The US Army used the following post-rebuild break-in procedure for the GMC 270 (CCKW and DUKW). It assumes you are using a dynamometer, but may be helpful for general guidance. The military wanted the engine to be fully broke in before putting the truck in service for obvious reasons:
1. Run for 15 minutes at 700 rpm with no load.
2. Adjust valves to 0.012".
3. Run for 30 minutes at 800 rpm at 10-15 hp.
4. Run for 30 minutes at 1000 rpm at 25-30 hp.
5. Run for 30 minutes at 1400 rpm at 50-60 hp.
6. Run for 30 minutes at wide open throttle with load adjusted to maintain 1800 rpm.
7. Run for 30 minutes at wide open throttle with load adjusted to maintain 2400 rpm.
8. Idle engine (no load) for 15 minutes.
9. Tune engine.
10. Change oil.
Taken from War Dept. Technical Manual TM9-1802A
