OS 46 AX knock
01-01-2011, 06:52 PM
#1
Thread Starter
OS 46 AX knock
I just noticed that when I flipped the prop on my OS 46 AX I got a knocking noise as the prop was reaching compression. I then noticed the knock as I started the engine but the engine seems to run fine and idles well. What might this be and should I be concerned? Engine has run great in my B-25 for three years.
01-04-2011, 07:15 AM
#2
My Feedback: (11)
RE: OS 46 AX knock
You are hearing the "over the center" action as the engine is turned over and the piston is stopped by the pinch at the top of the cylinder. When the engine's running, it won't be there.
Your engine is an ABC-type of engine. This means that the particular construction of your engine has an Aluminum piston, and a Brass Liner that is plated. Traditionally, the plating was Chrome, but some engines use a different metal, usually a nickel alloy.
An ABC-type engine will have the cylinder liner manufactured in such a way that it is tapered towards the top. This will cause the piston to jam or "grab" as it moves towards Top Dead Center. Some engines will actually seize if you turn them over when cold, and will require considerable force to turn them through the jammed area. You may feel something that you may interpret as play in the connecting rod. This is almost certainly not the case. What you are feeling is a slight springing of the connecting rod as you force the engine through the top of its stroke. The piston doesn't go all the way to the top, and the rod will flex slightly, which gives an "over the center" feeling. Many people have interpreted this as play in one of the rod ends.
When an ABC-type engine is run, the heat of combustion will cause the metal parts to expand. Because the piston and cylinder liner are made of different metals, they will expand at different rates. The cylinder liner will expand just enough to make a nice fit for the piston once the engine warms up. The liner is tapered because it is hotter near the top of the cylinder than lower. The different temperatures cause different amounts of expansion.
Your ABC-type engine must be broken-in at full throttle. You don't want to run it too rich. Too-rich running will not allow it to warm up to proper temperatures. Lean it until it is running just out of what is known as "four-cycle" mode. You can lean it to peak RPM as soon as the engine will accept it.
You should prop your engine to allow it to turn up. ABC-type engines work better at higher RPM. You should also use a fuel that contains castor oil in the lubrication mix. Castor oil helps the engine run well because it's still the better lubricant for ABC type engines. All-synthetic fuels have been known to cause difficulties in running, at times.
Once properly broken-in, your ABC-type engine will serve you well and deliver lots of reliable performance.
Your engine is an ABC-type of engine. This means that the particular construction of your engine has an Aluminum piston, and a Brass Liner that is plated. Traditionally, the plating was Chrome, but some engines use a different metal, usually a nickel alloy.
An ABC-type engine will have the cylinder liner manufactured in such a way that it is tapered towards the top. This will cause the piston to jam or "grab" as it moves towards Top Dead Center. Some engines will actually seize if you turn them over when cold, and will require considerable force to turn them through the jammed area. You may feel something that you may interpret as play in the connecting rod. This is almost certainly not the case. What you are feeling is a slight springing of the connecting rod as you force the engine through the top of its stroke. The piston doesn't go all the way to the top, and the rod will flex slightly, which gives an "over the center" feeling. Many people have interpreted this as play in one of the rod ends.
When an ABC-type engine is run, the heat of combustion will cause the metal parts to expand. Because the piston and cylinder liner are made of different metals, they will expand at different rates. The cylinder liner will expand just enough to make a nice fit for the piston once the engine warms up. The liner is tapered because it is hotter near the top of the cylinder than lower. The different temperatures cause different amounts of expansion.
Your ABC-type engine must be broken-in at full throttle. You don't want to run it too rich. Too-rich running will not allow it to warm up to proper temperatures. Lean it until it is running just out of what is known as "four-cycle" mode. You can lean it to peak RPM as soon as the engine will accept it.
You should prop your engine to allow it to turn up. ABC-type engines work better at higher RPM. You should also use a fuel that contains castor oil in the lubrication mix. Castor oil helps the engine run well because it's still the better lubricant for ABC type engines. All-synthetic fuels have been known to cause difficulties in running, at times.
Once properly broken-in, your ABC-type engine will serve you well and deliver lots of reliable performance.
