Lighter flywheel

As a track day junkie,I was always interested in running a lighter flywheel as I was advise by a mechanic who builds Racing LC8 motors.I was going to shave my KTM flywheel down but when I heard about the Honda 600 F2 F3 flywheel bolting straight on and was lighter,I purchased a second hand one in good condition for $100.This way,if it didn't work out I still could bolt my original back on.The only thing that you have to do to fit the honda flywheel without issues is to machine the six large rivets on the inside of the wheel down about half a mm other wise they will inter fair with the three bolts that hold in the stator.Once I fitted it ,I went for a ride and instantly the mod quickened the acceleration noticeably and increased engine braking.No problems like harder starting,higher idle,rough down low as other people have suggested on other forums but the weight saving was only 250g.So my next step was to shave as much weight of the honda flywheel as I could.After turning another 250g off(now half a kilo lighter than the KTM wheel)I had the balance checked at a local high performance engine builder.The balance was perfect so that was a relief when I redline the motor.Bolted back in and the response from the motor was immediate and quick.I run a 050 quick turn throttle,negative vacuum crankcase mod along with this latest mod and the acceleration is amazing plus gear shifting is smoother.I have just arrived home from a 500km test ride and have to say I can't weight for my next track day at the end of the month.Riding out of corner was insane.The rush of acceleration to the next corner with increased engine braking was a big plus in the tighter stuff.I have found that lugging around town smoothly can be achieved by riding just on or below 3000 revs.Next mod will be lightening the rods and crank at rebuild time.As my mechanic told me,these motors are so over engineered on the bottom end,there is pretty of meat that can be removed.This mod is not for everyone.The motor is very rely and would take quite a bit to get the hang of for general riding but I love it.Cheers

Very interesting. It's been ages since someone did a new thing to the SD engine and got positive reports to make. Do you think this mod is better just for balls out track riding or would it make sense on the road too?

Also, how long a job is it to change over the flywheel? Did you do it with engine in situ or have to remove the engine from frame?

Awesome work big fella!
I have a hunch the ZX10 may fit as well as the puller and stater are the same.
Got one in the shed but but cash is low at the moment

The flywheel comes off quite easily Bic. You just need a flywheel puller.

I'd be interested to see some pics Silverbear of where you machined the metal from the flywheel. This could indeed be a great cheapish mod. Anything that allows the engine to spin up quicker has to be a good thing on track. I would imagine it could make it a bit more snatchy at slower speeds though. Is yours a SD or SDR? Doesn't the SDR have a lighter flywheel anyway?

My bike is a 2008 SM but they are all pretty much the same and as you say Klinky,the flywheel removal should be pretty easy BUT!.First of all when you start the job,rest the bike up against the wall or what ever leaning toward the right so that when you open the alternator cover,no oil leaks out.Remove the bolt and washer holding the flywheel on with you rattle air,electric gun(if you don't have one may The Lord be with you).There are six Allen screws holding the dog housing thingymigig on the back of the flywheel which I unscrewed but I have a feeling I may not have needed to.To pull off the wheel a 16mm by 1.5 pitch grub screw is screwed into the end of the crankcase so a little is poking out the end.A 20mm by 1.5 pitch bolt is then screwed into the flywheel which pushes on the crank grub screw to force the flywheel off the crank taper.My flywheel thread had already been badly damage from a heavy handed mechanic so when I went to remove mine ,the 20mm bolt trashed the thread.I manage to machine down a three jaw puller to fit and applied some heat and very low rattle gun torque to remove it.Do not buy KTM puller unless money is no object.Buy high grade bolts from your local nut and bolt shop and turn the 16mm bolt into a grub by cutting off the head and cutting a key way for a screw driver to drive it in and out off the crank.If you go to the trouble off removing the alternator cover,it is a good time to change the balance shaft seal to prevent oil blowing out of the crankcase.I removed about 3mm off the outside of the flywheel but be careful because I actually exposed a tiny bit of epoxy which holds the magnet ring on.No big deal but looked a bit how's your father.To balance the wheel I had to machine a foot long axle with a taper to match the wheel.The other end of the taper had a 12 mm spriget to fit the balancer.I also balanced the KTM wheel just in case it was balanced with the crank.The KTM wheel was zero so as long as the new wheel was zero,no balance issues.I have gone a little out of my way but if you do buy another wheel to machine and you zero it in a four jaw chuck as I did, then you shouldn't have to worry about balancing.You will pretty quickly know when you ride the bike if something is out of balance anyway.Another trick to try at the track is to disconnect your charging system and gain the benefit of zero anertia at the magnets.You will have more than enough charge to run one session at total loss charge.I was told that in the Formula Extreme series in OZ which is now our top class,that the bikes are voltage tested after the the race because total loss electronics is banned.Must be good.Im sure Linga will know more.There is a neat little charging system out for pit bikes with a very small flywheel rotating inside a rotor.Nothing for big bikes yet but I shall keep my eye on the ball.Cheers