Spinners make an aircraft look fast, they also streamline the incoming air toward the cowling inlets and play some part in cooling the engine. They also protect, in case of a controllable propeller, the mechanics to control the propeller blades. And as a rotating propeller is hard to see, some paint a stripe from base to the tip which can be easily seen rotating.
Spinners also have been known to cause trouble. Tension cracks can develop where the spinner is mounted on the base plate, thus during preflight the pilot must check to see if any cracks did develop. this is because any spinner which is not in balance will create vibrations and possibly depart the aircraft a lot sooner than you may think.
Most spinners are made from metal or composites and ABS (acrylonitrile, butadiene, and styrene) has also been used in the past. It is prone to cracking due to UV radiation from the sun but with some acetone it can be fixed temporarily.
They basically consist of three parts: the spinner it self, a front bulkhead and a rear bulkhead. The propeller sits between the bulkheads and the spinner will be attached with fasteners to both bulkheads. It must be centered in such a way that it will not wobble when the engine runs. This would create vibration leading to fatigue and maybe a the loss of the spinner during flight.
Spinners can be bought from several suppliers, Aircraft Spruce is one of them. Some propeller manufacturers supply their propeller with a matching spinner so that they always fit and the builder only needs to install it. But if you want to install your own from a stock supplier then there's a challenge getting the alignment right.
As said there are two bulkheads and you should check to see if the rear bulkhead fits the spinner. It is no problem if the spinner does not sit flush, the protruding edge can be trimmed off later on. The inner or front bulkhead should also fit nicely into the spinner.
Spinners need two bulkheads. Some leave out the front bulkhead but the spinner will eventually fail at the rear where stresses might result in fatigue.
You need to measure the actual distance where this bulkhead sits as the propeller will need to go snugly between the bulkheads and any spacing must be filled to prevent stress.
The alignment of the bulkheads to the propeller hub is important as sometimes the propeller bolt holes in the rear bulkhead are over-sized to accommodate any prop extension or drive lugs.
You will need to save the cutouts for the blades as they can be used as a filler when the spinner is installed. This is to fill the gap underneath the blades. Some may opt to leave them out as inspection is a bit more difficult but it looks great when these gaps are closed.
The spinner will be drilled to the rear bulkhead on the engine. It must be fitted snugly to the bulkhead and the engine must be free to rotate by one hand (remove the spark plugs). This makes it easy to see if the spinner does not wobble. If this is correct you can drill evenly spaced pilot holes through the spinner into the bulkhead. Use anchor nuts in the bulkhead and stainless steel screws for the spinner.
The spinner is not the easiest part to inspect during preflight, especially the bulkheads. Look for the obvious signs of cracks, they might indicate stress where it should not be. Damage or chafing to the cowling could be caused by a spinner which is not aligned properly or an engine with too much movement in the mount, do have that fixed too. During annual inspection take the opportunity to remove the spinner and cowlings too.
Homemade spinners and bulkheads usually are more susceptible to problems as the stock spinners from aircraft or propeller manufacturers have the bugs already ironed out. But even they can develop cracks due to improper alignment and or stress during assembly on the propeller hub or rear bulkhead.
When spinners fail it is usually in the first few hours after installation indicating an alignment or stress problem. After that it is quite uncommon for a spinner to fail. Unless they are made from ABS, which eventually succumbs to UV radiation unless the proper additives are added to the mix.