HeadSmess
Well-Known Member
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- Joined
- May 17, 2010
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Because in Australia things are horribly expensive to manufacture - there is no real competition to reduce prices, unlike Asian countries.
Just to plate a Chinese 47mm cylinder bore with Nicasil will cost me $600 at my local plating shop.
^))????
hmm. you cant shout in numerals, so...SIX HUNDRED DOLLARS???? cant remember who i spoke to, but they said 180... that was for a BARE cylinder, but...so maybe they charge lots because of the chrome removal?
anyway...leakage.
the piston is 46.8mm
the bore is 47mm.
the clearance between piston and bore is then 0.1mm
theres 5mm before the first ring... so 5X0.1= 0.5 square millimetres as an orifice before it hits the first ring. (double this because usually the top land has slightly more clearance than the skirt etc...) (edit...theres one major flaw in this part, that i just realised.... circumference. so... erm... (PIx23.5x23.5)-(PIx23.4x23.4) = 1735-1720 = 15 square mm is the area of the gap between piston and bore. thats still only an orifice of 3mmx5mm say...)
the gap, ok...huge, at 1.9mm. cant say i have ever worried about measuring the gap myself. due to piston and bore clearance, maximum area of this gap is still... 1.9mmx0.1mm= 0.19 square millimetres. (so now we have a 15mm orifice restricted down to a 0.2mm orifice)
the gas then has to travel from this gap, through the passage way between the two rings from one locating pin to the other... still restricted to 0.1mm x the distance between rings. say 5mm. so, once again... an orifice of half a square millimetre, approximately an inch long (love mixing my measurements) then another gap in the second ring, of that previously calculated figure... 0.19 square mm.
not terribly large, is it? and the pressure is substantially reduced just by the first land, before it even hits a ring or gap!
i agree, new rings make an engine sit up and behave. the bore should be rehoned, the rings bed in nice and properly, and hey...its just like a new engine again. even if the bore was oversized, meaning the end gap may have been larger than specified when new...
an engine with worn out rings will be hard to start...its revolving rather slowly, so theres a lot of leakage.
but when spun fast enough they still start, and run...albeit, woefully.
usually, by this point...the bore is glazed, the rings arent sealing the same, you get leakage around the whole circumference...
no, i always liked the idea of thin rings, but now i had a good think about it...id prefer the wide dykes ring before using thin rings, if flutter was my major concern. which it is
experience on one or two engines, none of them being full racebred things such as a YZ250, NSR500 or the rotax 125 in a gokart, too mention a few... just a good old HT. like reading "spot goes to the zoo" and then being an expert on pachyderms and dromedaries...
and one more rant!!!!
you tell everyone that the piston and gudgeon pin are too heavy in the first place, use a hollow gudgeon pin, then tell us this minarelli piston and gudgeon combo weighs an extra 10 grams!
titanium just isnt going to make that much difference, i suspect... unless it weighs only 10 grams?
be better to drill the crank, wouldnt we? and live with the other (secondary) vibration caused by doing so?
regards making new cylinders altogether...we are all familiar with the lousy ports as designed on the 66. (isnt that what half this site is based on? http://www.dragonfly75.com/motorbike/index.html ) such as the piston never opening the exhaust completely. the tiny 16-17mm cross section intake port. the woefully finished and angled transfer ports, the lack of material in the stud area, yarda yarda, blah de blah blah... rhubarb rhubarb
patternmaker... roughly $5000 dollars to make a useful pattern. to quote..."how many do you want to make, and how many will you sell?"
foundry...about $20 to 150 depending on how many required, and reject ratio.
most foundries in australia are scared by anything less than 3mm thick! despite hobby machinists doing the same thing in their backyards for YEARS!
then the machining work... plating. stocking, advertising, shipping... its amazing these engines can be bought for under $150 at all!
gee im rude
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