Dear Tom and Ray:

Last night I was sitting on my porch having a cold one, and my girlfriend mentioned to me that she would like me to show her how to change a tire. As I began to confidently explain to her the basic universal mechanical principles -- you turn the bolts clockwise to tighten them, counter-clockwise to loosen them, etc. _ I remembered that the opposite is true for the hose connector on my propane grill. I then remembered that during my tenure as an esteemed lube-and- tire jockey at Sears in the 1970s, I and my air wrench often ripped the lugs off a number of Dodge Darts, which for some unknown reason, had reverse threads, albeit on only one of the front wheels. Can you possibly explain why Dodge did this? What were they thinking? -- Bart

Tom: This was another attempt by Chrysler to keep its wheels from falling off. Actually, the front AND rear wheel on the driver's side of those cars had reverse threads.

Ray: The idea was this: If the motion of the wheel happened to make the lug nuts turn, Chrysler wanted them to turn in a "tightening" direction rather than a "loosening" direction. And to do that, they reversed the threads on the left side of the car.

Tom: The logic is sound. But as you discovered yourself, the fact that it defied convention (they were the only manufacturer at the time who did this) made for many lugs being torn off of Dodge Darts by knucklehead 17-year-old tire jockeys.

Ray: It also led to Dodge owners giving up in disgust after trying to change a flat tire -- all the time having been unknowingly TIGHTENING the lugs nuts so much that they'd never come off.

Tom: So Chrysler eventually gave this up, mostly in order to stop annoying its customers. And also because it's unnecessary. Properly manufactured and tightened lug nuts should not come loose from mere driving -- although it's worth checking them when you service your car.

Ray: As for your propane tank, all combustible gas lines have reverse threads as a safety device. My oxy-acetylene torch has right-hand threads for the oxygen and left-hand threads for the acetylene so I can't accidentally reverse them. I guess it's one thing to accidentally tighten your lug nuts so they have to be cut off. It's another to blow up your home or place of business.

Wait! Don't buy another car without the mechanic's checklist that's included in Tom and Ray's pamphlet "How to Buy a Great Used Car: Secrets Only Your Mechanic Knows." It will help you get a good used car and avoid the clunkers. To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1998 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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