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Post by david3336 on Jul 19, 2011 21:59:37 GMT -5
I had posted eariler about a 2008 peace with 60cc not running well. ok it went out today so i tore it down and ordered 47mm bbk head combo, cam and 20mm carb. but when i tore it down the piston was beat up bad edges broke off to top ring what could have cause this i just want to make sure it was not causes by problem with crank before i put 250 bucks of new parts in it any help would be great and do you think i have a good plane to rebuild with the parts listed above all from scrappy. its the bbk kit and head combo and the a9 perf. cam 20mm carb will i need to order jets for carb or will it be jetted already for the build im going with thanks again
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Post by ctss on Jul 19, 2011 22:54:29 GMT -5
I see you are adding octane booster, that may have had something to do with your problem. Another problem I have seen that causes your problem is with where the gas is coming from.
If you are buying your gas from a pump that has three grades, but only one dispenser hose, then over a quart of the gas you are getting is what the last person bought, which is usually regular. I had to rebuild one guys engine that always bought 93 octane, but used this type of pump. He only got just under 1/2 gallon at a time, so all he ever got was 87 octane. A 60cc has way too much compression to use less than 93 octane.
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Post by mainepeace on Jul 19, 2011 23:02:36 GMT -5
Can you post a picture? Sometimes people's descriptions aren't as accurate. It *could* have been slowly destroyed by detonation. It could also have just been a bad piston from the factory that shattered. If the valve contacted the cylinder you most likely would have heard it, and you would see the scoring on the cylinder face, not to mention bent valves, but I've heard of stranger things. At this point it's pure speculation without a good picture.
Greg
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Post by meitianman on Jul 19, 2011 23:09:05 GMT -5
detonation and or excessive heat. picture would really help.
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Post by revets on Jul 19, 2011 23:20:40 GMT -5
Just rebuilt the motor on my 250. Make sure the rings are in the proper groove on the piston and the ring markings turned up. Not sure if your motor has been built before. Check your bore and piston size. Make sure they are in the specs. Put new piston with rings installed into cylinder and check the gap clearance of the rings. Might want to to check the wrist pin and rod. Make sure they are in the specs. Not sure what beat up your piston but make sure you check these things when you rebuild it.
Not much help but I tried.
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Post by noday on Jul 20, 2011 3:21:16 GMT -5
"when i tore it down the piston was beat up bad edges broke off to top ring what could have cause this i just want to make sure it was not causes by problem with crank" this describes what happens when the piston top hits the head. gas additives and detonation in general do not cause piston fracture. a picture would really help. the following pic (from another forum 49ccScoot.com) is of a piston fracture from a 50cc over reved. 49ccscoot.proboards.com/index.cgi?action=gotopost&board=4help&thread=2168&post=33178"It is odd that it happened, expecially only 580ish miles. Oh well, what better excuse do you need to upgrade. BTW, Not sure if I did that to it, but, And it would have been a youtube viral video had we recorded it. But, I was doing a Left side circle wheelie with it, and the throttle got pinned WOT, and it was going at about 20MPH circles around me, Kind of like a playground merry go round, and I was holding it in the air. It was the funniest thing, but it did take quite a slam when I...........simply let go. Also, this scooter is only 3 months old." Nitroman666  another pic from 49ccScoot. piston edge fracture on a BBK build, 83cc... the stock build was 47mm/72cc.... the poster "Speedy" had only 200 miles on this build.  |
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Post by david3336 on Jul 20, 2011 3:57:03 GMT -5
i wish i had a way to put pic up but right now i dont
i didnt use booster on this scooter just bought it a week ago
it has 5000 miles on it
it kinda looks like the top ring might have broke up and cause damage
no marks or damage to bottom of valves like they hit piston
when i took it apart the head and piston had strong burn smell realy bad odor and lots of black hard crusty build up on both parts
piston also had 5 or 6 kinda deep dings in scattered out places on top
hopes this helps i just wanna make sure it was not something going on in the bottom end the (crank) that caused this. other than the 2 or 3 time i heard the noise below, the engine run good strong and quiet.
compression test the other day was only 125 when i broke it down last night it was 85 engine would still idle but would not rev up
looks alot like this 2nd pic but edges or worst
the last 2 times i drove the scoot when going down a big hill if wot i would hear what sounded lilke a bad spark knock til i let off gas like over rev was happening
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Post by noday on Jul 20, 2011 4:51:39 GMT -5
when the RPMs are too high, it slings the piston out too far. the edge of the piston then hits the edge of the head & then causes the edge of the piston to break off if it only hits a little it may cause the piston to crack in the middle. the crack is in line with the wrist pin/holes, like the 1st pic. if there is wear between the rod and crank, that can also allow the piston to go too far and hit the head. here is a link about piston failure. perhaps it will help. www.boosttown.com/engine/piston_damage.pdfyou need to look at the whole piston carefully. ring failure is usually 2nd to lubrication failure. a lube failure will have other scuff wear signs on the piston and the cylinder bore both. contact areas between valve and piston are usually just shiny, not really damaged ( except for the part that breaks) unless the the valves are way out of spec and they probably were not as the engine ran OK for 5K miles. the stem of the valve may be bent and not move freely but otherwise not show a whole lot to a visual inspection. I have seen quite a few GY6 engines fail when going down hills at full throttle. because of the gearing, you push the engine rpms above 8K and something breaks. if the engine oil was not changed as needed, there may be excessive wear of the crank /piston rod bearing. this is a high load and wear area. no easy way to tell or measure in a GY6 unless the rod has obvious excessive play or a clunk. at 5K miles, I would certainly think about replacing the crank as well. the big problem is you don't know how well that engine was maintained & serviced or how hard it was run. |
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Post by mainepeace on Jul 20, 2011 11:32:57 GMT -5
That's some great info noday. Learn something new every day here.
Greg
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Post by noday on Jul 21, 2011 4:49:23 GMT -5
some more discussion.
for 4T engines we seem to rarely concern ourselves about piston protrusion from the cylinder sleeve. the 2T race guys (who rev in the 10K range ) always look at this and often use double base gaskets or even shims at the cylinder base to adjust piston top surface protrusion.
one can easily see in the 2nd pic that there was a lot of piston surface-head contact area. notice the part where there is no carbon ? that build lasted only about 200 miles from the poster's (Speedy) account.
in the 1st pic there is contact area at the bottom left of the piston, notice there is metal showing/missing carbon on both piston surface and in the valve cutout area.. the poster reported a single over rev episode.
part of the problem, if you can call it a problem, is that the small bore GY6 heads have small combustion chambers. there is often a small rim on the head at the edge of the chamber that allows early contact of the piston edge.
the big question is not "does this happen ? " but why in some scooters and not others ?
we know that the rod does not grow in length with rpms. the crank does not change in size.
the weakest link is the wrist pin and piston. perhaps the wrist pin flexes to allow a few thousandths of protrusion.
perhaps there is wear and enlargement of the wrist pin hole in the aluminum piston ?
over all failure mechanisms show us what to not do.
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Post by D-cat on Jul 21, 2011 9:48:42 GMT -5
It doesn't grow directly from rev, it does grow from heat. Not much, but you should pay attention to clearances on a build.
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Post by TERRA NUVO on Jul 21, 2011 11:28:01 GMT -5
noday
your a poet
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Post by ctss on Jul 21, 2011 12:16:50 GMT -5
The piston above with the crack across it hit the spark plug. You can see the indention is in line with the wrist pin, and not the valve relief. It could have been the wrong plug, the crush washer was left off, or worn rod and main bearings combined with high revs. I am posting one picture of a valve dropping, and two of a piston showing what happens when you run 87 octane at WOT in a 60cc engine that has 210 lbs compression.    |
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Post by noday on Jul 21, 2011 15:56:37 GMT -5
thanks for those pics ctss.
yes the cracked piston hit the plug.... but the plug impact did not fracture the piston...
the crack extends from the other side of the piston head & is wider on that edge. the crack does not appear to include the plug impact site.
a lot of failures will show multiple factors contributing to the final outcome.
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