trex banshee Posted June 18, 2007 Report Posted June 18, 2007 Mine is .042 but i run flat top pistons. What should mine be at? Its a trex topend with 12cc domes, on alky. Quote
David Keith Posted June 18, 2007 Report Posted June 18, 2007 .020 are you kidding me???????????????? That is way to tight. I really don't care how you setup your own engine, But please do not recommend this recipe for disaster to others!!!!!!!!! I cannot stress enough that .020 is way to tight!!!!!!!! Optimum would be right at .040 up to around .058 or so. This is right. You need more than 0.020 inches clearance because when you're engine is turning more than 10,000 rpm. The rod stretches, the piston stretches & the crankshaft actually flexs as well. All of this is why engines need more than 0.020 inches of clearance. Talk to some professional builders & they will verify this for you. There is static clearance & dynamic clearance issues involved here. Just imagine what might happen if the piston starts hitting the head... Quote
majicmike Posted June 18, 2007 Report Posted June 18, 2007 I dont have to imagine. Back in the day I had a 7mm cub built by K&T and they set it up at .028 and .032 and look out that motor ran for 15 min the first time the second time after I cought what happend and had him "fix" it it ran fo 30min the piston hits the head and then the skirt rocks into the side of the cylinder you hope the top of the piston breaks before the skirt. But anyway it hit the head and did not live long Quote
blowit Posted June 19, 2007 Report Posted June 19, 2007 .020 are you kidding me???????????????? That is way to tight. I really don't care how you setup your own engine, But please do not recommend this recipe for disaster to others!!!!!!!!! I cannot stress enough that .020 is way to tight!!!!!!!! Optimum would be right at .040 up to around .058 or so. Man do I get shit for the smallest thing on here!! The .020 is a "for instance" or "example" purposes only. We do complete stress analysis on metals here so we know exactly how much a rod will stretch at a given piston acceleration, mass, and rpm. I have personally seem motors with .020 clearance and do not hit so the assumption that .020 is too tight is here say. I am not going to quote expansion numbers because I doubt you would believe me anyway. Brandon Quote
jeepman380 Posted June 19, 2007 Report Posted June 19, 2007 .035"-.045" is what you should be shooting for. Quote
quad4914 Posted March 25, 2009 Report Posted March 25, 2009 Oh man thanks for the confusion. I'm building a 4mm, machinist by trade, needing to know what to set the squish at & I find this. I was thinking that I read in a book I barrowed a couple years back, performance 2 stroke tuning by A. Grahm Bell, that on a 2 stroke with a hemispherical dome you set the squish in a taper .007 on the outside ring edge tapered up to .022 where it drops into the combustion pocket. Thats the way I remember it anyway. I'm looking so I can be sure as i dont have the money to guess off my memory. I dont think blowit is that far out there. I'm sure flex varies from a stock crank to somthing like a sweet ass twister crank, forged vs cast, bore size has got to come into play too. A smaller bore having less piston rock. I'm just gonna buy the damn book. If you dont, play it safe .037 is the minimum building a n/a 4 stroke Quote
AKheathen Posted March 25, 2009 Report Posted March 25, 2009 haha, just read through that shit without noticing the year. :shoothead: good stuff Quote
'04limited Posted March 25, 2009 Report Posted March 25, 2009 haha, just read through that shit without noticing the year. good stuff me too Quote
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