylinder (above the exhaust port).

AT FIRST, WE WERE GOING TO JUNK IT

At first, we were going to junk the cylinder, head, piston and rings—but then we remembered Millennium Technologies. They had rebuilt our 2006 Yamaha YZ250F project bike a few months ago for our “Restore That Old Dog” story, and we decided to see if they could fix our YZ250.

This is the same cylinder head as above…only with less that $50 of work in it.

Amazingly, Millennium stripped, repaired and replated our damaged YZ250 cylinder for $249.95. Even though the cylinder head was a mess, Millennium milled it down and reshaped the combustion chamber back to its original form—for $49.95.

Our YZ250 piston looked a little worse for the wear.

The stock piston was replaced, along with the rings and clips, for $129.95.

We installed a Vertex Pro Replica piston kit, which came with rings and clips for $129.95. Our last expense was a $36.86 gasket kit. We did the labor ourselves (because a two-stroke is so simple that a monkey with a wrench could fix it nine times out of 10). The total cost of repairing and rebuilding our totally wasted YZ250 engine was $466.77.

WHO TO TALK TO

We shipped the cylinder, head and new piston to Millennium Technologies. Turn-around was one week. You can find out more about Millennium by going to www.mt-llc.com or calling (920) 893-5595.

For more about the Vertex Pro Replica piston kit, go to www.vertexpistons.com.

Cometic Gasket supplied the gasket kit. Go to www.cometic.com or call (800) 752-9850 to find out more.

 

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