Exhaust guides lead a very difficult life. These copper, bronze or steel sleeves sit in between the exhaust valve's stem and the cylinder head, and they must absorb not only the tremendous friction created by the reciprocating valve but a fair amount of the exhaust heat as well. Even a tiny imperfection in material, fitment or installation can result in a broken exhaust valve guide and quite possibly engine failure.
An exhaust valve opens twice for every engine revolution and may travel as much or more than half an inch every time it opens. The exhaust valves in an engine running at 3,000 rpm slide through the valve guides at about 500 feet per minute. Over time, this will cause the softer valve guide to wear away, becoming thinner and more prone to cracking.
Car manufacturers don't get things right 100 percent of the time. The original valve guides may have been too thin or made of an alloy that tended to work-harden and crack over time. The factory may also have miscalculated the guide material's heat expansion characteristics. Material casting defects aren't uncommon either; even a tiny imperfection in the guide metal's crystalline structure can act as a starting point for cracks.
Overheating can cause a multitude of problems, most of them resulting from heat-induced metal expansion. As metal heats up, its crystalline structure widens before breaking down completely and melting. Different metals expand at different rates. Extreme overheating can cause a valve guide to expand far more than it should (causing it to clamp down on the valve or become warped) or cause the head to expand away from the valve. Either way, the valve guide weakens and eventually becomes prone to breaking.
High Exhaust Gas Temperature
Valve guides act as a conduit to conduct combustion chamber heat away from the valve and into the cooling system. The extremely high exhaust gas temperatures experienced by highly boosted, turbocharged gas and diesel engines can create a temperature differential between the valve side of the guide and the coolant system side. The valve side heats up and expands more than the coolant system side, causing the guide to literally rip itself in two.
Under some conditions, the valve guide may go out of alignment with the valve seat. This lack of concentricity is usually the result of heat induced head warpage from overheating, welding on the cylinder head or improper machine work. Misalignment between the seat and the guide will cause the valve stem to flex every time the valve head meets the seat, putting a side load on the guide. This will cause premature guide wear and breakage, and in extreme cases can cause the valve itself to crack and snap. This lack of concentricity may also be a factory defect due to post-head-casting valve guide installation.