hello, i am an ase master tech with a 1979 300d. i work in a euro shop and decided to rebuild the engine in this car about a year ago. i had new bearings, new liners installed, and new 91.00mm pistons as well. the engine was fully rebuilt and assembled by a very experienced machinist that does all of the machine work for my shop. the engine always had a bit of an irregular noise to it, i always figured it was related to the 5 cylinder layout. after the rebuild, the irregular noise was still there, but since it had always been there, i again thought it was normal.

over the past few months, the noise has gotten louder. it is a very pronounced knock with that seems to happen every power stroke. the noise can be pinpointed with a stethascope to the #1 cyl, on the pass side of engine, right where the wrist pin is at TDC. it sounds loud at low idle,and it seems to quiet with load, although as the accelerator is backed off, it gets louder. i have removed the piston/rod assy for inspection and had the machinist replace wrist pin bushing (after finding out it was not replaced during rebuild) and rod bearings (just because i had an extra set) and verify the clearance ok. after install, noise is still there. cracking the injector line to #1 quiets the noise, but not completely. i am very frustrated by this, so if you think you can help me i would appreciate it. if you know what would cause that exact noise, i want to hear from you.

side note: why is the factory spec for cyl #1 bore .010mm larger than the rest? i'm not 100% sure this was done.

thank you
eric