Question: We recently took an extended trip with our Volkswagen camper, starting off with four good tires. A nail in the road ruined one of the tires, forcing us to buy a tire at a gas station. I bought a retread that was guaranteed to match the retread spare I was carrying. Upon arriving at home, I measured the circumferences of the two retreads and found a 1 1/2-inch difference. They are both P205/75R14. Will this cause unacceptable wear on my differential?--G.K.
Answer: It is not a desirable condition, but it would be difficult to say how much, if any, additional wear it would cause on the differential. A 1 1/2-inch difference in circumference is less than a half-inch difference in the diameter of the tires. That's about a 2% variation.
You could easily get the two tires to be exactly the same diameter by experimenting with the air pressure in the tires. By slightly raising pressure in the small tire and reducing it in the large one, you could probably reach an equilibrium with an adjustment of only several pounds of air pressure.
The cause of the problem is that the retreads probably came from different manufacturers, one of which delivered a tire that was not accurately sized. As you probably know, a retreaded tire has a new tread layer attached to an old tire body. Controlling the exact diameter in the production of a retreaded tire is more difficult than in a new tire.
Q: My 1978 Oldsmobile has 150,000 miles but is running smoothly because most was freeway driving. It receives regular checkups, but I was recently told there is too much play in the timing chain. Is this a common problem and what are the risks if I do not replace the timing chain promptly?--C.W.H.
A: A timing chain transfers rotating motion from the engine's crankshaft to its camshaft, which opens the engine's intake and exhaust valves. The camshaft must turn in precise coordination with the crankshaft so that the valves are opened exactly at the correct instant.
As the timing chain and its drive sprockets wear out, the chain gets somewhat loose. The risk you run is that the chain will become so loose that it will jump a tooth on the sprocket and lose its timing.
If the chain does jump a tooth, the least problem is that the engine would begin to run poorly or not run at all. The worst potential problem is that a mistimed valve could interfere with the motion of the piston, resulting in a broken valve or piston. That would result in an engine tear-down, which could easily cost more than a thousand dollars.
If your car has 150,000 miles, it's not unusual to have a timing chain wear out. Sometimes, a loose chain will cause a loud rumble during idle or when accelerating, but you cannot be sure until you inspect the chain by removing an access plate. If you have confidence in your mechanic, I'd follow his advice.
Q: Last year, I bought four gallons of anti-freeze at a close-out price. I have not used it yet. Will this anti-freeze retain its full strength if left unopened?--H.L.
A: The active ingredient in anti-freeze is ethylene glycol, which is a stable chemical. It should have a long shelf life, and keeping it for one year should cause no problem.