03 YZ450f cam caught in a journal. Cause?

Hi, I bought a yz450f, it had 80 hours on it since rebuild. I did like 5 hours of riding and then decided to check valve clearances as I was not sure how the bike was maintained. As I took the caps off I noticed these: cap bolts loose, intake cap bent, journals badly scratched overturned cam cog. As I bought the bike really cheap and was not aware of when the head was fully rebuilt I decided to buy a new one. A brand new head for my bike is on its way from the states but I am wondering what was the direct cause of that seizure? As I don't want that to happen with my new one.

These are my bets:

*The guy who sold me the bike told me the oil was replaced every 30hours

*wrong torque on the cap bolts

*overheated engine

*other serious mechanical failures like bad oil pump?? are they prone to failure?

As I drained its oil, it was nearly pitch black, nothing exceptional, paper oil filter was clean.the engine worked, wasn't noisy, checked the oil pressure by loosening the bolt in the head, the oil was pumped. The man who sold me the bike assured me it was rebuilt, he changed the piston, crank, intake valves, cam chain. As I took the head off, I saw a really nice cylinder, piston and a tight conrod which I do not want to replace. My question is would you strip down this engine before installing the head which cost me nearly 900$? I badly want to cut down the costs as the head has already stretched my budget and would like to install it without inspecting the rest as that will perhaps involve bearing and gasket replacement& others. I thought about taking off the right and left side covers, clutch then take a look at the pump and if fine put the head on. My question is would you put the head on without stripping it down?

At this point, it's a little late, but you could probably have repaired the head for under $300 to a good as new condition. Engine Dynamics in Cailifornia has done dozens of them, and any really good small engine machine shop should have been able to do the same thing.

You could, I suppose, use the new one, repair the old, and sell it, but that might not pencil out to much of a profit.

The most common cause of cam seizure is incorrect torque, either by tightening out of sequence, tightening to full torque in a single step, or just over-tightening. A thorough inspection of the oil pump is warranted, however, as is a check of camshaft to head clearance during assembly.

*The guy who sold me the bike told me the oil was replaced every 30hours

That's a problem!

The engine oil also lubricates the transmission. The gears eat the oil and it shears out of viscosity in a hurry. Also, the oil becomes contaminated by the wear of the clutch.

You should change your oil every 10 hours at the outside.

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