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How To Rebuild Your RC Engine - Reassembly - Part Three

Article by Kenny McCormick

This is part three in the RC engine rebuilds series. Parts one and two cover engine teardown and can be read here and here. We recommend that you read those two engine dismantling articles first before you read this one.

Now that your engine has been fully dismantled, it's time to replace any worn parts. In my engine all that was required was a new piston and sleeve. Yours may differ. If your engine failed simply because it wore out over many many gallons it's a VERY good idea to replace the wristpin, rod, and rear main bearing at minimum.

I'd suggest a front bearing as well, and if the new rod has any play on the crank it probably wouldn't hurt to grab a new crank as well. If your engine uses C-clips or other similar devices to retain the pin then buying spares of these is a good idea, I lost both originals trying to reassemble mine.

With your parts procured, lay everything out on the same workspace you tore the engine down on. If you are opting to replace the bearings, strip EVERYTHING else off the engine. You want the block to contain only the crank bearings. Take it to your oven and set it on the rack, front of the engine facing up. Set it to 300º Fahrenheit for about 20-25 minutes, or until you hear a light "plink" from inside the oven. When removing your parts, make sure you don't grab them with bare skin, they'll be piping hot.

This is what most modern engines require to do this.

Installing the new bearings is pretty much the opposite. First, put the block in the oven again and get it up to 300 degrees. While it warms up, slip the rear bearing onto the crankshaft and nudge it up right against the counterweight. Then, take both it and the new front bearing out to the oven. When the engine is done cooking, pick it up with an oven mitt and carefully slide the crank in. You will need to push on the counterweight a bit to drive the new rear bearing into it's home. Once it's in, slip the front bearing onto the end of the crank and lightly work it into place, ensuring it fully seats inside the block. Then, just let everything cool off.

If you didn't opt to replace the bearings, but did remove the crank, install the crank by simply pushing it back in. Don't worry about getting the drive washer back on at this time, we'll do that when we install the prop. Make sure, while installing the engine, that you lubricate the living hell out of everything. There is no such thing as too much assembly lube. You can use whatever you use for after-run oil as assembly lube, just be generous with it.

The new PS.

Now that the crankshaft is done, it's time to assemble the piston and rod assembly. Begin by freeing the piston from the sleeve. They come packaged one in the other, and in my case, they didn't want to part company. Be very gentle, and use PLENTY of lube. Do NOT whack it with anything, and do NOT try to force it. If you must, bake it at 250 degrees in the oven with the piston top facing up, it will just slip right out when the sleeve warms up enough. I was able to work mine out with generous lubrication and my thumb pressing on the top of the piston.


Once the piston is out, liberally lubricate the wrist pin bore. If your engine uses clips of some sort, install one in the hole without the cutout for a tool. Do so very carefully, so as to not scratch the outside of the piston. Then, lubricate the new pin and begin to slip it in the opposite side. Stop once it's just poking through and take the connecting rod. Be sure to make sure it's all oriented properly and put the small end of the rod onto the pin. Then, resume carefully working the pin into the bore. Once it's bottomed out, install the other retainer.

With that done, lube up the big end of the rod and the crankpin. Carefully drop the piston in from above, with the crank at top dead center. The piston will need to be lightly pressed against the back of the block in order for the big end to clear the crankpin. Once it's in position carefully slide it down onto the pin. With that done, set the engine aside and pick the new sleeve up.

The sleeve is probably the most critical point for assembly lube. Soak both sides. With it well lubed up, carefully turn the engine to bottom dead center and begin to insert the sleeve. The lubricant should ease slipping it in, if not, a hair dryer aimed at the cooling fins will help things along. Get the sleeve in until the ports just begin to slip into the block and rotate the sleeve to match the index marks. Then, turn the engine upside down and carefully allow the piston to drop down into the sleeve. Jiggle it a bit and the piston should slip right in. You may also need to guide it in with a finger slipped in through the backplate opening. Once the piston is inside the bore, slowly rotate the engine about halfway to TDC.

With the piston halfway up the stroke, start sliding the sleeve further into the bore. Only lightly hold the crankshaft, once you feel the piston begin to push against it let it rotate. The sleeve should smoothly drop right into the bore, and it should be indexed with the ports. Apply more assembly lube and gently rock the engine back and fourth through BOTTOM dead center a few times. This will make sure the sides of the piston are well lubed.

Now that the sleeve is in it's time to put the outside stuff back on. First step should be the cylinder head. Make sure your copper gasket is in place and lightly finger-tighten the screws. Then, in a criss-cross pattern, snug them up another quarter to 3/8s of a turn. This is all you need, tightening them further may strip them out or break the bolt heads off. With the head on, the next step is the backplate. If you have a gasket or elected to replace the o-ring, now's the time to do it. If you have an O-ring, a light coating of lube won't hurt and will help seal things up. Take care to mind any cutouts for clearances, and tighten the backplate bolts the same way you tightened the head bolts. Pop the carb on at this time as well, ensuring you press it into the carb opening while you tighten the cinch bolt to ensure it seals up.

Note for RC car and RC boat guys: If your engine has a pull start or onboard electric start, the backplate will also have a start shaft that has to engage the crankpin. Be sure to rotate this shaft a bit while installing the backplate, you will feel it click and drop further into the engine when it engages. Lube the bushing up, and it's probably a good idea to install brand new parts if the engine is high mileage. After this, install your starter mechanism. Your starter need not be tightened as much but I suggest threadlock on the screws and using a criss-cross pattern to ensure even one-way bearing engagement... oh, you did install a new one-way too, right? Yes? Good, good. Car, boat and heli guys, you will want to install your drive accessories now, as they are nigh impossible to install after the engine's in. Steps are the same as below, only installing the engine is last instead of first.