4Strokes.com Technical: How to Fix a Cracked or Broken Dirt Bike Engine Case by Brian10
From the dealer, the price of a case can be horribly expensive. If the case is shattered, it is a little tougher job. If it is just cracked, it is an easy job.
Shattered or Cracked Dirt Bike/Motorcycle Crankcase Repair
Remove the case, collect all of the pieces, clean everything with carb cleaner or brake cleaner. Take a Dremel or a small hand-held grinder, and V-out all of the edges along the pieces you want to put back together (this provides more surface area for the JB Weld to bond with). To hold all of the pieces in place and back the area to be repaired on the opposite side with a thin sheet of metal. I usually use aluminum so I can shape it to the original shape of the case. Hold this in place by using a couple of pop-rivets. Then get a cookie sheet and turn your wife's oven on to its lowest setting. Place all of the pieces in their respective places, and mix up some J-B Weld. I usually try to coat the inside of the case first then do the outside next. Cover all of the pieces with a thick coating of J-B Weld and place case on the cookie sheet. Place the whole mess in the oven on low temperature. In approximately 30 minutes, the J-B Weld will be cured and the heat makes it flow out into all of the crevices in the case before it sets up. Cool the case, clean up any imperfections on the inside that might interfere with engine internals (I've even re-machined in the oil splash guards that are on the inside of some cases). Now turn the case over and remove the aluminum that you use to hold everything in place, or not. If the aluminum fits really snug, it will just add strength. I try to fit the aluminum and then remove it and mix another batch of J-B Weld and do the outside of the case the same way and bake it also. If I end up using my aluminum patch, I put it over this fix, cure it then J-B Weld over the top of the aluminum. Just be sure to rough the aluminum surface up a little before you try to get the J-B Weld to stick to it. When you are all done, you can actually sand the J-B Weld and re-paint the case. No one will ever know you broke the thing. I perfected this method when a dealer wanted $80 bucks for a new clutch cover on my CR250R ten years ago. Since then, I've probably fixed a dozen cases this way. They have all been as strong or stronger than the stock set up when I was finished, and I've never had one leak yet.
Using J-B Weld, do the same sort of application as above. Clean everything with brake cleaner. Grind the edges of the crack back a little on both the inside and the outside of the case. Heat up the oven, coat with J-B Weld, and bake for 30 minutes. Clean the other side, J-B Weld, and bake 30 minutes. Sand flat, paint, and re-attach the case. Good as new!
J-B Weld flows out and works very well when baked in an oven at low heat. It also speeds the curing process and the stuff gets a hard shiny surface during the baking process. It works really well!
Related Forum Topic
Other Misc Dirt Bike or ATV Topics: Tig Welding Engine Side Cases
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