promocom

I was given an old KB 40 engine that was seized up. it was disassembled and soaking in antifreeze. All the parts look good except the piston and ring. The piston has hundreds of vertical scratches all over it. Can the piston be repolished? The sleeve looks good but the piston will fall right through with little resistance.

A. J. Clark

Is the ring on the piston when it falls through the sleeve.

promocom

yes it does have the ring on AJ.

speedracerntrixie

My Feedback: (29)

Mecoa has parts listed. The K&B 40 uses a low tension dykes ring that expands and seals during combustion. It won’t have the same compression/seal as a normal piston ring.

http://www.mecoa.com/kb/40/40-parts.htm

tedsander

I'm no engine expert, but speedracerntrixie makes a great observation. You won't feel much resistance with that kind of ring until the engine has compression. Even then, if turned over slowly, it may not seem like much. But with a sharp flip, it will feel much greater. It should give a nice "pop" feel even without fuel or the glow plug lit. The compression forces the ring out to the walls, making the seal much tighter. A faster flip overcomes any small leakage it might have.
Scratches on the piston body itself, while signs of wear/maybe abuse, have no impact on the above. Scratches on the ring, or just wear, will allow too much air to leak past, and then even a sharp flip won't provide enough compression. If such is the case, even if the sleeve looks good, I'd likely replace both it and the ring. But with that much wear or abuse, then the connecting rod bearings become suspect too....
And low compression might be the head seal or the glow plug seal, and not the ring/sleeve....or3 maybe all of them!

mgnostic

I wore out two rings on my K&B .40 before the sleeve was too worn to use again. And it still might go with with a new piston and ring. The point of this is that a new ring is 16 dollars plus shipping. Go ahead and spend the five bucks on gaskets as well. Treat it as a new engine break in since the new ring will have to seat itself. It's easy to spend enough on new parts to buy a running engine but it is an educational and, at least for me, an entertaining experience.

A. J. Clark

Just put it back together. Run it and see how it does.

promocom

With nothing to lose, I lightly sanded the piston and ring with #500 grit and then steel wool and it looks great! so yeah i will put it back together as it is.

promocom

I ordered new bearings cause they were in terrible shape. I assume I can test the motor on the bench in a vise when its back together? What kinda voltage do these glow plugs use btw? Is it regular 2 cycle fuel mixed at some ratio? sorry for all the questions...lol!

speedracerntrixie

My Feedback: (29)

Negative on the vise, you can easily distort the case. Glow plugs are 1.5v, fuel should be 80% methanol and 20% oil. Most common oil package for that engine would be 15% synthetic with 5% castor. You need the correct oils that mix with methanol. Adding some nitromethane will increase power somewhat but max would be 15%.

mgnostic

Building on what Speedracer said, if you are going to try to get the engine running then spring for the appropriate size engine mount or a test stand. By the time you cinch a vice down tight enough to hold onto a running engine you will have ruined the case. One thing glow motors have in common with electrics is that they require a certain amount of infrastructure. At a minimum you will need a fuel tank and fuel line that are glow fuel (methanol) compatible, a connector or glow starter for the glow plug. and some means of holding the throttle at whatever position you want it to stay. You will see people using fancy electric fuel pumps but after nearly 40 years in the hobby I still use veterinary syringes for anything smaller than a 12 ounce fuel tank. Watch a few youtube videos on test running engines and you will get an idea of what you need. You will need to make certain that your glow plug is good. It should glow a nice orange with 1.5 volts. Do Not use anything more than 1.5 volts, it will burn out in the blink of an eye. Seriously, an old stuck motor is not your best introduction into glow power but if you can get past the learning curve they are rewarding and a lot of fun.

tedsander

Using a vise is also an excellent way to guarantee a trip to a hospital! A bit of searching will come up with how to make a test stand (or buy one). Can be as simple as bolting the lugs to blocks of wood, and bolting those to a solid, unmoving, base, Fancier versions allow for adjusting to fit different engines, places to hold a fuel tank, guide throttle pushrods, etc. You want it very solidly mounted, and to never get near a spinning prop.

Your reference to 2 cycle fuel and mixing might imply you are thinking of gasoline. NO, NEVER. and as speedracerntrixie noted, the oils are NOT petroleum oils. Buy specific fuel made for 2 cycle glow engines. Then no mixing of anything needed.

promocom

Good to know!