on mine ive changed the profile of the exhaust port a little, from an oval shape (see port map thread) to an almost square with 4mm radius corners - ive done away with the arched roof of the port and made it much flatter, with a width of 30mm.
if you stick a finger in the port and feel the inside, there is a definate rched step on the floor as well - remove it.
there is no point opening out the manifold end of the port unless you have modded the pipe flange & header to suit.
instead of raising it i took 1mm off the top of the cylinder - the standard piston stops 2mm short of the cylinder top.
this raises the exhaust but also raises the boost & transfer ports to give 190 exhaust & 124 transfer durations.
this combined with opening out the tiny rear boost port as far as the gasket will allow has made it very different.
it still has good power low to mid rpm but has a definate power band now!
that's on th 47mm cyl btw but i copied the same porting to the one with the big piston in, without the -1mm decking.... still havent got to road test that one due to ice & snow!
next time you have the engine out for bearing replacement it is well worth looking at the reed valve recess and smoothing out the airflow a s much as you can.
you will find, when fitting the block, it is a total mismatch with the ports, with a large square bridge in the middle which needs to be removed.
the arched floor of the intake recess should be ground flat, remove the step and round off the corners.
with the casings off, fit them up to the underneath of the cylinder and have a look at the port mismatching!!!
there is a lot of metal that can come off!
have fun
@reveen - there is plenty of meat on the polini 47mm (67cc) cylinder, enough to open it out plenty.
have a look at my pug thread & you will see what i've done to that.
an fr80 / gt185 piston will go in - standard is 49mm but they are available in oversizes up to +4mm.
mitaka part number 12110-35623 will definately fit.
you can get that from any mitaka dealer or off ebay - search suzuki gt185 piston.
(you will need the piston before taking it for rebore btw.
that's a 50mm piston which will give you 77cc - this needs a =3mm rebore on the 47mm cylinder, taking 1.5mm off the spigot & wall, so there is plenty of metal left for future overbores.
regarding piston - bore clearance, i went with 0.0025 but that was a pure guess lol, hopefully i got it right lol.
i went a bit ott with mine tbh and took it out the full 6mm to fit a 53mm, 86.44cc piston but there isnt much spigot left lol - also the boost port wall is like .75mm thick so i think going for the 50mm would be a wiser option.
the critical dimensions are the gudgeon (wrist) pin diameter, which is the same, and the pin to crown height, which is slightly higher on the new piston, but not enough to be a major problem.
easily fixed with a 1.2mm base spacer and 2 standard base gaskets.
the ring stops are in the right place between the boost and transfer ports - you will also need to cut 8mm off the skirt and copy the piston port from the original piston.
this gives the desired port timings as aboveand makes the engine very motocrossy!
this needs no mods to the crankcases to fit, like other 80 kits, it will run with the standard crank (dont know how long for lol) everything else is the same.
the only other thing to do is dish out the cylinder head to fit the new piston crown as the shape is different than the standard one.
oh... and check the spark plug doesnt touch the crown lol... mine did so i fitted a 1mm copper gasket.
sounds like a lot of work but is definately a mod worth doing, but i wouldnt dvise doing it on a new kit.... i just happened to have an old scored cylinder to experiment on.
total cost of this mod was half the cost of a new bbk.
