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Post by 90GTVert on Aug 28, 2016 18:29:28 GMT -5
Hi I've been watching this thread with a lot of interest as it evolves into the weight of a piston and how to reduce it. Somewhere on youtube there is a video showing the weight of the three pistons for a GY6 50cc, a 39mm is 63 gram, a 47mm is 84 grams and a 50mm is 95 grams. STG weighed the 50cc Airsal piston and it was 66.5 grams, not sure if it was weighed the same way or not but if close I gotta believe it isn't much more than a 47mm piston weight. farmer54
All pistons won't be created equal. Here are a few I weighed. I don't recall which 50mm I weighed, but I have an Airsal 50mm BBK here so I can put it on the same scale as shown below and let you know. 47mm was an NCY. 52.4mm is Naraku. If the 50mm isn't the Airsal, it's a generic Chinese kit with no branding.  |
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Post by 90GTVert on Aug 28, 2016 18:44:59 GMT -5
The Airsal was still sealed so it definitely wasn't what I weighed. This Airsal piston is looking better and better...  |
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Deleted
Deleted Member
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Post by Deleted on Aug 28, 2016 19:06:24 GMT -5
The Airsal was still sealed so it definitely wasn't what I weighed. This Airsal piston is looking better and better... So Brent, which Airsal piston was that? 72mm? 50mm? Man, that is about the same weight as stock right? There is more to just the weight of the piston on the crank as we know, the combustion impulses will be bigger as well but man oh man, this is what I was trying to get at. Surely using this piston would be a great advantage over the heavier versions right? I had never heard of these. Are they expensive? Super expensive? Thanks for showing this. Bill |
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Post by 90GTVert on Aug 28, 2016 20:28:44 GMT -5
50mm Airsal piston. Mine is from an entire Airsal BBK, but it looks the same as the piston being sold on it's own. I bought the whole BBK on sale for $50 a couple of years ago. Now the piston costs $41. www.ebay.com/itm/Airsal-50mm-Racing-Piston-139QMB-GY6-Scooter-Engine-Performance-/182197560625?_trksid=p2352135.m2548.l4275It should be closer to the same balance of a stock piston/crank. It should reduce stress from accelerating and decelerating the reciprocating mass, and should improve reliability of bottom end components. Some say lightweight pistons will make the engine respond more quickly. It may, but I'd expect rotating mass like the flywheel to have a greater effect there. Maybe it frees up some energy that isn't being wasting on moving a heavier piston to make more power, or maybe that is what would equate to better response. At any rate, as long as it's sturdy enough, it should only be beneficial to reduce weight. |
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Post by humanshield on Sept 22, 2016 21:07:48 GMT -5
Almost 30 grams is a huge difference.
The Airsal piston could make quite a difference in how long a 50mm BBK lasts.
The dome might possibly need some work to lower compression on smaller valve heads tho.
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smokeymcpot
Scoot Junior
Got A Scooter Heart-ache
Posts: 10 
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Post by smokeymcpot on Sept 24, 2016 1:14:36 GMT -5
Hello guys +1 for me on failure after install...I installed a 50mm bbk on my jonway 50cc,I must put this out there that I'm a noob to the gy6 engine, so I was installing the kit with very little tools & knowledge, sad to admit, but I like to say the poor MUST Improvise.
I did the install, ran the scoot umm idk 3min.and it died there. I did WOT but I wonder if that was the issue. Only 340m on the engine.I've been working on it lately but I dont know if my cylinder is wOrth replacing,so I made a HOMEMADE HONE,with a baby bottle brush as drill attachment and wrapped 3M 220grit around it. used hot glue to keep it together.I got compression now (since that has been issue), but damn no spark (always had spark)
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Post by humanshield on Sept 24, 2016 12:42:39 GMT -5
Hello guys +1 for me on failure after install...I installed a 50mm bbk on my jonway 50cc,I must put this out there that I'm a noob to the gy6 engine, so I was installing the kit with very little tools & knowledge, sad to admit, but I like to say the poor MUST Improvise. I did the install, ran the scoot umm idk 3min.and it died there. I did WOT but I wonder if that was the issue. Only 340m on the engine.I've been working on it lately but I dont know if my cylinder is wOrth replacing,so I made a HOMEMADE HONE,with a baby bottle brush as drill attachment and wrapped 3M 220grit around it. used hot glue to keep it together.I got compression now (since that has been issue), but damn no spark (always had spark) Sorry you're having issues. In this case, I would attribute this more to user error than a problem from the kit itself. Probably would have happened regardless. Practice makes perfect. Read the section here on engine building. Take your time. Ask lots of questions. |
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smokeymcpot
Scoot Junior
Got A Scooter Heart-ache
Posts: 10
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Post by smokeymcpot on Sept 24, 2016 15:50:27 GMT -5
Thanx I have to admit I always figured that,due to lack of knowledge and basic principles of a gy6 engine. I have been using my trial and error process to get everything together. GOING SLOWLY BUT SURELY.
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Post by humanshield on Sept 24, 2016 15:58:29 GMT -5
Thanx I have to admit I always figured that,due to lack of knowledge and basic principles of a gy6 engine. I have been using my trial and error process to get everything together. GOING SLOWLY BUT SURELY. No worries. Lots of people here who've BTDT. Ask...ask...ask....no question is a bad question Starting with base gasket, then the jug, then the head, rocker assembly etc. Get each step right and you'll succeed. |
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Post by humanshield on Sept 24, 2016 16:13:46 GMT -5
By the way, when you get it all done and running right, you'll be thrilled.
I have a 50mm BBK, A9 cam, 20mm carb and 21mm bvh and my scoot is a blast to ride.
But be patient. Even experienced builders have to go through the process of tuning it after it's done and that can be a pain but just ask and you'll get help
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Post by humanshield on Oct 18, 2016 9:11:11 GMT -5
A potentially new piece of information surfaced lately that "Might" explain some of the problems some people had with early failures of 50mm BBKs.
Of course, the heavier piston is a factor but some people seem to have far a far better experience. Why?
Recently in another thread someone put together a 50mm BBK and installed a stock 5cc head. In other words, they did not use the 8cc 21mm valve head such as came with my 50mm BBK.
That means their compression was probably too high. I would imagine over 230psi. I haven't verified this but it makes sense. All that compression would make it faster....for a short time. But all that compression also generates a lot of heat and puts a lot of strain on the internals.
So while this is just speculation at this point, it's something to look out for.
If you're installing a 50mm BBK, you need to get the 5cc volume, 21mm big valve head to go with it or otherwise lower the compression a bit. 180 - 200psi is more than enough.
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Post by 90GTVert on Oct 20, 2016 10:57:23 GMT -5
I spoke with the only big local (hour away) shop 5 or more years ago briefly when I was first trying to investigate the 50mm failures. They said that they used to install 50mm kits only, but eventually learned that they had to sell and install big bore heads along with them because so many came back with failures and signs of overheating. Since changing to that strategy they said they had less trouble.
I don't really see that as a huge revelation. Compression gets ridiculous if you keep a stock head on some of the bigger bores. You could potentially move into compression ratios like pro-stock drag engines (~15:1-16:1). These little air cooled engines on pump gas aren't going to take that and their weak starting systems probably don't like it much either.
It's good info, but not the reason behind so many failures. Too many have put together engines with larger heads or modified the chambers themselves and still had failures. The shop reported success, but they aren't going to tell me if they have engines dying currently from their mods. They are in a college town where mostly students ride and go a mile or so back and forth. I suspect it would take quite a while to see many with even 3,000 miles on them from students that aren't there the whole year. They went out of business as well, so they didn't have multiple years after this to find out if it really worked for them long term. It sounds like they just knew that scoots weren't coming right back with issues.
Still, good things to keep in mind are compression ratio and piston weight. A lighter piston and reasonable compression ratio should improve longevity greatly compared to a heavy piston and ultra-high compression.
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Post by tortoise2 on Oct 20, 2016 11:41:23 GMT -5
Wonder if a "ballpark" static balance could be achieved on a crankshaft/rod/piston assembly by attaching the woodruff-key-indexed flywheel and drilling the heavy-flywheel-point to remove weight? |
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Post by humanshield on Oct 20, 2016 17:35:10 GMT -5
From what I've read (I think) Installing a heavier piston causes more force both on the inward and outward directions of travel vs stock. So to counter balance the heavier piston going outward, you would actually need more weight on the opposite outward lobe of the crankshaft counterbalance. No? You can't really balance the force of the extra weight out, just dynamically balance the running engine (as I understand) Bottom line though is even if you do, you're still adding stress to the bearings that were designed for small forces. Bearings need a certain amount of surface area to handle the expected loads. Engine balancing is pretty darn complicated.... Balancing Engines linkCould you please read and comprehend it all and then come back and explain it all to me in English?  |
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fergulious
Scoot Member
Posts: 59
Location: Thomasville NC
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Post by fergulious on Oct 21, 2016 10:42:54 GMT -5
I installed my 50big bore cyclinder last night, i was very surprised how well it does speed is great way more than i thought it would have. Hopefully i can make mine last 5k plus miles.
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