# KTN LC4 flywheel weight



## John Conroy (Dec 29, 2016)

I was asked to make a flywheel weight for a KTM LC4 super-moto bike. He got the plans off another member of a forum he frequents and supplied a 1" thick slice of 5" round 1018 cold rolled steel. The dimensions for the weight are 125mm OD and 17mm thick. It has six m6 threaded holes to fasten it to the flywheel and a centering hub that it fits over also on the flywheel. The plans show a bolt hole circle diameter of 61.5mm.








Being a skeptic I double checked the dimensions on the flywheel he sent me with the material.

I mounted the flywheel in my super spacer and used a coaxial indicator to find the center of it. I had to indicate off the OD of the hub as there is a keyway on the id of the hole. Short video of that process.





I moved the mill x axis over and used a center finder to locate the hole position.







I measured the radius as 30.66 mm not 30.75 as the plans stated so I went with that in my build.

This is the outside of the flywheel, the starter sprag is mounted on the inside and will be removed and replaced with this weight along with all the other electric start parts.







This is the sprag hub.







I faced off the slice of steel in my 4 jaw chuck so I could dial it in perfectly centered, my 3 jaw has about .002" runout.







Plans call for a 1.240 center hole so I drilled it to 1.125 and used a boring bar to finish the hole.







I used a boring bar to machine the recess that fits over the center hub to .175 depth and 3.069" which is what I measured the OD of the hub on the flywheel at. I wound up with a nice snug fit.







I mounted it in the super spacer and drilled and tapped the six bolt holes.







It fits perfectly and the bolt holes line up exactly, that was a relief.

I bolted the weight to the flywheel and mounted the flywheel in the 4 jaw and centered it indicating off the center hole in the weight I bored earlier. I turned down the OD to 4.921" and the thickness to .670" as per the plans. Here's the finished product next to the old starter sprag hub. The sprag hub weighs 645 grams and the finished flywheel weight weighs 1382 grams. This should make the engine much smoother a low rpm and help to control wheel spin.







A fun project! This is the 8mm of steel I machined off the 1" slice to get it to 17mm thickness.


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