Chainsaw Repair

The pin should be about the diameter of the ring groove. I use a little piece of gas welding rod. You use a drill press to drill a perfectly straight hole about .003" smaller than the size of the pin in the ring groove. You want to locate it where there are no ports in the cylinder, like in between the transfer and the exhaust port. This is critical. Then I use a bit of JB Weld on the pin, and with the piston held in a padded "V" block, very carefully drive the pin into the hole with a hammer and a small pin punch. (It's a three handed deal.) Start it straight and don't miss, or you'll ruin the ring land. Usually you size the pin to be the full depth of the ring land plus the distance of the hole you drilled, so as to be able to handle the pin and drive it in properly. But the pin has to end up being only 1/2 the depth of the ring groove. So you have to very, very carefully grind or file down the pin until it is only sticking up halfway into the ring groove. Once again, this is critical as if you don't get it right, you will either break Frank's special ring when you assemble the engine, or have a tight spot there because the pin is holding the ring up. Not good! The other thing is most people don't realize that the piston ring must seal on the bottom of the ring groove in the piston as well as against the cylinder wall. If you damage the ring land, or groove, the ring won't seal well and the engine will be down on power.

Note that the Homelite 30cc engine, which is identical to the 25cc except for the bore size, has a pinned piston and an exhaust port without a bridge from the factory. It weighs exactly the same as the 25cc, and is a better candidate for your airplane engine. As of the date of this posting, I have been seeing Homelite 30cc engines in the Ryobi brand of string trimmer, and in some of the Homelite brand handheld leaf blowers.

The pin should be whatever you can find that is about the same diameter as the width of the ring groove. It is a bit tough to drill a hole in a ring groove that is larger than the diameter of the groove, and often times when you try to do so, you will break a bit and/or damage the ring groove. That's why I suggest measuring the width of the groove, and then planning to use a pin that is close to the same diameter. Do not quote me on this, but it seems as though the last time I did one, which was some time ago, I used a gas welding rod with a diameter of .063" to make the pin.

Then, since you want a press fit in the aluminum for the pin, I suggest precision measuring the pin with a micrometer, and then using a drill bit that is .003" less than the diameter of the pin for your hole in the piston. You will really need a drill press and a "V" block for the piston for doing this procedure, as the hole has to be perfect. If done by hand, it will surely not be perfectly straight nor round, and you will not be able to trust the press fit of the pin. That could be disastrous for the engine.

The depth will vary with the different pistons encountered with the various Homelite 25cc engines. In some cases, you will break right through the ring land while drilling, and of course, that is as deep as you can go. In other cases, the drill will go partly into the piston crown, and partly into the interior of the piston. Once again, that will determine your depth, as once the hole breaks through to the interior of the piston, don't bother drilling any farther. And lastly, the drill might go entirely into the piston crown, and never break through to the interior area of the piston. In that case, I would drill the hole, let's say, approximately the same depth as the depth of the ring land.

Use a tiny bit of JB Weld on the pin when punching/pressing it in place. Just be sure to clean any excess JB Weld away before it dries. And when you grind that pin down to be only half way protruding into the ring groove, be super careful that you do not damage the ring groove while doing so.

Vee block source . . .
http://www.plastixrevolution.net/products/soft-jaw-vise-blocks?utm_medium=cpc&utm_source=googlepla&variant=365317043&gclid=CIb89t3Tw8QCFZcdgQod_ogAOQ

To align the chuck perfectly with the "V" block by chucking a bit in the drill press, and lowering the chuck while adjusting the position of the "V" block until the drill bit hits perfectly in the bottom center of the "V". I then, using a very sharp fine point punch, center punch exactly where I want to drill the hole in the piston. (Remember that this must be in a spot of the ring groove that will not align with any ports in the assembled engine!) Then, using a drill bit less than half the size of the hole I eventually want, I carefully position the piston in the "V" block aligned with the drill bit and drill the hole. Using a high speed on the drill press chuck, and very light pressure when drilling the hole helps keep it on track. Once the small guide hole is drilled, I follow up with the final size drill I want the hole to be.

Chuck a pin vise into my drill press to drill very tiny holes. Just be careful, as some of the cheaper pin vises do not actually center the drill bit well in their chucks. This is not too noticeable when using the pin drill by hand, but it will really wobble when chucked into a drill press.