[tjohnst3]

So I bought a 2007 CRF 450R project bike off Craigslist for a good price ($500), complete with an engine that totally exploded (I'll refer to this as engine #1). It also came with a second complete bottom end (engine #2), bought by the seller off of eBay which was in running condition. He never rebuilt the bike, so all I got was several boxes of parts to make a whole bike, plus an extra bottom end, and both a 2007 and 2008 frame.

I'm not sure what happened, but the engine in the bike exploded (according to the seller at high revs while doing a wheelie). Piston turned sideways in cylinder and jammed. Cylinder head beat up by debris, valve fell off and bounced around, piston wedged in cylinder, and the crank rod broke off the piston. It also cracked the left case. Catastrophic engine failure to say the least!

I bought this bike as a project, figuring worst case scenario I could part it out on eBay and at least get my money back. I also have a 2008 frame and plan on rebuilding the bike in that frame, since I have a title for the 2008 frame and no title for the 2007 (someday might want to make street legal - I am in Texas after all).

Anyway, I will post updates of the rebuild as I go.

Does anyone have suggestions on what my best bet is for the most cost effective rebuild?

I'm not sure if I can get the piston out of the cylinder, and if I do -- would it be possible to repair? Probably not.

The cylinder head is definitely junk. I took apart the #2 engine bottom end and I'm evaluating what kind of bottom end repairs to do.

What do you guys think I should do with the bottom end -- new bearings? New crank? Just bearings?

I'll start this post off with the bottom end, and build the engine and bike back together from there and post photos as we go!

http://imgur.com/a/hyYJ9

[JimDirt]

Welcome to the site !!

Wow , there is a lot of devastation in those pictures , it looks like it might be a good idea (if your worried about cost) is to buy everything that is damaged (with the exception of piston/rod/crank , etc) used from eBay , as that cylinder i would not trust even IF you could get that piston out

I would look for a cylinder and the case parts , then go with new , for the crank assembly and piston , Ken here has really nice pre-built heads with trick valve and valve seats , from mild to wild , and/or stock heads , since if you got a used head , you would have to have it re-worked anyway , this will save some extra money and time as well , since all you would need to do is pay for it , and its shipped to your door ready to install , but that is how i would go about it , after looking at the pictures , there does not seem to be much of anything salvageable in that mess , the only other option would be searching eBay for a good price on a complete engine , but the only issue will be no guarantee it will last past first fire up

The first thing you need to do is do a complete inspection of everything damaged and evaluate what is actually salvageable and what is junk , this will be your basis for what can be used and what needs to be replaced with new

[tjohnst3]

Yeah, I'm kind of looking forward to doing a rebuild - that way I know for sure what is in there. Plus a great learning experience!

I think you're right -- I will look around on eBay and probably get a top end ready to bolt on... new head, cylinder, and piston. I'll have to look at the camshaft and see how it looks.

As far as the bottom end goes, I have the #2 engine which the seller says he had bought off of eBay. It came off a running bike. Story goes that his mechanic tore it down for inspection, and that's when he ran out of money.

First question -- do I need to replace the crankshaft? What about the bearings - they feel smooth? Not trying to skimp, but also not trying to unnecessarily spend $500 on a bottom end kit.

[JimDirt]

If your talking about the crank of the engine that disintegrated , your better off with new (Hotrods ,etc) , i would not trust that crank any farther than i could drop it , and anything that was connected to the crank would be questionable at best with that much force being slammed onto everything , i would expect a quick demise of crank , bearings , and its obvious the rod is toast , the cost to replace a rod onto that crank would almost be equivalent to a new crank/rod setup , some things you just dont want to skimp on , or hope that its good , its a major effort to fix if its not , and could even lead to damage like what your already looking at , this would be a sketchy thing to "sort of" repair ...follow ??

If its off the eBay engine , the issue is , you still don't know the history of it , so its still a sketchy thing , it could be good and your fine , or it could be a bomb just waiting to let go just like the one your replacing ..... its your call , but without knowing the history of either engine , i would be skeptical