kmagg wrote:
I am in the process of repairing a CRD for broken rockers. After reading the priceless info here, I ended up pulling the head to send to the machine shop for new exhaust valves just to be safe. This machine shop works on pretty much any kind of internal combustion engine up to and including 1000+ HP cummins engines along with Duramax's & Powerstrokes. The machine shop did the work and afterwards reported the results to me. They ended up having to ream pretty much all the valve guides in the head because they were all tight. Before this the engine had no head gasket or overheating issues. This was all for preventative measures. So, I am wondering if the tight valve guides could be a contributing factor to broken rockers, broken exhaust valves and other CRD maladies?
kmagg:
I believe the following are all related...
1) ...cylinder head gasket failures.
2) ...cracked cylinder heads.
3) ...broken and bent exhaust valves.
4) ...cracked and broken ceramic glow plugs.
5) ...worn out and broken rocker arms.
In the case of the worn out rocker arms, a contributing factor appears to be the pollutants being pumped back into the engine.
So far, there has been no evidence presented that conclusively proves the exhaust valves themselves are to blame for their failure.
The preponderance of anecdotal evidence points to the cylinder head warping for one reason or another. If you twist up any cylinder head enough, any of the above problems can occur. Your machine shop reporting that your valve guides had to all be reamed is clear evidence of this. It is not normally possible for tolerances to get tighter as an engine wears out in its lifetime, so the only explanation I can come up with is that your cylinder head is warping and twisting.
When a cylinder head warps and twists, valves can become misaligned to the valve seats and the valve stem to valve guide clearance, (already one of the closest tolerances in any internal combustion engine); can get tighter as the cylinder head metal that supports the valve guides begins to bind them up when it warps.
The fact that the exhaust valves appear to be the problem almost 100% of the time in CRD engines lends additional credence to my explanation here. Exhaust valves, the exhaust valve seats, the guides and all of the metal immediately surrounding these parts are the hottest part of the engine during operation, creating the greatest differential in temperatures with the parts and metal next to them. This - in turn - creates the greatest opportunity for parts to start warping and twisting, (and cracking, in some cases).
ENGINES LIKE THE V.M. MOTORI R428 IN THE LIBERTY CRD USE CONSUMER GRADE ARCHITECTURE; THAT IS; ALUMINUM CYLINDER HEAD ON IRON BLOCK CONSTRUCTION. A PROPERLY OPERATING ENGINE COOLING SYSTEM AND MAINTAINING FACTORY DESIGN OF THE COOLANT FLOW ARE ESSENTIAL REMEDIES TO AVOIDING THE ABOVE PROBLEMS.