why is my engine blown

[(F5)]

After making sure the crank is still true. Polish the bearing journals too.

[Neil E.]

I'm sure you guys are right on the temperature. I'm just used to having the oven at 350*F for everything. How else do you know it's done unless it's smoking when you open the door? (I'm single)

[grachal]

So today I slapt it back together, and took the bike for a quick spin. Runs like it used to, but I have a bad feeling about the motor.
When driving I hear a slight whine. Comparable to what a lengthened timing chain would sound like.
At a standstill I can't hear it, and I can't tell where it comes from exactly.
Main bearings must be fine now. They are not loaded up or anything, crank spins super easy and freely.

[Jacob 'Berg]

Originally Posted by (F5)

After making sure the crank is still true. Polish the bearing journals too.

+1 on checking the crank.

I would start looking for the root cause of the main bearing failures, rather than repeating the same steps and expecting a different result.

Here are some possible causes for main bearing failures:

Dirt 45.4%
Misassembly 12.8%
Misalignment 12.6%
Insufficient lubrication 11.4%
Overloading 8.1%
Corrosion 3.7%
Improper Journal Finish 3.2%
Other 2.8%

[grachal]

I did not have the possibility to properly check if the crank is still true with a gauge, but if it was not, you would expect it to slightly bind when turning it by hand in the assembled case, right? The journals were polished, as suggested.
Thank you for the possible main bearing failures. I guess I'll start looking at some exploded drawings, to confirm proper assembly

[grachal]

I did not find anything wrong and I do not feel like pulling the motor back apart. I'll just send it till she blows, lets see
DSC_002345.jpg

[grachal]

One more question. Bike ran fine the last 400 kilometers, but I noticed, that my coolant turned black. It looks like dissolved aluminium particles in there if i would need to guess. Any idea what this comes from? I have no water in the crankcase or in the oil. But the water level got slightly less, about 150ml since the rebuild. Maybe it burns the coolant due to a leaky head gasket and this again leads to carbon in the water

[Zman]

Could it be the plastic impeller gear ?

[grachal]

Thanks for your suggestion. I checked the impeller and it is good. Flushed the radiators and it showed that it was not as bad as it looked before. Clear water with some dark sediment in it.
Maybe some Oxidation residue from the radiators (cheap chinese ones) or the engine itself?
I refilled the system and i'll check again later.
But the next thing that concerns me is clutch rattle getting worse. The sound does not change when actuating the clutch and also no difference if its in gear or not.

All in all not a very pleasing experiece so far. Never had that amount of uncomfortable events with my KTM SX