high comp piston yx140

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pidasmuz

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Hi I have a question regarding the fitment of a high compression piston to an otherwise stock yx140 engine +( oko26 s38 m96 ) (outer rotor kit ooracing)

This piston I bought from DHZ ( 56mm 13mm pin).

can I fit the piston with no worries or is there anything else I should do?

thanks in advance

Pete
 
Light hone so the rings seat and your good, maybe new gaskets depending how careful you are. Mind you I re used gaskets on my sons 160 and it leaked so its probably worth the 30 odd bucks for a gasket set for piece of mind.
 
yea should have mentioned I have gaskets coming with it.
can I use sandpaper to rough up the cylinder or should I really use a honing tool?
anyone done this mod before and had good results? ( the piston not the sandpaper!)
 
I would be dummying it together with some thin blue tack or clay over the top of the piston,
Assemble the engine, tighten the head bolts, and check the rocker arm to valve clearances.
Turn the engine over by hand 3 or 4 times and then take the head back off.
You need at least 1mm between the valve and piston, check how thick the clay/blue tack is.
 
Piston arrived today. it is just a standard piston!
Called DHZ they told me it is a standard yx140 piston as they come high compression from new.
Bit disappointed as I was expecting compression gains.
I don't think they should advertise them as high comp if they are really just standard.
stripped engine for no reason.
 
wouldn't that end up retarding the cam timing slightly. chain tension issues?
i'm just annoyed. might just end up ordering a proper high comp piston from somewhere else.
 
ok so I stripped the engine down today, honed the bore (looked ok). rebuilt it without a base gasket to up the comp a little, just a thin smear of silicone. cam timing spot on. adjusted valve clearances. spun the engine by hand not issues.
started it and ran it up.
loud tapping noise upper cylinder area. not rocker tap rechecked. not chain rattle.
I don't believe its valves hitting piston.
possibilities: pinking detonation due to upped comp?
new steel head gasket interfering with piston at tdc?
piston skirt hitting crank counter weights at bdc ( I didn't line pistons up next too each other
before fitting, just measured diameter)

there was no abnormal noise before rebuild just a bit smokey
any input would be great
 
I'd be checking the valve clearances again.
Make sure it's at TDC when checking.
Push the kick start lever down until it start's to get hard, then leave a little pressure on it and it will slowly turn a little more.
Now check that the marking on the flywheel lines up with the mark in top of the case.
You need to turn the flywheel clockwise a bit to line it up
Now check the valve/rocker clearances
 
yea checked them again, then tightened them up too much but noise is still there.
set them back to spec.
might be detonation, I'm running it on 91.
I will try again tomorrow with 95-98 see if that makes any diff
seems a bit loud to be just pinking tho...
might pull it down again and fit a base gasket tomorrow if fuel doesn't help
 
you should measure the piston to head/valve clearance before lowering dec height (removing base gasket)
 
Might sound like a silly question, but did you make sure the piston was in the right way around ?
And did you check the squish, the clearance from the outer edge's of the piston to the head ?
 
yea piston right way around:)
didn't measure the squish
stripped back down this arvo, no signs of piston to valve touch, no piston to head marks either.
Found the problem, the steel headgasket wasn't centred on the bore.
the dowels held it in place so that at tdc the edge of the piston made contact with the gasket.
I opened up the dodgy dowel with drill hole to get gasket centred, slammed it back together now it's mint.
ran it up with no loud tapping
no smoking either. a few heat cycles and it'll be ready for a hoon.
it was amazing how loud the noise was from only the slightest interference
 
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