ABC ENGINES COMPRESSION

Is the resistance you meet when moving thhe shaft near the TDC an indicator of the engine's condition?

I have this .32 that has been going places and it still requires some force to get to the TDC,.

On the other side my .25VF doesn't resist s half as much as the 32, and it's nwe and was propery broken in. And it sure delivers the goods!

Anybody can tell me someting?:

Ion (just curious) Muniz

Good question, Ion. I have seen the same effects on different engines.
My Aero Tiger 36 squeals when turned through compression slowly while others are just "tight" in varying degrees. My OS 25 VF is not as *tight* as the Aero, but it runs just great.
As long as the engine id performing, and you will tell if you ever do wear one out, I don't consider it anymore.
My poor old OS 32 F is getting on it's last legs, compression wise (at least 10 years old and flown to death), but it still hauls the mail pretty good, and is as easy starting as ever.
I wouldn't worry about that part of comparision. ABC/AAC/Abn engines sure don't *seem* to exhibit the same feel as the old ones did.
Bill <><

I don't take any notice of how much "pinch" there is so long as the engine still starts easily and runs the way it should. I've got a 3rd hand (at least) SC 1.08 that leaks compression like an old Fox 35 but starts every time with one flick and runs beautifully. Neither of my 40VF's or Irvine 40RLS have got any pinch but you couldn't ask for better running engines.

LAST EDITED ON May-30-04 AT 04:01 PM (CDT)
 
Tightness at T.D.C. is a good thing for ABC & AAC engines. The newer engines ABC & AAC are built with a taper in the sleeve. The smallest dia being at the top of the stroke. This is where it is needed most for a good compression seal. The newer motors such as the PA's and the Ro Jetts etc. require more break in time and they are built to last. If you break in an AAC properly you will find even after a year of full use the motor will still be tight when cold. Another advantage is, should you have a lean run the sleeve will expand accordingly to the temperature, thus returning to normal when cooled down. The motor is not ruined as in the days of the old Mc Coys etc. Motors like the the OS have much less taper and probably more clearance between the piston and the sleeve, they are easier to break in. They are made this way on purpose so they are more user friendly to the beginner or the modeling public and he can get in the air faster. This does not mean they are bad motors, just a different type. Many OS's have been broken in in the air. This is not a wise decision when dealing with the AAC or the ABC. It is best to break them in on the stand first so when you put your first flight in it will be a good one. When first breaking in a AAC motor it will run a little erratic until the piston and the sleeve develope a good seal. Stalker recommends 1 1/2 to 2 hours break in time. Most people don't have the patience to do it correctly and they end up blaming the manufacturer when their new motor doesen't function properly. It's important to follow the manufactures instructions.

>Tightness at T.D.C. is a good thing for ABC & AAC engines.
>The newer engines ABC & AAC are built with a taper in the
>sleeve. The smallest dia being at the top of the stroke.
>This is where it is needed most for a good compression seal.
>The newer motors such as the PA's and the Ro Jetts etc.
>require more break in time and they are built to last. If
>you break in an AAC properly you will find even after a year
>of full use the motor will still be tight when cold. Another
>advantage is should you have a lean run the sleeve will
>expand accordingly to the temperature, thus returning to
>normal when cooled down. The motor is not ruined as in the
>days of the old Mc Coys etc. Motors like the the OS have
>much less taper and probably more clearance between the
>piston and the sleeve, they are easier to break in. They are
>made this way on purpose so they are more user friendly to
>the beginner or the modeling public and he can get in the
>air faster. This does not mean they are bad motors, just a
>different type. Many OS's have been broken in in the air.
>This is not a wise decision when dealing with the AAC or the
>ABC. It is best to break them in on the stand first so when
>you put your first flight in it will be a good one. When
>first breaking in a AAC motor it will run a little erratic
>until the piston and the sleeve develope a good seal. It's
>best to follow the manufactures instructions to the letter.

Ron B.
F4Fguy

Gregg:

Not to nitpick but,well,to nitpick,the real answer is in the piston/sleeve materials,and the fact that they have different thermal expansion coefficients.The taper is chosen so that,at temperature,the bore and piston diameter and resulting fit is a straight line,or nearly so.This usually results in an interference fit at TDC on a cold engine.

The fits and tapers required are different for different sleeve materials(Al,brass,or bronze),and different silicon/aluminum alloy pistons but,the bore plating material has little or no effect on the thermal expansion.The piston always runs hotter than the sleeve.Al/Si alloys have significantly lower thermal expansion coefficients than other alloys.This is why the Al/Si alloy is used for the piston,so the piston can be fitted closer and still not over-expand the bore.The big issue on break-in is to bring the fits to those best suited to the operating condition.

Unlike the older iron/iron engines,AAC,AAN,ABC,or ABN should be broken in at as near as possible to the expected running condition.This,generally,means a lot of short runs at or near the in- air condition,with complete cool-down between runs. Avoid overheating as this will accelerate wear,but also avoid running at too light a load as the wear pattern will be altered by an undesirable thermal profile.Actually short tanking and flying with a size smaller prop is an excellent break-in procedure.

Most high quality iron bore/iron piston engines have some taper so that the bore is straight at operating temp.This extends even to full size aircraft recips,some having as much as .020" of "choke"at the top.

We have broken in a number of LA 40's very carefully. Start up the new motor, run it for a minute. If it sounds OK, stop it, refill the tank, and fly it, usually on a slightly smaller than normal prop, say a 10/4 instead of an 11/4. After half a dozen or so runs, put on the bigger prop and fly. They all run pretty well and generally start hot or cold in a couple flips.