Home Forums 2-Cycle Racing KT100 SA Blueprint Reply To: KT100 SA Blueprint


Walt Gifford

Got the cylinder on and the reeds move freely and seal quite well.

Time to get a head in life. Keeping with junk parts theme of this build I have gathered several heads over the years from various places.

Top left is a rare find, an old 9CC head with no yamaha lettering and uncut the plug depth is .580″. Top right is the 11CC new style die cast head that came on the engine plug depth .550″. Bottom left is an old black fin head (SKS) Heli coiled with a very round and smooth combustion chamber about 11CC maybe made for enduro racing on gas with a pipe, plug depth .650″. Bottom right is an old style 11CC red fin head (Emmick) plug depth .540″.

The heat and pressure under the head nuts make the aluminum squeeze into the hole so, I like to spot face the top. First I drill with 11/32″ then I use a counter bore with a special pilot I made that’s .333″ diameter. The plug boss is cleaned with scotchbrite and not cut.

Next I mounted each head on my home made billet mandrel and found the bottom two in the pic above to be off square and off center so much I can’t re-cut them using this method. I think some one re-cut these using a crooked spark plug for a mandrel. Note to self: don’t but heads on eBay lol.

I’m saving the uncut head for an alky motor and the new style head was only out .005″ so, it seems to be the best candidate even though I wanted to use one of the denser castings.

I want to run this engine on high test pump gas and possibly use a full wave pipe. Here I’m cutting the squish band to 17 degrees. I inked the gasket area so I can make a good measurement after each cut. More later.


FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience