Ring Filer Death, taxes, and gapping rings. Those are the immutable laws for engine builders. Rings have to be gapped to specific tolerances (provided by the manufacturer) so that they close up and properly seal off the cylinder bore when the engine heats up. The ring filer pictured is a top-of-the-line electric model with a motor that does most of the work for you, but there are also cheaper, hand-cranked models that do the job just as well.
Height Micrometer If you are installing your own valves and springs, the height mic is the quickest and most accurate tool for checking the installed height of the valvespring. You simply install the mic in place of a valvespring and then run it up until it has seated the valve, locks, and retainer. Spring height must be consistent across all your like valves (intake or exhaust) for you to have consistent spring pressures. If it isn't, your machinist probably cut a seat too deep. Knowing your spring height also helps you determine if you have enough valve travel for your cam.
Ring Compressor There are several versions of piston ring compressors, and they vary greatly in price and ease of use. You need a ring compressor to hold the piston rings tight in the ring grooves for the piston to fit in the cylinder bore. Generally, the tapered-sleeve-style compressors (right) are quicker and simpler to use. You just slide the rod and piston through the top of the sleeve, place the lot of it over the cylinder bore, and press the piston on in. Tapered ring compressors are also the best at getting a ringed piston into the bore without causing ring damage or breakage.
Connecting Rod Vise During the process of sizing your rods and installing the bearings, you will have your rods apart two or three times. A rod vise holds a rod securely so that you can properly torque the rod bolts and then get the rod cap off again without harming either the rod or the bearing.
Telescoping Gauges If you can't swing the cost of a dial bore gauge (depending on quality, they vary anywhere from $150 to $1,000), you can get by with a pair of dial calipers and a set of telescoping gauges. The gauges find the maximum inside diameter of an object and lock in place. You can use your dial calipers to measure the gauge to get your ID measurement. They're good for tight areas where it is difficult to get a dial caliper to fit.
Feeler Gauge This tool is used for those areas where the clearances are simply too tight to use a micrometer. A feeler gauge is most often used to determine end gap on your piston rings (slide the ring in the cylinder bore without the piston and insert the feeler into the gap) and for setting valve lash. Each of the feelers is marked with a specific thickness.