I have heard that a common problem with the HT4100 besides the head gasket issue was a Knock that developed in the bottom end. If I'm not mistakenwas this an issue with the Main Bearing having excessive clearance? IS this true? Currently my 1984 Cadillac DeVille RWD has developed a low end knock. This noise ONLY occurs after the vehicle warms up AND is in gear at a complete stop. The noise was reduced with heavier oil and it does not make any noisewhiledriving. In other words it seems that when the car is driving and oil pressure is increased the noise is not present. Only at a stop in gear and after it warmed up.

Hope to hear from you soon!

 

Actually its a thump not a knock. After further research I found this was a very common problem and your car go on driving fine. One suggestion was to use Rotella 15W40.