The Offshore Voyaging Reference Site

Cummins Marine 6B5 Diesel Engine Manufacturing Problems

The Problem

When doing a scrupulous 50 hour check of our then brand new engine, I (John) found a bolt in the bilge. After much twisting of myself into awkward positions with a trouble light and a mirror, I finally found where it had come from: the oil pan. Things only got worse.

When I reinstalled the bolt and tried to torque it to the required value, it turned out that the threaded hole in the aluminum engine front cover was stripped. Wait, there’s more:  When I tried to torque the other front and back oil pan bolts that fit into aluminum (the side bolts go into the steel block), four more were stripped. My surmise is that the factory worker assembling the engine forgot to set his or her torque wrench to the correct setting for a bolt going into aluminum and over-tightened the bolts.

Manufacturer’s Response

Cummins accepted responsibility without an argument and agreed to have the engine repaired at any authorized Cummins dealer.

The Outcome

We wanted to get this done right and so did a lot of research before selecting Billings Diesel and Marine in Stonington, Maine. Billings jacked the engine off the bed, helicoiled the holes (a steel thread insert that can be used to fix stripped threads) and tightened the bolts to the correct value. A good job, quickly and professionally done.

Three months later all the bolts were loose again and the oil pan was leaking. Another dealer took the engine off the bed, replaced the gasket and tightened the bolts.

The third time this happened, after replacing the gasket and tightening the bolts, we put Red Loctite on the bolts.

Cummins paid each time, although they did try to argue the third time.

Lessons Learned

  1. Check the engine over carefully and frequently during the run-in period.
    Brand new engines, even those from quality manufacturers, have a surprising number of problems early in their lives.
  2. Not all dealers authorized for warranty repair are equal.
    We found two really good ones by doing extensive research and recommend that, if you have a warranty problem, you do the same.
  3. Be really tuned in to unusual noises.
    We recently saw a big diesel, with less than 150 hours on it, going through a forensic disassembly that showed that it was just about to fail catastrophically—probably by throwing the connecting rods out through the side of the engine—when the owner, hearing a ‘funny noise’, shut it down. When it was disassembled, it was found that the bolts holding the bearings that join the connecting rods to the crank shaft were only finger tight. Another torque problem, this time the opposite of ours.
  4. Give your engine a good workout before taking off.
    Try and put at least 200 hours on a new engine before taking off on an extended cruise far from help; 500 would be better.
  5. If using helicoils, use a thread lock material such as Loctite.
    Bolts will loosen out of helicoils from vibration more easily than from a normal thread. In fact, we use Loctite on just about anything that could possibly back out.

Wait, it gets worse.

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