Natedawgrebuilds
Member
- Aug 27, 2016
- 23
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Replace the crank. The damage that comes from the cam chain jumping a tooth is significant and expensive. With a crank gear this worn it is a real possibility.
This is a common wear point on the early YZ250F cranks.
Make sure and check the water pump seal and bearing while you have it apart. You don't need a case splitter for these engines, but you will need the tool to hold the inner clutch hob so you can break the center nut loose. Make sure you use a high quality 1/4 " torque wrench when you put the cam caps back together. Getting that part wrong can be expensive.
You'll need a proper inch/lb torque wrench to reassemble the cam caps properly. A ft/lb torque will work for the rest of the engine. I usually use the Precision Instruments brand torque wrenches, but I also have a 1/4 Proto inch/lb wrench that has proven to be very reliable over the years. https://www.amazon.com/Stanley-J6060A-Ratcheting-Micrometer-10-50-Inch/dp/B002FCQGWM
Snap-On and the major tool brands all have good models as well.
Yes the cam caps I'm referring to hold the cams in place in the cylinder heads. They also provide the bearing surface that the cams run on, which is why getting them torqued correctly is so critical. I've seen lots of Yamaha cylinder heads ruined because people didn't get this right. It's not difficult, it just takes some care. Given the age of the engine it's fairly likely that it will need new valves, new springs, valve seals and the valve seats resurfaced. If you end up going this route do yourself a favor and just spend the extra money and get a complete Kibblewhite racing spring kit and Black Diamond Stainless Steel valves.
http://shop.kpmivalvetrain.com/c/dirt_yamaha_wr-yz-250f-2001-2013
I've built hundreds of four-stroke MX cylinder heads using these Kibblewhite components and getting 300 hours out of the setup is fairly common. The OEM parts work very well, ,so if you are a budget you can use them as well as long as you have freshly machined valve seats. Just bear in mind if you ride a lot you'll be going through the process again much sooner with the OEM parts than with the Kibblewhite parts.
There is almost nowhere in the engine that you will be using Loctite. Most bolts that get torqued in the engine need to be clean and lubed prior to torquing. This is especially true for the bolts that hold the barrel and cylinder head to the cases. The OEM service manual does a pretty good job of describing what lube should be used for a specific fastener so try and follow their advice. If a bolt is designed to be torqued dry (no lube) the manual will usually call that out. Either way the bolt and threads need to be as clean as new. I use ARP Ultra Torque lube for most critical bolts when I'm building an engine.
http://arp-bolts.com/p/arpultratorque.php
There is lots of good technical info about how fasteners and torque work at the ARP link above.
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