Model Engine Company of America
3-24-2026
DON'T DESTROY YOUR HP or WEBRA Rotary Valve Four Stroke

WE CAN'T STRESS THIS ENOUGH! Unlike poppet valve four strokes the HP VT's and WEBRA 4 Stoke engines have a rotary valve which requires an oil film to seal properly. A minimum of 20% content, 22% is best. Caster oil is the preferred best, a 50/50 castor/synthetic only if 22% or more oil. More then 50% castor is better. If the oil content is too low the engine will not draw fuel well and idle properly This is due to leakage back into the intake manifold disrupting the incoming fuel/air mixture.

You also risk insufficient oil in the crankcase and cause premature wear on the crank pin, connection rod bearing, and piston. Not to mention excessive wear on the valve and gear train on all the VT's. There's no gears in the WEBRA 40, 60 & 80, just the WEBRA 90 has a gear train in the head.

The only oil available to these components is from blowby from the piston and valve or from the exhaust gases. The is the reason for a good percentage of Castor oil. Unlike synthetic, Castor oil does not burn as readily as synthetic, so the muffler pressure nipple supplies the unburned Castor to the components mentioned above.

On the VT the pressure nipple must be connected to the head and the nipple on the lower crankcase must be an open vent. There is a passage through the gear tower shaft. For muffler pressure a tee fitting between the muffler and head may be used. DO NOT try using the nipple on the lower crankcase, it must vent to open air.

 

Use a good quality fuel with the formula printed on the label. See also Fuels
 
This is what can happen to your crankshaft.
This example came from using 2% castor and 16% synthetic with a little Castor added.

The hardened steel crank pin wore down from a .232" diameter to .205" diameter.

The additional slop caused the rod to twist and break the piston which has score marks and confirms lack of oil resulting in the failure of the piston.



Just a note for reference: Breaking in an engine by spinning the crankshaft while submerged in a hot oil bath or just spinning the crank with a drill press is the absolute worst practice. All the components are working in the wrong wear/stress direction so resulting wear patterns are detrimental to engine life.

 
The pistons below are from the same engine. You can see the score mark from lack of oil. As the clearance between the rod and crankpin opened up to the rod lost its geometry with the piston twisting and rocking so much it broke the piston at the wrist pin lands.
   

This is the fuel used in the above with the customer adding 6 ounces of Klotz castor to a gallon. Please note Torco makes good fuel, just this blend is not good for rotary valve four strokes.

You would have had to add 12 oz of caster oil to the Torco fuel to meet our minimum of 14 oz required per gallon.

 
The above information is provided as a guide. Since MECOA has no way of determining the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage to person or property from the use of this information.