R/C CAR STAFF

It’s inevitable. At some point, your two-stroke nitro engine will wear out, run poorly, and require a rebuild, or a complete replacement engine. It’s up to you. For some racers, a replacement piston/sleeve/rod set makes more financial sense than an entire replacement engine. For those of you who’d like to rebuild rather than replace, we’ve written this “How-To” to walk you through the process. It’s not nearly as difficult as one might think.

1. Clean it up. The last thing you want to do is disassemble a filthy, grimy engine and contaminate its innards with dirt and debris. Before you disassemble, plug the carb neck, exhaust port, and fuel inlet and clean ’er up. Use a brush to knock away the major stuck-on gunk, and then spray a toothbrush down with motor-spray and get in there for the fine cleaning work. Keep scrubbing until the case and carb is clean, and there’s no chance you’ll transfer gunk from your fingers onto the new parts.

2. Tear down. Remove the cooling head and head button. Make sure the piston is at top-dead-center (TDC) by rotating the crankshaft until the piston is at the top of the stroke, and then pull the back plate out. Wiggle the sleeve out with your finger, or if it’s really stuck in there, insert a zip-tie into the exhaust port and turn the crankshaft to bump the sleeve up. Once the sleeve is out, you can turn the engine upside-down and drop the piston/rod assembly off of the crank pin. But before you do that…

3. Take a photo, it lasts longer. Before you get too jiggy-wid-it with the rebuild, do yourself a favor and snap a quick photo of the original piston/rod’s orientation inside the engine with your camera or cell phone cam. This pic will come in handy when it’s time to build the new piston/rod assembly. Pistons almost always have cutaways for the crank weight’s clearance, and an oil-hole in the rod that must face forward. The photo will provide this reference.

4. Pre-assembly. It’s time to build the new piston/rod assembly. You must purchase a new piston/sleeve set, plus a new wrist-pin (the pin that holds the piston to the rod) and circlips (the clips that hold the wrist-pin inside the piston) separately. Spray the components down with motor spray, and wipe them with the clean cloth to remove any machine dust and debris that may have snuck its way into the package from the factory. Then oil everything down with assembly oil.

5. Installation. Using your piston/sleeve Glamour Shots photo, build the new piston/rod assembly in the proper orientation. If the wrist-pin is hollow on one side and solid on the other, orient the solid side towards the exhaust port (back of the engine). Then take a deep breath, and install the small circlips. I like to use my fingernail to work the circlip into the piston, using extreme caution not to eject the circlip and shoot it into Neverland (see June’s “Driver’s Stand” for more info). Don’t use any tools that can scratch the new piston’s delicate outer finish. Use a small flat-head or toothpick to press on the circlips once inside the piston to ensure that they “snap” into place and are secure.

6. Final assembly. Oil up the lower rod bushing and crank pin, and drop the fresh piston/rod into the engine.

You’ll need to work the angle to wiggle the new con-rod onto the crankshaft. I like to turn the engine upside-down when doing this, as it makes alignment easier. Rotate the crankshaft to position the piston at BDC (bottom dead center), apply several drops of oil to the sleeve (inside and out), and gently work the sleeve into the engine block. 

Take careful note of the index notch in the top of the sleeve; this notch mates to an index pin in the block for proper alignment.  Rock the crankshaft lightly left and right to work the piston into the sleeve (don’t force it!), and finish by pushing the sleeve completely into the block. Reinstall the head button, checking to make sure the shims are all there, and reinstall the cooling head. Drop a few drops of assembly oil onto the rod, bushing and upper bushing, and reinstall the backplate with the engine again at TDC (or as high as it will go). The engine should be completely reassembled now.

7. Break-in. Follow the break-in procedure you prefer to break-in your rebuilt engine. Although the rest of the engine is used and broken in, the most important fit has yet to be made—the tight fit between the new piston and sleeve. Take it easy, and break-in the engine as though it were brand new. Now you’re ready to hit the track with a rebuilt engine that should be as good as new.