OzVMX Forum
Clubroom => Tech Talk => Topic started by: pancho on April 19, 2012, 11:59:11 am
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The piston from my RT1 on inspection today showed my excuse for hitting the ground so smartly on Sunday.
It is quite apparent to me that the main cause of the lock up would be the use of a worn out piston running inside a cylinder bore which was honed out to a size maitaining the correct minimum clearance. Bearing in mind that a worn piston is no longer the correct shape to accomodate expansion of the heavier parts of the piston.
It was interesting to see various conditions evident.
I used Castrol Synthetic two stroke oil [cost me $27:00 at the local servo, that hurt] at 25:1 mix. The piston is well coated with lube but the plug is not wet and not stressed.
Believe it or not its a Champion N9Yx. which I have found to be most reliable in this and also my XT slider.
There is no sign of overheat under the piston crown.
There are two small sieze spots on the boss area around the gudgeon pin.
Last time it "stuck" year before last I filed the hot spots away keeping in mind that removing too much from this area might interfere with port sealing, but I should have removed more.
Any how I will utilise my new +1.5 piston and get this near 1M o/s bored to suit.
Being no great fan of 2/ s I none the less found it interesting especially after reading the thread on here re two stroke oil ratios effect and performance.
cheers pancho
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If the bore was honed to the "correct miniumum clearence" for the piston if it was old or new it would not matter. On cast iron liners the bore wears more than the piston. Sounds a lot like not enough clearence.
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wally clearence is your freind on the big old 2 stroke bikes for dirt track.when i used to road race a yz400 in a tr3 frame it hadthe max mx ware clearence to start with it never nipped up
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YEp id say the same either you piston is on the piss or your clearance are up the shit. Either way that equals your forked.
Dirty method to check sizes for me is if i can pull it out the top completely you have way more than enought clearance, if its only able to get up 1/4 out way its way too tight.
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Had the crankcase main bearing lock up mid air once on a CR 125 most embarrassing not to often that happens thankfully
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PS.... your oil ratio wont fix tight clearances, but can assist wide ones ;)
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pancho,i wasn't kidding when i suggested you lift the needle,sorta a common problem,on full noise down the back straight,you shut the throttle to set the corner up?,bingo instant seizure,no more cooling fuel mix,instant piston expansion,last lap too,she's (yer beast) cleared itself out previous laps,& is running tad close to the bone,(always jet for the last lap ;)),put piston back in,& try again,(if as you say piston/bores at minium clearance,why waste a bore??),seems odd piston grabbed on pin bosses?(Lozza,feel free to enlighten us :D),unless as you intimate pistons no longer "cam ground",but perfect circle??, :P
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Dunno Mick untill I see a pic ...................around the pin boss is usualy the most stable part of the piston.
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eggsackery!, :P
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back in the early 90's i put a used tt500 piston in my sr500 which has done 60,000kms on that piston and instead of taking piston to bore measures on the final hone after reboring i used top ring min end gap measured in 4 places in the bore so there was no taper etc,maybe that could work for you ;D
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WALLY
wet and dry the high spots off the old piston and wack it back in if you can make sure there is no alloy on the barrel to pick up the rings it will be ok.as i said my yz400 was mx worn out clearences for road racing you know full noise just like nepean . amd put i bigger piolet and lift the nedle as mick says.go to nepean with firko and do some plug chops get it right and we can have a rematch down the straight at nepean see if you can catch my cz this time
jim
454
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If the bore was honed to the "correct miniumum clearence" for the piston if it was old or new it would not matter. On cast iron liners the bore wears more than the piston. Sounds a lot like not enough clearence.
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I just destroyed my last comments by pressing a series of wrong keys, however the thrust of it was- the cam grinding is obviously no more, hence skirt clearance needs to be much more to compensate.
I see this as the main problem.
so more or less--- lots more overall clearance is necessary.
pancho.
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pancho unless u tell us the piston to cyl wall clearance u have know then we really cannot help u.Yamaha specs r 2.5 thou piston clearance for a stock yamaha piston will be different for a wiseco.cheers
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wally listen to me and mick we are dirt track racers dont use mx clearance you will nip it upp all the time.if you have time come over to kirrawee and speek to gerg ball bring your piston and barrel he will fix it
cheers
jim
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I just destroyed my last comments by pressing a series of wrong keys, however the thrust of it was- the cam grinding is obviously no more, hence skirt clearance needs to be much more to compensate.
I see this as the main problem.
so more or less--- lots more overall clearance is necessary.
pancho.
Whoever measured the piston before they honed the cylinder would detiermine the cam on the piston. The clearence at the bottom of the skirt is what needs to be measured. all the ovality is in the top of the piston, the bottom of the piston,(well there is only the front and back) is round.
Mick said the answer already your jetting is the problem.