First, consider the size and capabilities you need. Was your old rig big enough to do the work, or do you need more power? Don't be fooled into thinking that bigger always is better. Too big a rig will make some locations inaccessible or require a lot of winching, plywood and site restoration. Too small, and you'll have to “run it up in the curve” so far that it will wear out quickly. Is it time to make the change from a kelly rig to a tophead? If so, bear in mind that these two types of rig drill differently and take a little getting used to. There always is a learning curve to a new rig, and major changes in rig type tend to steepen this curve. Each rig does some things better and some things worse than the other; it's just a matter of playing to the strengths of the design.
After figuring out what you need, read the ads in the National Driller, talk to the brokers, go to conventions and generally keep your ears open. There are enough rigs out there to find the one you want.
There are different reasons why someone would sell a rig, and figuring out the reason will help you in your selection process. Sometimes the rig just is plain worn out, in which case I hope you buy it by the pound! Sometimes the rig needs more rebuild than the owner wants or is able to do. If you can do it, you can save a chunk of change. Don't worry too much about the truck that it's mounted on - 75 percent of the used rigs I've looked at need remounting anyway. Keep in mind that a good rig can wear out four or five trucks during its useful life. Besides, you can mount it on a truck that is suited to your operating conditions. Sometimes, a person will sell a rig because a specific contract that he bought the rig for is done. This can be a very good deal; the seller already has significantly depreciated the rig, and it's probably in pretty good shape.
Fairly often, we see rigs sold because the owner dies or retires. These rigs generally fall into one of two categories. The rig may be the best rig the driller has ever owned in his life, or it may be in about the same condition the he is! If you are buying the rig from his widow, keep this in mind: Lots of drillers don't have any structured retirement fund and maybe not much life insurance. This may be all the money the widow is going to get for all her husband's work. You might be able to buy the rig for pennies on the dollar, but you wouldn't want that to happen to your wife when you're gone. Offer a fair price; if the rig is what it's supposed to be, it'll pay for itself. Leave the poor mouth con game to the used car guys!
When you have selected a rig, a careful examination is in order. If it's a rig you are well familiar with, look it over. Drill with it if possible. A couple hundred feet of hole will tell you more than a checklist the size of the Houston phone book. Think about the mechanical problems you've had with your present rig. If the new rig is a similar make and model, look closely at these areas to see if the former owner has worked on them, too.
Look closely at the components that add up to a major expense to rebuild. A mud pump gear-end rebuild definitely will hurt your feelings if you don't expect it. Fluid ends almost always need to be rebuilt on used rigs, so be prepared for that. Check the rotary table closely; the bearings and seals are notoriously expensive and hard to change. A good, easy check of the rotary is to check the pinion shaft and u-joint. That sometimes is one of the hardest places to grease and often is forgotten in routine maintenance. If the u-joint is dry and the pinion shaft loose, it's a pretty fair bet that some other important parts of the rig have been “overlooked,” too.
Another place to closely inspect is the derrick and crown. If it is stock, look for serious bends or breaks. If it has been modified, look very closely and consider a magnaflux inspection. This part is going to be over your head, and if it ain't right, you are gonna have a bad day with a scrap iron necktie!
Lastly, if you are not familiar with the rig or have any doubts, consider getting a professional appraisal. Most of the rig brokers can make a decent appraisal and tell you about the ins and outs of a particular rig make and model, plus any mechanical deficiencies of a particular unit. It's well worth the money spent. I've done many appraisals, and my customers have never griped much if I told them that a rig was junk or if it was a heck of a deal. Money saved or money well spent is worth it.