QDear Tom and Ray:

I am looking for a floor jack for my personal use and need some advice on how big a jack I need to get. I know that a two-ton jack would be plenty to work on a regular car -- e.g., a Civic, Altima, etc. However, we also happen to own a Chevy Suburban with a gross vehicle weight of 7,000 pounds. I tried calling some shops and was told by some that a 11/2- or 2-ton jack would be enough, since the jack will not be lifting the whole truck. But there were some that said I need to get a jack that is big enough to hold the whole weight of the truck, which would be more like 3 or 31/2 tons. What do you recommend? If I have to get a 31/2-ton behemoth, I probably would not bother working on the Suburban on my own. If I plan to work on trucks and SUVs in the future, do I need to get a jack with a higher range? -- Isao

ATOM: Yes, you do. Our rule of thumb is that a floor jack needs to be rated for at least three-quarters of a vehicle's gross weight. So, according to our rule, a 11/2 -ton (3,000-pound) jack could lift a car that weighs as much as 4,000 pounds -- or two mothers-in-law.

RAY: So, if you want to work on your 7,000-pound Suburban, you'll need to get a 3-ton jack, Isao.

TOM: Here's why: It's true that a floor jack does not have to lift all of the car's weight. It has to lift half of it -- taking two wheels off the ground. But when you operate at the limit of the jack's ability, two things happen.

RAY: One is that it gets very hard to lift the car. But second, and more important, you leave no room for error. And unless the jack is absolutely, perfectly centered and the weight of the car is perfectly centered (which you can't always count on), the jack's linkage bars can begin to distort under the maximum weight, causing the jack to flip and the car to come crashing down.

TOM: It's never good to operate anything at the absolute limit of its ability. For instance, some scientists say a man can go seven days without water in 90-degree heat, as long as he doesn't move. Does that mean we should all lie down and wait seven days between drinks?

RAY: Similarly, you don't want to be under a Chevy Suburban that's held up by a jack that's quavering at the limit of its abilities.

TOM: So, you have three choices, Isao. You can get a three-ton jack. Or you can let someone who has one work on your Suburban. Or you can buy a set of properly rated ramps, and use those to get enough clearance to work under all of your cars.

Dear Tom and Ray:

Yesterday, my son broke off a spark plug in his 1989 Ford F-150 with a six-cylinder engine. It broke off flush with the hole. Egad! How do we get the rest of the spark plug out, and what other concerns must be kept in mind? -- Dave

TOM: Well, it depends on how he broke it, Dave. If he broke it by turning the wrench the wrong way (trying to tighten it instead of loosen it), then you may be in luck.

RAY: In that case, you would use an extractor -- something like Easy-Out -- which taps into the remaining piece of spark plug and grabs onto it with sharp, fluted edges. Once the extractor is attached, you simply put your socket wrench on the end of it, unscrew it in the proper direction, and the whole thing should walk right out.

TOM: If, on the other hand, it broke because it was rusted and fused in there, and he had the 7-foot ratchet extension on it and was walking around the car trying to unscrew it, the job gets a lot harder.

RAY: If the plug broke despite the fact that you were unscrewing it the right way, then you have to drill it out. This is a job best left to professionals, Dave. It's not for the faint of heart -- or the faint of talent. A mistake here can cost you a cylinder head.

TOM: What a mechanic will do is drill out the broken piece of spark plug, along with the existing threads. The result will be a smooth hole that will be bigger than the old spark-plug hole. He'll then use a tool to cut new threads for an insert, and the spark plug will go into that insert. Then you'll be all set.

RAY: He'll also have to fish out all of the metal shavings that fall into the cylinder during this operation. We get most of them with a magnet. If you don't remove them, they could cause serious engine damage.

TOM: So, you have to start, Dave, by holding your kid's feet to the fire and finding out exactly what he was doing when the spark plug broke. He'll be embarrassed to admit it if he was turning the wrench the wrong way, but that will actually make his life a whole lot easier.

RAY: This is one of those rare cases where stupidity pays off.

TOM: Aha! I've been looking for cases like that.

Got a question about cars? Write to Click & Clack in care of The Post or e-mail them by visiting the Car Talk Web site at www.cartalk.com.

(c)2004 by Tom and Ray Magliozzi

and Doug Berman