Originally Posted by Christie McGill

Mustang engines probably have roller cams, so you could break them in with cyn.
"Break in rockers" Never heard of such a thing ? Maby you mean valve springs ? To remove the center valve spring on a dual spring is a standard practice for break in if you are running dual springs.

Originally Posted by mrford49

If your engine is a brand new crate motor you can break it like gearhead351 and odom1957 told you. But if it is a rebuilt engine that you had it bored and put in a new set of pistons with moly rings and also a cast iron type cam and lifters and stock style rockers. You should break it in using this method so not to do damage to any metal to metal parts. Remember a new engine will create a lot of heat (friction). Stay away from synthetic products it will cause galling of metal parts and your bearings, and as mentioned the rings will not seat to the bore. I use ONLY racing oil for my break ins. You should never have to retorque any engine bolts after they where torqued and long as everthing was cleaned and lubed correctly before installation. Most moly rings are LOW friction. Make sure your cooling system is up to the job before starting your engine because you will be running it for and hour or two to break it in correctly. So make sure your timing can be set right away to much advance is better that being retarded. I always set my using total timing of 30 degrees for break in time. The (break rocker arms) that where mentioned is to put less pressure on the valve train to keep the springs from wearing out to soon from the break in heat (friction). 15 to 20 minutes is NOT enough time to break in a rebuilt engine. I owned a Speed shop and built and installed over three thousand engine in vehicles. I built most of the engines myself. We tried many methods but what worked best was to run the engine at 180 degrees to 200 degrees temp for a minmum of one hour if the engine had a billet roller cam and if it was built with a cast cam we ran it as long as two hours to break in all metal to metal parts. Keep your eyes on the oil pressure and the water temp and oil temp. If the engine starts getting to warm put a big fan in front of the vehicle and cool it down DO NOT TURN OFF THE ENGINE. Now if it is an all out race engine things are differant. Thats because it is built with all the proper specs for all parts. Blueprinting! So it is clearanced for a lot less friction meaning horsepower. Now a race motor can be broke in with only 15 to 20 minutes of break in time. you should change the oil and filters after you are finished with the break in. The performance bearings made today are a lot better than they used to be with better coatings. Now after that is done break in the engine like odon1957 told you and it will give you a lot of great years of service. Remeber all factory engines are broke in before being installed or shipped out to the customer. After about 5000 miles you can start using synthetic oil in your engine only if it is a stock type engine. A blueprinted race engine put synthetic oil in it right away. The reason you run the engine at 1800 to 2000 RPM's is to keep the load off the valve train and helps engine run cooler. Make sure your fan is sucking plenty of air thru the radiator. Sometimes with big block engines you have to use water to help it keep cool for the first hour, so be ready to use a water hose to keep it cool. I use water only to break in a new engine. after break in I then install new antifreeze. The only bolts that should need to be retightened are header bolts. But a lot of guys like to check every bolt its up to you. Its better to be safe than sorry. remember not run the carb to lean at break in time it is better to be a little rich with your mixture. Most injected engines will not have problem. And I always add extra oil and temp gauges to monitor the engine while breaking it in. Remeber to leave the engine at steady RPM during break in time lowering the RPM as time goes on no lower than 1500 RPM's. Good luck with your project. Ron