This name is used for two types of tools, either a wrench designed for turning spud nuts or a wrench with a spiked end. Both of these have very specific uses in plumbing, metal working, and electrical work.
Plumbing Spud Wrenches
A spud washer creates a tight seal around the outside of pipe fittings such as the inlet hose connector and the tank on a toilet. This washer is held on by a spud nut. These nuts are generally made of brass or plastic making them very easy to damage. Spud nuts can also be very large to accommodate wide pipes all but requiring a specialized tool to turn them.
Multi-size: The simplest wrench, this uses stepped jaw surfaces to accommodate a wide range of nuts.
Adjustable: This tool closely resembles a pipe wrench, except the jaws are flat to avoid damaging the surface of the spud nut.
Socket wrench: Most spud nuts also have a small tooth placed at a ninety degree angle from the wrenching surface. This allows the nut to be turned using a compatible socket.
Electrical and Metal Working Spud Wrenches
Like plumbing pipe, electrical conduit also uses spud nuts. However, these nuts are used to hold lighting fixtures, not seals.
Since these nuts don’t have to deform to create a tight seal, they are made of stronger metal that can be turned with regular hand tools. However, these nuts are almost always round requiring the use of a spud socket wrench.
A metal or iron-working spud wrench is any tool which has a spike on one end. Sometimes called a podger, the spike is inserted into bolt holes to line up metal pieces before inserting a bolt through them. The opposite end may be an open-end wrench, box-end wrench, or ratchet. Electricians also use this tool for lining up conduit and fixture bolts.
In other words, an electrician could use a metalworking-style wrench to get a piece of conduit in place and then switch to a plumbing-style wrench to attach a spud nut.
How do I know which spud wrench I need?
It’s just as likely that you will find the wrench size by the name of the product you’re working on as you are the size of the nut. There are toilet, sink, and even garbage disposal spud wrenches. Spud nuts used on these plumbing fixtures are standard, so all but the oldest plumbing will work with these universal wrenches.
Most electrical spud nuts are 5/8″ or 3/4″.
Metalworking spud wrenches will have a spike tapered so that the widest diameter will be slightly more than the stud that would match any bolt head that would fit the wrench end.