Engine Changes and Modifications

Well, the engine has been modified and run. I made many small changes, mostly that will affect long term operation.

The first change that I made was to the crankcase. The gear on the crankshaft fits very tight, and it was always put on after the crank was installed in the crankcase. The hole that the crank went through was smaller than the gear, an assembly/dissassembly required the gear to be removed. The hole for the crankshaft was made slightly bigger than the gear.

The next thing I did was make a new crank-pin. The original crankpin was made from cold rolled steel. After the first run, I noticed some wear on it. (the first run was only 10 or 15 minutes) So I made a new one from drill rod, and hardened it. That stuff gets really hard, so it should last a long time with little wear. The crank-pin is just pressed in, so it is not a big deal to change it.

pic of crank

Next was to put a brass bushing on the big end of the connecting rods. The rods are made from aluminum and seemed to have a lot of wear from the first run. I don't have any bronze to make them from so brass was my next choice. I also put an oil hole in the big end of the rods. I noticed from running without a back cover, that oil blows past the rings and pools in the crankcase. The oil holes in the rods are positioned so that the oil will go into the hole when the rod hits the pool of oil.

pic of bushing

I cut the length of the cylinders down 0.070". This raised the compression ratio to 9:1.

I also trimmed down the rocker arm supports 0.030" the rocker arms were already at an angle before the valve opened. This made them more parallel with the head.

I made a new axel for the middle gear of the cam gears. The old axel was a loose fit with nothing to hold it in place. I made a new one with a flange on it that hits the inside of the front cover and keeps it from coming out when the engine is running. This was made from drill rod and hardened.

I also checked the balance of the crankshaft and rods. It is my understanding that the crank counter-balance is supposed to balance out the weight of the rods and wrist pin. I don't have any kind of a balance machine, but roughly, it seems that the counter-balance on my crank only balances out one rod! And I have noticed that the engine does vibrate quite a bit when running. So I drilled out each wrist pin to reduce the weight. Now I need to figure a way to add some weight to the counter-balance. I think I will try adding some weight at the propeller. I know that it will not be quite as smooth but it will be much easier. Below is a pic of the new wrist pins.

pic of wrist pin

I was concerned about the camshaft wearing from the lifters. The camshaft is made from drill rod and hardened. I checked the cam after the second run and there is still no sign of wear at all! Just a slight shiny ring where the lifter rides. No I did not make the gears, I got them from Small Parts Inc.

pic of cam

After all of the modifications were made the engine was assembled and run again. The engine started easily, and ran quite well.The prop was an 18 x 6 Master Airscrew The max RPM was 5800 when I put in some glow plugs without an idle bar. The slow idle was less than 1000 RPM !! The best idle was at about 1500 - 1800 RPM. I changed the prop and put on an 18 x 10. The max rpm was 5000. I am still using the OS-40 carb. I think that the small carb makes a better idle by having a much higher vacuum at the carb. I really don't know. But I do have an old carb from an ASP 75 that I am going to try soon. It is a much bigger carb. The engine was run thru 3, 10 oz. tanks of fuel. I let it run at about half throttle for several minutes, then opened it up, and it doesn't load up like a 2 stroke engine - it just goes. It will idle for an unlimited time without stopping. I have no experiance with 4 stroke model airplane engines, but I know that a 2 stroke engine will not normally idle very long, and if it does, it will not run very good when you open up the throttle. None of these characteristics seem to apply to this engine. It seems to be very reliable running. The only thing that I have noticed so far, and it is pretty obvious, is when the throttle is opened up. The engine takes 2 or 3 seconds to reach its peak rpm. As far as power, I don't know how much power this engine has. I do know that at least one commercially built 1.2 C.I. engine turns the 18 x 10 prop at the same rpm.

The engine still will not run without glow plug current. Ok, it will run, but it sputters and misses. But it is better than before I changed the compression ratio.

I have also checked the valve timing with the timing specs that the camshaft was made from. I am not sure why, but the timing is much different than what it should be. The timing should be a few degrees different because of the slack in the push rods, lifters, and rocker arms. But this should only be maybe 5 degrees or so. I seem to be as much as 35 degrees off. This is not a minor thing. It seems that a new camshaft is in order. (It still seems to run good)