Yamaha engine KT-100 Tech


The crankcase

Measuring each case halves is useful to determine the crankshaft end play. Add the 2 mesures together and substract the crank size. The end play should be between 0.005” and 0.010” (Photo 1-2).

Yamaha engine KT-100

Yamaha engine KT-100 Photo 1

Yamaha engine KT-100

Photo 2

To change the case bearings, remove the old oil seals and place the case halves upside-down in the oven (clean it before) to allow the old bearings to fall out.  Once the old bearings are removed, put the new ones in place while the aluminum is still hot. Add a bit of 2 cycle oil as case halfs are still warm. Once the case halves are cold, insert bearing grease in the oil seals and put them in place.

To ensure a perfect seal, sand the case halves with a 400 grit wet paper.

High temperature silicone (Photo 3) should be used to avoid any leak.  Put a tiny layer on each case halves before closing it.

Observe the correct bolting sequence.

Yamaha engine KT-100 Photo 3

Yamaha engine KT-100 Photo 3

Yamaha engine KT-100 Photo 4

Photo 4

The crankshaft

Before installing the new connecting rod, it alignment should be assessed and corrected if necessary.

When rebuilding your crankshaft, great care is required.  The crankshaft width will be base on the crankcase width.  It should allow an end play between 0.005” to 0.010”.

The maximum deflection at each end should not exceed 0.001”.


The cylinder must remain stock exept at the intake and the exhaust were it is possible to make some modifications.

The height of the cylinder can vary within the technical specifications.  Two measures have to be respected.  First the cylinder must be low enough to allow the piston to travel at least 1.045” from the top dead center (TDC) to the opening of the highest exhaust port (Photo 4).  To measure it, bring the piston to the TDC and put the dial gauge to 0.  Lower the pistion from 1.055” and if a 0.010” filler gauge goes, the cylinder is too high. Second, the piston is only allowed to travel 0.875” from TDC (Photo 5).

To avoid excessive wear, the exhaust port bridge must be recessed (approximately 0.0005”).

Yamaha engine KT-100 Photo 5

Yamaha engine KT-100 Photo 5

Yamaha engine KT-100 Photo 6

Photo 6

Piston and ring

A 0.030” chamfer is allowed.

The Yamaha, KSI, and Burris piston are cam grounded so the piston to cylinder gap has to the measure at the skirt.  The gap should be between 0.0025” and 0.003” (Photo 6).

The correct ring gap for racing is between 0.008” and 0.015” (Photo 7). Use the piston to push the ring in the cylinder (in about one inch). This will insure you that the ring is straight. Be careful not to reverse the ring when the engine is reassembled.

Yamaha engine KT-100 Photo 7

Photo 7

Note that the arrow on the piston’s top should point at the exhaust.

The cylinder head

Yamaha engine KT-100 Photo 8

Yamaha engine KT-100 Photo 8

The squish band should not be smaller than 0.035”, otherwise the piston may touch the head at high revolution.  To check the squish band, install the head on the cylinder, lower the piston, insert a piece of 0.100” solder (Photo 8) through the sparkplug hole, and make certain that is touches the cylinder wall.  Rotate the crank until the piston is at TDC and the solder is pounded flat.  Then measure the solder. This will give you the squish band’s measure.

Yamaha engine KT-100 Photo 9

Photo 9

To measure the combustion chamber volume, buy a burette of at least 13cc, fill it with automatic transmission fluid to the zero mark, bring the piston to TDC, and fill the combustion chamber by pouring the fluid through the spark plug hole.  The legal minimum limit is 11cc including the spark plug hole.

Be aware of the correct bolting sequence.

The ignition system

The standard gap between the rotor and the stator is 0.020” (Photo 9).  The wider the gap, the more retard, and the narrower the gap the more advanced the timing will be.  A wider gap may be preferred for fast tracks, and a narrower gap may be preferred on tighter tracks.

Yamaha engine KT-100 Photo 10

Yamaha engine KT-100 Photo 10

The carburator

Be aware of the gasket order when reassembling your carburetor.

The fulcrum arm level is 0.055” without the gasket (Photo 10).  To adjust it, bent it with a small screwdriver.

Yamaha engine KT-100 Photo 11

Photo 11

The carburetor must « pop off » at 10 to 12 pounds and hold 8 to 9 pounds for road race. The higher the « pop off », the leaner the mixture.  If the carburetor pops too low, try to extend the spring a little. If it doesn’t work, change it.  If it pops too high, compress the spring. (Photo 11).

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