When looking at the engine compartment of any aircraft, one can become bewildered by the amount of wiring, tubing, hoses and controls needed to operate and run the engine. Double that if its a twin aircraft. Installing an engine is a complex task, but taken step by step and subsystem by system it can be done in a reasonable amount of time without any difficulty.
Installing an engine for the first time can be daunting but with the help of other builders and maybe a friendly aircraft mechanic its not that challenging. You will have to plan ahead and get all required items, tools and parts layed out in an orderly fashion in your shop, saving a lot of search time when you need them.
It also pays to talk to other builders of the same type of aircraft, this will help to avoid some of the pitfalls you might encounter during this task. If possible, make a lot of pictures of their installations to study and for inspiration.
Before installing the engine you will need to check when the airframe is ready to accept the engine mount and the engine itself. These items are quite heavy and will change the weight and balance. If your shop does not have the room to fit the engine and fuselage then it might be helpful to move the project to a more roomier location. You will need some space to walk around the fuselage and move things about.
As already said, the engine is quite heavy, and hoisting it by yourself to install it on the fuselage might be overdoing it. Get some help from friends, a good hoist and some strong slings should the engine not have a lifting eye. Be careful not to trap any delicate engine part under the strap. It will get damaged.
Now you need to make sure that you have enough room to maneuver the aircraft and engine hoist around. For some aircraft you might want to bring it into level flying attitude or lift the tail if its a taildragger. Chock the wheels and immobilize the tail of the aircraft, especially so if the tail section and or wings are not installed.
Always use new shock mounts when installing or replacing the engine. I would go for new bolts, washers and nuts too, its really cheap insurance. Buy a good and calibrated torque wrench as you will need this to properly torque the bolts, rubber bushings will continue to compress so a properly torqued bolt is very important.
Tighten the engine bolts uniformly with the correct torque value, re-torque after the engine has run a couple of hours. Again, remember to DO NOT over-torque these bolts!
After the engine is mounted it is electrically isolated from the aircraft fuselage due to the rubber shock mounts. Therefore you will need to attach a firm ground strap to provide an electrical connection from the engine to the fuselage. The alternator and starter motor will need this to operate properly.
Air cooled motors need baffles to guide the incoming air over the cylinders keeping them from overheating. The oil radiator will also be installed in this area as it needs cool air too.
Liquid cooled models like the Rotax will sometimes use incoming air to cool the cylinders, most notably the Rotax 914/5 and some 912(i)S engines have forced air cooling with a composite guide installed over the cylinder barrels. I would recommend always using such an air guide.
At minimum you will need at least a throttle cable, but Lycoming and Continental and some diesels need a mixture cable too (to stop fuel flow). Those of you installing a hydraulic controllable propeller of some sort will have to install that cable too. Electric controllable propellers will need wiring and control box in the cockpit.
Make sure that you order the correct length of control cable, maybe a bit longer so that it can be routed out of the way of the hot exhaust and other essential items.
Now is also the time to install other control cables too: carburetor, cabin heat and cowl flaps might be on the list for your aircraft.
Oil lines, vacuum systems, fuel supply and the primer line need hoses and tubing to operate and they also need adel clamps to fit them securely to the engine mount.
Last but not least you will need electrical wiring, and it definately must be aviation grade. This is fire and chemical resistant insulated wiring, Teflon/Kapton/Teflon type of insulation and those derivatives are the best. Stay away from household grade wiring as the insulation will shrink, melt, short circuit and burn easily.
You can test this with a soldering iron: heat the insulation with it and if it does not react to the heat you might be able to use it in the engine compartment.