It’s the engine that more than doubled Cummins’ share of the Class 8 market in just eight years’ time. It was extremely durable, powerful, and returned best-in-class fuel economy. It’s called the Cummins 855 Big Cam, and it dominated the heavy-duty diesel segment in the 1970s and 1980s. But, far from perfect, this 14.0L inline-six isn’t without its shortcomings. Many of them have surfaced in recent years due to age, but others have existed since the 855 (Big Cam and Small Cam) was still relatively new. From NVH and father time-induced oil leaks, PT pump failure to liner pitting, and the problematic low-flow cooling system they came with, there were problems and solutions to address. We’ll cover all of them in detail here—and then wrap up each summary with the best solution(s) for this time-tested workhorse.