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Post by rcq92130 on Oct 31, 2014 18:56:21 GMT -5
The mystery of why the BBK had gone bad (low compression, blowby) are becoming more clear as the pix of the piston and cylinder liner make clear. But I'd very much like to hear from anyone as to WHY this likely happened so the new BBK (just installed) does not suffer the same fate. The piston WAS installed "backwards" (indent for intake valve down & that for exhaust up) - but there does not seem to have been any valve-to piston contact ... and the piston seems to be symmetrical other than the indents, so i doubt being backwards caused any 'wobble'. Pix of piston from 180 degrees apart:  and from the other side  Notice the lower compression ring - pushed and stuck all the way into the grove (and, seemingly, ground down). With great effort i was able to remove this ring - which looked not bad (and the right inner & outer diameter) once removed. But it was indeed jambed in tight into the groove! Here are 2 pix of the cylinder liner--- scored, of course, but only a few that could be felt to the touch:   And here is a pic of the gudgeon pin --- discolored from heat on both ends.  AGAIN -- ALL THIS HAPPENED IN ONLY 600 OR SO MILES. Did NOT run low on oil; did NOT have the incorrect oil in the crankcase. I'd dearly like to not have this happen again, but am not sure WHY this took place. Any experts able to shed light on this? |
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Post by alleyoop on Oct 31, 2014 19:16:01 GMT -5
Aren't you the guy that put the piston in wrong with the Intake side down and your still wondering why the hit the fan. Come on Bro your lucky that is all it did usually more damage is caused when you put a piston in wrong. From the looks of it it was vibrating quite a bit in the jug probably due to the different pressure put on one side of the piston due to being upside down.
Alleyoop
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Post by rcq92130 on Oct 31, 2014 19:36:42 GMT -5
Yes, I said this at the very top of the post.
My question was - and is - if this would cause this kind of damage. The piston appears symmetrical (other than the indents, which are 1/10th of an inch different), so it's not clear why the piston would wobble (unless a valve struck it, which does not look like happened).
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Post by alleyoop on Oct 31, 2014 19:51:18 GMT -5
The Valve might not have hit the piston but the different pressure put on that side from the size of the valve compared to the indent would cause it to vibrate and smack on the sides of the jug. Doesn't take much to have the piston wobble in there even to much play on the crank will cause that . Also that sucker was running really HOT you never see pins discolored like that, blue is danger zone.
Alleyoop
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Post by rcq92130 on Oct 31, 2014 19:53:45 GMT -5
Oh. Thanks! I was hoping that might be the whole problem, but didn't understand why it WOULD be (and the burning on the gudgeon pin made me think of "oil shortage' of some kind.
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Post by rcq92130 on Oct 31, 2014 19:54:39 GMT -5
Hot as in "lean mixture", or hot as in "needs an oil cooler"?
Or just "hot' as in the rings were not conducting heat away from the piston becasue of the damage?
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Post by alleyoop on Oct 31, 2014 21:37:40 GMT -5
Hot because of the imbalance of the piston moving left and right due to being mounted backwards.
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Post by scooter on Oct 31, 2014 22:17:51 GMT -5
Aren't you the guy that put the piston in wrong with the Intake side down and your still wondering why the hit the fan. Come on Bro your lucky that is all it did usually more damage is caused when you put a piston in wrong. From the looks of it it was vibrating quite a bit in the jug probably due to the different pressure put on one side of the piston due to being upside down. Alleyoop Is there any chance he wasn't getting oil to his wrist pin? |
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Post by alleyoop on Oct 31, 2014 22:59:01 GMT -5
And why would he not get any oil to his wrist pin it is a splash system there are no oil passages going up to the pin. Unless maybe he ran it without any oil. So lets just say the boogie man did it and rebuild and hope he does not mess it up again.  |
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Post by scooter on Oct 31, 2014 23:03:59 GMT -5
And why would he not get any oil to his wrist pin it is a splash system there are no oil passages going up to the pin. Unless maybe he ran it without any oil. So lets just say the boogie man did it and rebuild and hope he does not mess it up again. That's what I was asking, as I don't know how it gets oil. I assumed it had a channel in the crankshaft. |
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Post by ccr on Nov 1, 2014 0:07:37 GMT -5
here ya go. this one best matches your description  and if i were to give it my expert opinion i would say the piston was upside down in the cylinder. (i did this myself the very first time i built an engine. mistakes like that are made by everyone on their first try. thankfully i had someone to correct my mistake at that time.) and /or the cylinder head was not torqued down equally |
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Post by rcq92130 on Nov 1, 2014 12:14:04 GMT -5
Thanks, crr
Yeah - as I said at the top the piston WAS in backwards .... and I was HOPING that was all that caused the problem, though I wasn't very confident of that since the piston looked pretty symmetrical (and thus it wasn't clear why it would wobble). But hopefully that's all the problem is and it's now solved.
Just like Scooter said, I was worried about lack of oil delivery after seeing the gudgeon pin and how it was burned - thought maybe it was freezing up and preventing the piston from rocking correctly (and thus hitting the cylinder wall). It was REALLY a surprise when Alley explained there isn't even an oil delivery path within the rod, and the design depends on 'splashing" to get oil to where it needs to be. I'm kind of amazed this gets a good enough supply to those parts that need a constant 'film of oil' ... but I guess it works OK. Kind of amazing.
But, I guess (hopefully) it's only because of the "backwards" installed piston ....
Thinking of doing a write up for others so they can avoid this. I'm not exactly a novice (compared to most) in tearing down / rebuilding engines ... have done so with a V12, a BMW 6, a Ferrari, a Honda Goldwing, a couple of british sports cars ... And (from the markings on the piston) I STILL made this mistake. A better write-up might help someone avoid things like this (I was using only the Scoot Dogs video, which glosses over a lot).
Anyway, thanks much for the info!
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Post by ccr on Nov 1, 2014 13:15:44 GMT -5
the first time i built my engine i used a koso 124cc kit and installed the rings in the wrong order and the piston upside down. there were no markings on the rings and for some reason i thought the arrow on the piston face meant "this way up".
after putting it together and tightening the cylinder head down without a torque wrench i started her up. and it was so much stronger than the original cylinder i gave it a quick 5 minute blip around the block. the morning after there was a whole engine case of oil on the floor beneath it. DOH!
so i took it to my friends shop an we tore it down and figured out my mistakes. after inspecting the connecting rod for play i decided to replace the rod and crank bearings just to be on the safe side. that's one thing you should always do when you bore up an engine that's already got a bit of mileage on it. peace of mind.
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Post by rcq92130 on Nov 1, 2014 13:39:41 GMT -5
Wow! Mine lasted 500 miles or so before biting the dust!
Yeah, in an earlier Thread (2 days ago) I showed photos of the new piston i was about to install - identical to the failed one already in there - and Shannon helped identify I had probably installed the wrong one upside down. Like yours, these 60mm pistons only had an arrow (and on the other side "60"). I didn't interpret the arrow as meaning "Up", but rather pointing to the intake side. Most pistons I've played with are MARKED "in" - so it made sense if they were not THAT explicit they would at least be showing with the arrow which side was 'intake'.
Shannon though the indent on the other side looked slightly larger - thus for the larger intake valve - and sure enough, it's 0.1" larger. Since there was no evidence of contact with the valve, though, I was not comfortable this explained the rapid failure - didn't want to fix this one problem only to find out another thing entirely caused the failure.
Well, anyway, the scooter is back together (this time with the arrow pointing to exhaust) and as soon as the battery is charged will give it a try. Hope the issue is resolved!
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Post by rcq92130 on Nov 1, 2014 13:41:36 GMT -5
ps: probably should ask:
The compression rings were marked "1T" and "2T". I assumed the "2T" was the lower compression ring, marked side up.
Hope that is correct.
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..BAD TO THE BONE..
