I just did a compression check on one of my .46 LA engines. I added a small amount of fuel in the ex. port, rotated the prop to TDC and noticed a small amount of bubbles on the left side of the piston-liner. It holds for quite a while before totally losing all compression. This engine was bought used but the P&L look fine, no scratches or score lines. Should I run it on a test stand and see what happens. I would rather replace the parts now before I put it in a plane. How much air leakage, if any, is acceptable.

T.I.A

Rich

Sounds pretty OK.  I have a Cox .049 that has almost no compression unless primed and flipped hard.  It starts and runs very well most of the time.  When it is acting up it is not the fault of the engine, it is the operator or dead batteries, which I guess is still on me...

Static compression on high speed engines is not completely meaningful.  If the bleed down takes time, then at 11k rpm there is not enough time to loose compression in the first place! 

With ABC or ABN (as OS uses) construction the piston and liner change size as the engine heats up.  A static test while cold is also not very meaningful.  It might be more beneficial to pop the head and carefully examine the liner for the nickel wearing through as a determination of if the engine needs parts or work.  With ABN the piston and the cylinder have similar life spans.

Phil

Put it in a plane and fly it. You'll like it a lot! Read the pinned setups for the .46LA at the top of the page.  Steve

As Steve said, go out and fly it, it will probably work fine. A lot of LA engines can wear in the P/L dept, but keep running well enough to fly. It doesn't take much effort to put in a new P/L if you have to.
Go and give it a whirl, chances are it will keep running for a long time, so what if you have not got the last 5%, you probably would not notice that anyway.

Andrew.

Andrew - is there a Tower replacement p/c for an LA46? For which Tower engine? Might need to know this one day.

NO  there is not

Hello Ian,
Just to put the record straight. I meant buy a standard OS p/l set from Towers. Even with the postage it is a cheaper bet than buying in the UK unless you are near a model shop that stocks them! Towers don't do a p/l set of their own as Randy says. Sorry for the confusion.

Andrew.

Hi Phil

NONE of those links  are the same as a OS LA 46, and  they are also  not  plain bearing , those are dual ball  bearing engines, with  hugely different piston / sleeves , as compared to a LA plain bearing 46

Regards
Randy

I misread Andrew's question.  I thought he switched it to asking about the TOWER branded 46, which is not an OS engine rebranded, but some other engine rebranded.  Tower lists the 46LA parts in stock currently.

http://www3.towerhobbies.com/cgi-bin/WTI0005p?&I=OSMG0047&P=Z

My brain must have misfired, OS parts are easy to get from many other venders too...

Phil

Compression ratio is a theoretical constant that assumes that there are no losses, but in a less than perfect world we are forced to deal with this.

So respectfully I disagree, true comp ratio does regard engine wear.

The closer you can get the engine to run to its mathematical comp ratio the stronger it will be.