Blown Motor Crankshaft Inspection  By  joaoeu (Read 1,362 times)

Why did you rebuild the engine in the first place ?
Did the ebay kit come with an oil pump ?
Did you hone the cylinder ?
Do you own a tork wrench ?
Did you oil soak the cylinder , pistion, rings and the cam bearings ?
Did you turn the engine over by hand to make sure you were getting oil to the cam ?

Please don't get me wrong here you could have gotten bad parts to start with .

John

Take it all the way down and inspect clean reassemble with good parts.

2nd this

$5 or $10 is not worth cheaping out on , then having to do it again .

I have seen specs somewhere , to check crank clearance . I can't remember which forum I have seen it on or who's tech section otherwise I would post it .

John

Thanks for the suggestions guys.

Yeah, i did blow up the crank pretty good.
I hope the case is still in good condition, otherwise I will ask a friend of mine who has a tig-welder for help.

No dropped valves, thought that doesn't mean it wasn't the valve spring's fault (i think).

The reason why I changed it is because it was a plan for a cheap bike, with a backup plan for a lesson bike for the mechanics.

Observations:
1. Signs of interference of the intake valve with the piston (bottom edge scratched & non-carboned edge)
2. Slight Dark mark on the piston
3. No significant damage on top of the piston
4. Piston completely ruined in the bottom, with rolling pin rod completely severed
5. Cylinder below the gasket level, that protrudes into the crankcase, is at least 3/8 severed
6. Did not see the crank-rod, so i'm assuming it's ruined.

Background
1. Never a rode a bike or scooter in my life
2. Bought "broken" 150cc scooter (lance venice aka bms romans) for $100 as 1st bike on the cheap
3. Plan was to either fix cheap, or at least be a "lesson bike"
4. Was told "engine top" needed replacement, but when I opened, no visible signs of piston or valve damages (remember, i never one of these things before)
5. I went ahead and replaced with $80 ebay engine-top kit, re-using crankshaft & rocker-arm assembly
6. It started rather easily since the first time
7. I always thought it had an excess knocking noise (only confirmed it when I swapped in the good motor)
8. I even re-checked the valve gaps, but always thought there was something wrong with the rocker arm having an excess spacing. In TDC, the rocker arm had a slight drop on the valve, I had to "lift" it up to make the gap measurement.
9. In retrospect, as a GUESS, i think the "new" valve springs were out of spec.

As to some of the objective questions you guys asked of what I did or did not do:
1. Piston Rings Gap: I checked ONE ring, maybe I should have checked all for verification
2. Why Rebuild: Previous owner said it needed a engine-top rebuild, and I was determined to learn the engine mechanics
3. Oil Pump: Kit came with an oil pump, but did not swap them (I thought I needed to remove the flywheel and did not have the tool for that)
4. Cylinder honing: No, I did not hone the cylinder
5. Torque Wrench: No, I only did it by feel. Now I've found my torque wrench
6. Oil Soak: Did not soak neither cylinder, piston, rings or cam bearings. Although I reused the old camshaft. I did oil the rings to make them easier to slip-in.
7. I did not turn the engine over by hand WITH oil in the sump to check oil supply to the cam.

Bad news is, I'm 80% sure I ruined a good used engine.
It was a $100 bike in good registration standing.
I used a newer motor from another $100 bike that had registration penalties.
So I'm still ok being $300 into it, with another set of tires that are over 50%, extra CDI, extra coil, extra battery.

Good news is, it's just making me more curious to dig deeper and dissassemble the crankcase now.