Depends on the engine and what you do with it. Most likely failure is a broken conrod if you have it set too lean with too small of a prop. Another possibility is a bearing failure on some engines. (never heard of an OS bearing blowing up, but it's possible) When new, there is more friction in the conrod connections than after the engine has broken in. Also, new engines often have burrs or very small bits of metal left over from the manufacturing process. These all go away pretty early on in the break in, but if you don't have enough lube, it could lead to problems.

The smaller engines can generally turn up higher than larger ones as well. So 24k on a .15 is not the same as 24k on a .25.