No doubt you've popped the hood on your 4x4 and looked at those metal tags riveted to the firewall and on the radiator mount. And if you're the nosy type, you might find another one on the inside of the doorjamb, or maybe on the windshield pillar post.

In fact, if you want to waste a perfectly good Saturday, get a flashlight arid crawl under your rig and shine that light everywhere.. You'll see mysterious tags on the trans and the rear end. Or, if there aren't tags, you'll find a string of numbers embossed on the parts that mean utterly nothing to you.

And that tag under the hood is virtually incomprehensible. A typical tag on a vehicle might read like this:

1FMDU1 5H7HLA55237 89SM 1152 H9 K

Here's a breakdown of a typical Ford code tag and how to translate it. Each manufacturer has its own system and code numbers.

Now what is all that gobbledygook? What does it mean to you? More than likely, nothing. But to the trained eye of a mechanic, it's a wealth of information.

You see, he knows the secret codes. At a glance for example, he knows that those first three digits "1FM" mean that this is a multi-purpose Passenger vehicle made by Ford Motor Company, USA. If those first three digits read 2FT, it would mean the vehicle was a complete truck made by Ford in Canada.

An incomplete truck (one used as a base for a small motorhome) would have the first three digits read 2FD,' and would also have been made in Canada.

So how do you find out about these codes?

First off, chances are you're not going to find them in the typical next- to-useless owners manual that comes with most new vehicles. These goofy things tell you all kinds of important stuff like: "Be sure to close your door before you drive off." and "Avoid jackrabbit starts."

For real hardcore information, you're going to need a genuine shop manual. Your FLD (Friendly Local Dealer) can order one for you-as can many companies listed in Hemmings Motor News (available at finer newsstands or from HMN, PO Box 100, Rt. SW, Bennington, VT 05201, 800-227- 4373), and chances are it'll cost a bit more than you might expect. Plan on spending anywhere from $35 to $60 for a real manual.

Your next best bet is to get one of the Chilton or Clymer manuals for your model. While not quite as thorough as a big shop manual, they're packed with useful information and are low priced. Plan on spending between $l5 and $20 for a Chilton or Clymer.

But if you're after the secret codes, there's a section in the genuine shop manual titled 'identification codes." This is the mother lode, the key to interpreting those cryptic tags.

HOW CAN THIS HELP YOU?

Muscle-car fans will recognize this Chrysler favorite. The code tells usthat this is a Charger with a 426 Hemi under the hood.

Sure, you can be a real whiz at parties with your dazzling knowledge of secret codes, but what good is all this know-how? It can be worth its weight in uranium if you're the typical off-roader who's into any sort of competition, fixing up an older vehicle, or just trying to save a few bucks by getting parts at your local junkyard.

By knowing how to read these secret codes, you can tell in a moment exactly what you're looking at, where it came from, what it's for, and you'll even have a good idea as to its interchangeability value.

THE CHEAP WAY TO ACQUIRE CODE INFORMATION

Want to change your gearing with a junkyard rear-axle swap? All you need is a chart like this one that covers Broncos and full-sized Ford trucks.

You'll want to have a good cross-section of codes for truck parts if you have a rnid-'60s Chevy and a late-'70 Ford 4x4. You certainly don't want to buy dozens of manuals. But you can go to your local library; a well-stocked one will have shop manuals. Many libraries also have coin-op copy machines, or you can usually check out the books and copy them yourself.

Put all of these copies into a handy three-ring binder and take it with you to the junkyard. You can now blithely walk up to a bare rame with a motor in it, check out the codes on the ID plate, and know exactly what you're looking at.

THE POSSIBILITIES ARE ENDLESS

Lets say you're building a mud bogger, or just some sort of homemade off-road fun machine. Use of the secret codes can even tell you the capabilities of a braking system. Remember our example code? That fourth digit, the letter "D" tells you that the braking system is designed for a vehicle weighing between 5001 and 6000 pounds. Here's the Ford hydraulic brake guide for lighter vehicles:

CLASS A: Not greater than 3000 pounds
CLASS B: 3001-4000
CLASS C: 4001-5000
CLASS D: 500 1-6000
CLASS E: 6001-7000
CLASS F: 7001-8000
CLASS G: 8001-9000
CLASS H: 9001-10,000

Just in case you were curious, a Class 8 air-brake system will work quite nicely on a 50,000-pound vehicle, so monster-truck drivers should certainty take notice. The code letter is "B' for that set of stoppers.

VIN NUMBERS

Code numbers are usually stamped on the housing of a transmission. This Ford chart is simple and covers about 25 years of trannies.

The Vehicle Identification Number (VIN) is used by various state departments of motor vehicles. For vehicles from 1968-1980, most often it's a metal tag located on the drivers side and fastened to the top of the instrument panel.

The first digit is the model year of the car (0 for 1970, 4 for 1974 and so forth). The second digit is the assembly plant code where the vehicle was built, the third and fourth numbers are the body-code designations, the fifth digit is the engine code, and the last six digits are the consecutive numbers (which start at 100,001) for the first car of a model year built at each assembly plant.

Beginning in 1981, the serial numbers jumped to 17 or more digits or letters, so you really have to pay attention when translating these codes. Why so long? Well, the government mandated things like the type of restraint system to be included in the codes. Go figure.