What is the possible cause of a knocking noise in the bottom of an engine when starting in cold weather?

A knocking noise in the bottom of an engine when starting in cold weather can often be attributed to worn main bearings. When the engine is cold, oil is typically thicker and does not flow as easily, which can cause inadequate lubrication of the bearings. This situation can lead to increased clearance in the bearings. When the engine starts, the lower oil pressure combined with the cold, thick oil may not provide sufficient cushioning between the bearing surfaces and the crankshaft, resulting in a knocking sound.

Worn main bearings specifically are designed to support the weight of the crankshaft and help regulate engine motion; if they are worn, they can create a gap that allows for movement or "play," leading to the knocking sound as parts hit against each other during operation, particularly noticeable during cold starts when the engine hasn't yet reached optimal temperature and viscosity.

In contrast, while worn wrist pins, oil leaks, and clogged oil filters can contribute to various engine noises, they are less directly associated with the knocking specifically associated with the bottom end of the engine during initial cold starts. Worn wrist pins relate more to noise in the upper part of the engine related to the pistons. Oil leaks generally would not cause a knocking sound but could lead to other issues like insufficient oil pressure.