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Post by bigkahuna427 on Mar 1, 2012 20:07:41 GMT -5
What should this clearance be on a 2 stroke 50cc? What would be the loosest measurement you could live with on a new piston and cylinder.
You thoughts?
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Post by jmkjr72 on Mar 1, 2012 20:17:23 GMT -5
right from every yamaha service manual for hm 2 strokes
0.03 ~ 0.05 mm (0.0012 ~ 0.0020 in) 0.10 mm
(0.0039 in)
anything more the .08 mm and i would find a diffrent pistion
if you go on the loose side there is less chance of seize but you will have piston slap
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Post by bigkahuna427 on Mar 1, 2012 21:40:01 GMT -5
So what would you say to this.... New piston and cylinder for a chinese scoot. The piston is so loose I really do not need to measure it. I tell them it is really loose. They want me to measure it. I do and I can slip a .010 INCH feeler gauge in there pretty easily. No sense in going higher the thicker the gauge the harder it is to conform to the shape of the cylinder but I am guessing I could go a few thou more. There has been several emails over the last few days but this is what I get.
"Hello,
Unfortunately, we can only go by the measurements of the products on the shelf. The parts on our shelf match to manufacturer spec. In fact, with a tolerance of 0.010 the part we sent you is well within tolerance as well."
We have had quite a few emails thus far and they certainly don't seem to want to make this right. I am ready to throw this thing in the trash and be done with them as it is just not worth my time. Not a small company either. I am trying to be reasonable with these guys giving them every opportunity to make this right. I think you could go with a .010 INCH over sized piston and be right on the money with this cylinder! They have a whole shelf full of these???
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Post by jmkjr72 on Mar 1, 2012 21:48:30 GMT -5
send them the yamaha spec and tell them that it is unaceptable
plain and simple send me a kit that has less the 0.10 mm of piston to wall clearance or give me my money back
to include shipping to and from as they darn well know its out of spec and are trying to sell crap
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Post by bigkahuna427 on Mar 1, 2012 23:04:26 GMT -5
yep well already done that...
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Post by bigkahuna427 on Mar 2, 2012 7:04:56 GMT -5
I really did not have high expectations knowing it was a chinese sourced part. I really hope this guy does not know what he is talking about because this is a company that is probably considered by most to be reputable. I will give them one more try today.
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Post by 90GTVert on Mar 2, 2012 8:55:48 GMT -5
The generic 1E40QMB manual and the Vento manual I have say .060-.070mm is standard and .120mm is the service limit. The Zuma manual I have says .035-.040mm, limit .10mm.
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Post by reveeen on Mar 2, 2012 9:49:48 GMT -5
It all depends on the quality of the piston. A "forged" piston does not expand as much as a "cast" piston, and of course there are different qualities of "cast".
I would be tempted to heat the piston in question with a heat gun, or maybe real carefully with a propane torch (or even 400-450 in an oven) and measure it. *Remembering* a poor quality piston will melt from the application of a propane torch (so be careful).
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Post by bigkahuna427 on Mar 2, 2012 9:56:13 GMT -5
This is .010 inches..... Keep in mind that the feeler gauge is 3/8 in. wide. When you get that thick the feeler gauge does not conform well to the piston the clearance is actually more. I am saying like .014 inches. I just do not want to try to jam the feeler gauge in there and mar the piston, no point in that. Am I correct that this is way excessive? All of my experience tells me that it is, but, this guy says it is not.
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Post by 90GTVert on Mar 2, 2012 11:40:36 GMT -5
Well, .01 inches is more than double any service limits we've posted from manuals.
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Post by bigkahuna427 on Mar 2, 2012 12:04:23 GMT -5
Well I was not putting that engine together that way...
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Post by 90GTVert on Mar 2, 2012 13:39:05 GMT -5
I'd like to see what specs they have that this is well within. Are you certain the seller is clear you mean inches and not mm? I dunno, it's hard dealing with stuff like this sometimes because sellers don't always know everything about what they sell. They don't know if you're some guy that's never seen an engine or a life long technician, and they're prob more used to dealing with the former. I hope you can get something straight.
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vvtr
Scoot Enthusiast
Posts: 243
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Post by vvtr on Mar 2, 2012 14:48:44 GMT -5
tbh on most performance 2t's, the service limit for piston-bore clearance is 0.005", so to say a 10 thou clearance is within tolerance is bollox?  who's tolerance lol! you'd have to be running pretty hot to avoid a lot of piston slap & eventually oval out the bore. the mega bore i just did in my pug i went with a 2 1/2 thou clearance but tbh it's too tight - i'll probably hone it out another thou to 3.5 - 4.0 thou before it does any serious mileage. 4t's run with a film of oil on the bore so you could possibly get away with a larger clearance - but even then, 10 thou is massive. the main issue with cheap pistons is the alloys they are made from - a high quality piston from wiseco or mitaka or jap oem will have a very high silicone content in the alloy and so will expand much more uniform than one made from a low silica alumina alloy, therefore you can get away with running tighter clearances... too tight a clearance on a high silica piston will result in a '4 point seizure' where you get almost symetrical contact piston to bore and minimal damage (if any) the piston will expand in an egg shape rather than perfectly round with a low grade alloy & you are risking a front to back seizure which will usually wreck the bore and or rings! (dont ask how i know lol!) |
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Post by bigkahuna427 on Mar 2, 2012 20:53:57 GMT -5
I really do not understand this into the trash it goes it is a loss. Here is what I was told.
"Yes, .010 inches. Crazy, right? The TB engine is actually an old Suzuki moped architecture, it isn't even a scooter engine like a Minarelli or Morini 2t engine. The TB engine is substantially less efficient than either of those two designs, and makes quite a bit less power to boot."
"I haven't seen a TB60 kit with .002 clearance yet, all of the ones we've ever seen from any of our vendors over the years have all had a large clearance gap."
I think everyone in that thread is referencing the Minarelli engine, hence the Yamaha reference. It is the default 50cc 2t nowadays.
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Post by bigkahuna427 on Mar 2, 2012 20:59:53 GMT -5
Oh, by the way guys just wanted to let you know I got these things to accelerate much faster from a dead stop. Push with both feet!
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