Test running a freshly rebuilt engine is as important as building one. As a liability claims specialist for three companies, I have seen the results of many engine failures due to improper test running or no test running at all. This is a very critical time for a freshly rebuilt engine.
Even before you fire it up for the first time, you want to know that the results of the "Rebuild" were successful. To do this you just perform the same tests on the engine you would to determine if the engine needed rebuilding.
Before any tests can be preformed, you need to be sure the ignition timing is correct and fuel mixture is correct or a hair on the rich side. If this engine has been modified during rebuild you need to set the timing close to standard specs for start up and the fuel should be set rich. Always work from too rich toward what is needed for the modified engine. Timing will usually go from standard toward advance settings but don't count on that as your modifications may require a more retarded timing.
1. Perform a "Wet" and a "Dry" compression test. This confirms that the basics are complete. The test may show a little low on several cylinders and may even show a good variance between the wet and the dry tests as the new rings may not have seated yet.
Warning!!! If you used cam lube (as you should have) and used any lube other then the oil then the oil you intend to use on the bearings etc. Do NOT use Castrol Syntec oil. I painted my cam lobes and lifter faces with "Kent Cam Lube" and painted my bearings with a 50/50 oil and STP mix and it turned the Castrol Sysntec oil into some unknown substance that was obviously not oil in only 20 minutes of test stand running of a fresh rebuilt engine. I contacted Castrol as I have been a user and supporter of Castrol Oils for over 50 years and they brushed me aside with stupid statements like "Castrol Oil has a shelf life of only three years and I should not have used Castrol Oil in a modified engine and for those reasons, they refused to test or even look at samples of this oil.
When less then a teaspoon of Cam Lube and STP are used on a rebuilt engine and it turns 4 qts of Castrol Syntec oil into something other then oil and Castrol don't want to even look at it, tells me they have something to hide. Also it proves their responsibility to their customers is ZERO.
2. Run an oil pressure test with a manual gauge before you install the engine. With the plugs out you should spin the starter until you finish filling the oil filter and get oil pressure. Many engines can have the starters bolted up without a transmission. If you can't do that, use a drill motor to spin the oil pump. The pressure will vary with different engines but you should see 35 PSI or more with the starter or drill motor. Don't start an engine to get oil pressure!!! Unfortunately, most mechanics do it this way. Sooner or later, this practice will bite them in the butt. This is especially true if they assembled the engine lubing everything with just engine oil. By the time they decide something is not right, the damage to the bearings has already occurred. The practice of spinning the oil pump with a drill motor works well if you see that there is an immediate oil pressure on start up. A friend used that method all the time and on one engine that gear drives the distributor and a hex shaft inserted into the bottom of the dist. drives the oil pump. He started the engine and didn't check the oil pressure. Unfortunately, the hex shaft he installed was a little shorter than needed and he locked up a couple of bearings before he noted anything wrong.
3. With the oil pressure test completed, look at all the oil lines an any known joints that have oil pressure going between any cases for any signs of oil leakage. Even though you only used the starter or a drill motor to test oil pressure, any major leak will show. You don't want to find a loose screw in type oil galley plug between the engine and transmission on a road test. Or a oil filter block gasket not correct after the car has gotten on to an expressway.
4. Coolant test next. If you don't have a pressure tester you should try to borrow one. Many mechanics just use the running of the engine to test the coolant system. This is OK but a lot of time and worry can be avoided with a coolant pressure tester. I will cover both tests for those who don't have a pressure tester and don't rebuild enough engines to warrant buying one. First with a pressure tester, fill the radiator and expansion tank if it has one and clean under the car or move the car to a clean spot. You might think that a little room for heat expansion would be needed but you will find that there are many air pockets in a cooling system at this point so the level will drop as soon as you start the engine. First look at the cap and note the pressure rating and confirm it is correct for the car. Then use the pressure tester to see if the cap releases at the correct pressure. Then pressurize the cooling system. Run the pressure up a little over what the cap is rated for. Now, take a brake. Check on the pressure every so often and pump it back up if needed. If you tester is not leaking, it should stabilize soon and hold a pressure. Look under the car for any wet spots and backtrack up to any possible leaks. A mechanic's mirror and a light are useful at this time. If it holds pressure over a coffee brake or lunch, it is a good one. Leaks can still develop when an engine heats up but you have a good start if it holds cold. Next, run the engine a little until you get a normal operating temp on the gauge. Check for any leaks and road test the car. As soon as you return with the engine still running do another close inspection for leaks. let it cool down a little and check and fill to the correct level as most of the air pockets will be flushed into the radiator. Be aware that some systems are strange in that they need to be bled of air when first filling. Some have bleeder valves for this and others require that you loosen a heater hose or other hoses. One strange one like the VW Rabbits require that you remove a lower hose and fill the hose as well as the radiator.
Without a pressure tester, fill the radiator and only half fill the expansion tank if it has one. With the cap still off start the engine and set it at a fast idle. You may see the coolant level go down very soon so keep it up by adding coolant. Use a 50/50 water/antifreeze or a 40/60 mixture (what ever is recommended for your area). As the engine warms up you should not need to add coolant. Put the cap on and run it a while longer watching the temp gauge and looking for leeks under the car. Engine temperature may run just a slight bit higher than what you seen on that engine before rebuild as more power and the strain of brake-in will produce a little more heat than normal. This should stabilize and show a normal temp soon. keep the first road test short so any leaks that develop can be corrected before much coolant loose can cause any damage. A good rule to follow on a road test (or any time on the road) is, If you note over heating on the gauge pull over at the closest service station. If you notice a loose of oil pressure, reach for the ignition switch as fast as you can (Being very careful not to lock the steering) as you pull over to the side of the road.
Damage to an engine when it starts to over heat is measured in minutes. Damage to an engine when it looses oil pressure is measured in seconds.
After a successful road test it pays to go back over and retighten hose clamps an give a good visual check for coolant, oil or fuel leaks before calling it "Complete".
A "brake-in" period is needed to get the rings to conform to the cylinder walls. Bearings usually don't need any brake-in, although it is still a good idea because if anything is not right it will usually show up in a brake-in period. A common problem is coolant and / or oil leaks. After assembly and getting heated and cooled several times a joint can develop a leak so it pays to keep a close watch for leaks after a rebuild. As a line mechanic I told every customer of mine that they must come back after 300 to 500 miles for an oil filter and oil change to get the "Brake-in" oil out of the engine. There are a lot of metal shavings made by a fresh engine rebuild in those first few hundred miles. The old "wives tails" of "If you brake it in fast, it will run fast" is not true. A freshly rebuilt engine may be tight and a fast brake-in MAY loosen it up sooner but the damage inside is not worth it.
There are reasons to need a fast brake-in but damage is still done. For example, When I was racing AMA flat track, a friend rebuilt his BSA Gold Star just before a race but he didn't get time to brake it in. He said he would just try to get in some easy running during practice at the track. He had just put in new rings on the old cylinder and they would not seat which made it burn oil and fouled the plug. He went to a local grocery store and bought a can of Bonamy cleanser. He set the RPM at a high speed and held a teaspoon of the cleanser up to the open carburetor and the engine was billowing blue smoke out the exhaust. As he got the teaspoon close to the carb. the cleanser was sucked into the engine and the blue smoke quit immediately. While this solved his immediate problem, it did cause damage to the engine as noted on tear down after that race. The same is true on a fast brake-in. A good "rule of thumb" on a car engine is, "Don't let it idle for long periods of time, Don't apply heavy loads at low RPM and don't run at high RPM for long periods of time." Gradually increase the RPM over a 500 to 1000 mile period. On most four cylinder engines I tell customers to not apply any load under 2000 RPM and don't run the RPM over 3500 RPM for the first 300 to 500 miles. On a domestic V-8 engine I would just lower the RPM limits by 500 to 1000 RPM. Then gradually increase RPM after that and change oil and filter often at first and never go beyond 3000 to 4000 miles between oil and filter change no matter what the manufacture says.
A few final tips, even though many head gasket manufactures claim "No re-torque necessary" I still re-torque after 3 to 5 hundred miles. (always a cold engine). If the head studs extend to the bottom of the block through the coolant, be sure to install an anti electrolysis in the coolant. (this is available from most truck parts stores).