Why do some spark plugs have a screw-on contact terminal?

Because they are also used in applications that use the threaded stud as the contact. Many snowmobile and military engines use the threads to retain the wire. Keeps the wire on regardless of vibration.

small engines use them.

Doc wrote in news:9dd9d43a-b2c7-4378-a717- snipped-for-privacy@b6g2000yqd.googlegroups.com:

For the reasons given by others. Very old cars (Ford Model-T, for instance) also used this type.

I've been told not to use the screw-top types on modern automobile engines. If the screw tops happen to be loose (or come loose), they can cause arcing and ignition problems.

yeah in theory, but in reality, they cause more reliability problems than they solve. screw-on terminals aren't typically weather sealed in the way that push-on terminals can be. and the threads themselves can be problematic too. when trying to use a push-on over a screw cap [as is typically the case with modern applications], if the cap isn't absolutely tight, there can be sparking and erosion under the threads causing all kinds of intermittent misfires that are otherwise hard to diagnose. to make matters worse, caps have a nasty habit of loosening over time [thermal cycling] and what may once have been "fixed" can soon become a problem again.

bottom line, i won't use them - only one-piece capped plugs for me.

Use jam nuts on them and they won 't lossen. My 1914 Model T Ford car has the old style spark plugs. The spark plugs also can be easily disassembled with two wrenches for easy cleaning.