Used or abused?

Radio Control Car Action, Oct 2003 by Bess, Stephen

EXPERT ENGINE ADVICE

How to buy smart when you buy a "pre-owned" engine

If you're in the market for a hi-po nitro powerplant but your budget can't take the sticker shock of a new engine, you might consider buying a used engine from a friend, or an online auction site or bulletin board. Though the prices may be right, what you buy may not be all it's promised to be. Before you plunk down your greenbacks, you should realize that only by physically examining an engine can you determine whether it's all it's claimed to be.

By doing the checks described here, you'll quickly be able to determine whether a particular used engine is worth buying.

PISTON AND SLEEVE WEAR

This quick check can be done with the engine assembled; the rest require some disassembly.

Until a bolt-on "gallon-o-meter" is invented, there's no way to know how many gallons of fuel have passed through any engine, and can you trust the seller's claims? An engine advertised as "Hardly used; less than 2 gallons," will, however, show less wear than an engine that has drunk 8 gallons. A new or fairly new engine's piston/sleeve fit (this determines compression) is tight, and it loosens with use.

To check an engine's piston/sleeve fit, simply turn the crankshaft or flywheel at least one full revolution and check its resistance. As the crankshaft of a "new" or "hardly used" engine is turned past the top of its stroke, it should present plenty of resistance. A distinct "thump" should be noticeable when a glow plug is installed in the cooling head; if there's little resistance, the engine needs a new piston and sleeve.

CHECK THE PISTON HEAD AND HEAD BUTTON

To do this, you'll have to remove the cylinder head. Check the piston head and head button (the area into which the glow plug is threaded). The head and head button on an engine that has always been properly tuned will be perfectly smooth. If you see pits or missing metal on either, you'll know that the engine has been run overly lean, has blown plugs, and/or has detonated and been generally mistreated.

Piston/sleeve sets cost between $100 and $200 plus (for certain competition engines), so think twice before you buy a used engine with pitting and/or poor compression.

CHECK THE FRONT AND REAR BEARINGS

Engine bearings develop play as they wear, and this can cause the crankshaft to wobble, and at high rpm it can snap; it can also cause clutch-bearing problems. An engine that's run without an air filter can suck in dirt that will contaminate the ball bearings, and this also causes excess play. As the balls inside the bearings wear against their races (the cages that hold them), the inner race begins to wobble and feel gritty when it's spun. New bearings spin smoothly.

To check the bearings, completely remove the piston/sleeve assembly and the crankshaft. To check the front and rear bearings, insert your little finger into the bearing race and spin the bearing. If it feels gritty, or if the inner race wobbles as you apply force with your finger, your bearing is probably toast and should be replaced. A good bearing feels smooth and tight when you spin it.

CRANKSHAFT

The crankshaft's surface is an excellent gauge of how much an engine has been used and how well it has been maintained and protected from contaminants. Remove the crankshaft from the engine, and inspect its surface for scratches, discoloration and rust. Pay special attention to the area underneath the induction port; if the area around the crank's circumference is scratched, the engine has ingested dirt because it has been run with a poor air filter or even without a filter. Rust on the crankshaft shows that the engine has been stored with fuel inside its case or that it has been improperly stored during the winter.

Next, inspect the crankshaft for discoloration and deep grooves where the bearings would usually be seated when the shaft is installed in the engine. The shaft's bearing areas should be clean, and its surface should appear consistent with the rest of the crank. Any bluish-purple shaft discoloration or polishing in the areas where the bearings seat indicate that one or both of the bearings should be replaced. Significant polishing or discoloration shows that the bearing is seizing and not spinning with the shaft. A slight polishing might be acceptable in sport engines, which are assembled with looser tolerances, but not in high-performance engines.

CHECK THE CONNECTING ROD AND WRISTPIN

When they replace a piston and sleeve, most racers don't think about also installing a new connecting rod (conrod) or a new wristpin (this attaches the conrod to the piston). Most assume that these components must be good if they aren't broken. But a used engine that seems to be "like new" because, among other things, it has a new piston and sleeve, may still require work and expense to bring it up to spec.

Conrods take a lot of punishment, and over time, they can stretch or even break (if they're very worn or have been abused). How can you tell if the conrod is so worn that it should be replaced when the engine has good compression? You have to remove the backplate and check it.