I just built a dyno of my own design using parts and materials I had on hand. I used a Tower engine test stand bolted near one end of a 1" aluminum channel 18" long. I drilled a hole thru the channel for a 1/4" bolt to act as a pivot right under the engine. The pivot swings on a base mounted to the bench. A bolt attached near the other end of the arm serves as a leg that applies the force required to balance the torque generated by the spinning prop. The distance from the pivot to the bolt pushing on the scale is 10". The bolt rests on a digital postal scale. The force on the scale times the distance from the pivot point to the bolt pushing on the scale is the torque. The scale is pretty accurate and automatically zeros out any load acting on it when turned on. So the torque arm can have a load due to the setup not being perfect and the torque will still be accurate. The other source of error is engine rpm. I have a TNC tach so that is covered.
A 1 hp engine turning 10,000 rpm generates 0.5 ft lbs of torque. So typical glow engines will produce loads on the scale in the range of 4 to 30 ozs, right in the working range of my scale.
I have only run the stand briefly and am working on some problems. The engine load is not steady and produces a vibration in the arm pushing on the scale . I am trying to damp this by putting a slab of foam on the surface of the scale. I need to velcro down some things that keep sliding around but the whole works looks feasible so far.
I will get some pictures if there is any interest.
I ran my OS 50 at about full throttle and got roughly 80 ounce inches of torque at 12,000 rpm for about 1.2 hp.