I have an OS 46AX and have about 15 hours of flight on it now. The manual says it is normal for the engine to feel tight and not able to turn over when cold (when its not running). Also when it was new i couldn't turn it over by hand when cold. Now i can. Although it still feels tight at the compression stroke but it turns over.

Is it because of some left over castor oil inside or have i lost compression? i always run it on the rich side. also I fly at a field with plenty of sand around.

G'day Mate,
That is completely normal, I have two 46AX's, & they were both tight when new unrun, but they bed in & turn over easier, but with plenty of compression.
When you flick the prop over compression, does the prop bounce back & forth? If so then bearings & compression are OK.
Only start to worry if it won't start & run smoothly, then you may have a problem.

Yup, the prop does bounce back and forth right after the compression stroke. so that means i still have good compression. (phew)

I am worried about the sand that we have at our field. i hear the silica sand, if it gets into the cylinder, will straight away scratch the cylinder walls causing a loss in compression.

Just for general knowledge, what other methods are there to test if an engine has good compression?

Cheers

you may want to do a search for carb fillters the was a thread on rcu take a look around then you wont have to worry about sand..

You might be able to take a patch of material from womens stockings and put it over the inlet and hold it in place with an o-ring. This is just a GUESS now as I haven't tried this myself but it should work.

Also http://www2.towerhobbies.com/cgi-bin...?&I=LX9160&P=7

Bruce.. won't they tend to make the engine run a tad rich?

Dick.

Isn't that why they put on a needle valve? But you are correct in that he will likely need to retune slightly but that is still better than a new piston/liner.[:@]

I use the Bru Line filters on all my glow engines here in the desert southwest - very dusty here.

I have not noticed these in general use though. I guess the cost of a new cylinder liner & piston or bearings is not that much.

I just makes sense to me to have air filters on engines.

I've detected no difference in tuning with/without the filter.

Dave

The tightness mentioned in the manual is for a new unrun engine and is quite normal for ABx type engines. This "pinch" will get less (and in some cases completely disappear) as the engine runs in. But this brings up a point, never turn over an engine straight out of the box just to play with it. Right then it's got the tightest "pinch" it's ever going to have and this pinch is an interference fit between the piston and liner. The oil that's covering all the surfaces is only an inhibiting oil to stop any rust and isn't designed to lubricate. If you turn the engine slowly through that pinch then chances are you'll hear the piston squeak. That's because there's more than enough time to squeeze all the oil away from between the top of the piston and the liner so it's running dry, metal to metal.

Only turn them over after you've put some fuel (or oil, preferably castor) into the cylinder and then turn it over quickly, not slowly.

Definitely use an air filter like the BruLine fine filter especially around sand. Dirt is death to an ABx type engine, ringed engines will handle it better.

"A little castor oil" actually increases compression over a "dry" cylinder. (Old tractor tick for disguising a needed ring job - pull the plug and squirt Marvel Mystery Oil in the cylinder).

Anyway, pull the glow plug and turn it over, then put it back in and compare the two. It should be harder to turn over with the plug in. Also, try doing it without the leverage of the prop. Seems tighter then, henna?

hmMm [8D]

I tried flipping over the engine through compression without the propeller attached. Turning over only the prop shaft with my hand was actually impossible!

But i plan to take this advice about the air filter. Its strange none of our club members use it since our field is all sand, no grass. Our runway is rolled flat every week. i can live with a slight decrease in performance (if at all) from the air filter.