I took apart a Homelite 25cc engine and found brown glazing on the cylinder wall,and the outside of piston.I've never seen this in an engine before.I used a green Scotch Brite pad and got the piston clean,but it didn't come all off the cylinder wall.Is there anything else I could try to get this off ? It was sugested to me to get a brake cylinder hone and run it down into the cylinder. http://www.ebay.com/itm/220-Grit-3-S...item27e7171d06 The rest of the engine seems ok.What causes this in an engine ?

Lots of reasons. Blow by from a bad piston ring (worn ring) can do it, as can high operating temps and/or poor oil. A cylinder hone will ruin the cylinder in a heart beat. Just use #600 sandpaper kept real wet with acetone, and don't worry about getting every little bit out. Then install a new Frank Bowman high performance ring, (about $11.00), and you'll love how the engine runs.

Frank Bowman also has a very good mod for the Homelite 25cc engine. If you send him your piston, he will install a pin in the ring land so that the piston ring cannot rotate, and then he will make a special ring for it to accomodate the pin. This ring will prevent blow by, add power, and it is a very good modification for the Homelite 25cc engine. Once set up this way, you can grind that bridge out of the exhaust port for much improved flow. I've done a bunch of them that way. I hate to quote numbers, but I have seen gains of 600 to 800 rpms with these mods if used with an 11mm carb and a free breathing exhaust.

AV8TOR

Thanks AV8TOR for the tips.I have 600 wet/dry sand paper and have some acetone and will give it a try.

The specified gap for the ring in these small engines is very, very small at .003", which is about the thickness of a sheet of paper. Every small engine I've ever taken apart had a too wide ring gap, and would benefit from a new ring. Frank Bowman uses a very high quality cast iron in his rings, and actually recommends a ring gap of .000". (That's right; no gap at all.) A too wide ring gap costs considerable power and makes an engine harder to start, tune, etc. I have seen an apparently good running engine gain 800 rpms when the stock worn ring was replaced with a Bowman ring. It's the best $11.00 a person can spend on this hobby to install a Bowman ring in their engine, and everyone raves about his rings. Many people remove the stock ring from their brand new DLE's and other engines and replace them with a Bowman ring from the "get go". (Note: I have no interest or connection with Bowman and his rings, I just have lots of very good experience with them.)

AV8TOR

Commercial standard ring gap for air cooled engines is 2 thou per inch of bore size.

I got the 25cc Homelite all cleaned up as I posted in another thread and used the 600 wet /dry with the acetone. and got almost all the glazing out,but as you said don't worry about getting it all.I think I'll make this engine into an airplane conversion as I already have three water cooled Homelite boat engines.On another Homelite I converted I got a lightened and balanced flywheel off Ebay,but can't find one yet,so maybe it's time to try an electronic ignition and go with a Bowman ring as suggested.The mount and prop hub will probably come from Carr Precission unless you guys know somewhere else that has good prices,since Wacker Engines is no more.
I also have a 30cc Homelite,a 28cc McCulloch,a 28 Husquvarna Mondo and a bunch of 18cc to 25cc Weedeater brand trimmers.Some will be fixed as trimmers,but some will be given a new job on the front of an r/c airplane.

The Homelite 30cc engine is a good one, the McCulloch only fair. The Husqvarna should be great, but the Weedeaters not so good. They are very cheaply made and don't usually make much power unless modified extensively. Just letting you know....

You'll like that Bowman Homelite 25cc mod, and you'll love electronic ignition. More power, lighter weight, better idle and great starting. It's all good....

Try www.extremepower.net for the ignition system, and www.lambertsrc.com for the prop hub(s). (Tell Ken I sent ya.)

AV8TOR

You have to have a little ring gap one or two .000.Ask Frank about this.
BC

Actually it was Frank that told me zero, but I always shoot for about .001"or .002", just for cylinder and temp variations if nothing else... Bad things happen when rings butt ends. Ask anyone that's ever run an engine on nitrous or a blower with too tight a ring gap; it's not pretty and it is accompanied by a severe pain in the rear pants pocket area, followed closely by a divorce.

AV8TOR