Broken Crank 3406

First thing that comes to mind is the front damper, if it has been damaged at some time, it can break the crankshaft, I wonder i fthe truck has ever had front end damage, also damaging the vibration damper?

Most high speed crankshafts, especially long ones, need a system to damp torsional vibrations while they are running at certain rpm ranges. The rubber mounted weights bolted to the front pulley damp the vibrations before they can build up to destructive levels. An old, stiffer or softer than normal damper will not function like a new one. As catsilver mentioned, external damage to the damper might cause a sudden failure in the damper function, followed at some point by crankshaft failure.

Some engines probably should have a new one installed at overhaul or certain age/hours. I know some of the larger old JD tractors with the 531 and 619 6 cylinders had a bad reputation for crankshaft/bearing problems and some mechanics will not warranty an overhaul unless the damper is replaced.

First thing that comes to mind is the front damper, if it has been damaged at some time, it can break the crankshaft, I wonder i fthe truck has ever had front end damage, also damaging the vibration damper?

I remember my Diesel Engine Instructor who worked for Finning ( Cat ) for years told us about a D8 (I believe ) that had a miss in #6 cylinder. After eliminating other possibilities he pulled the side covers off and found that the crank had broken just behind the #6 main bearing and turned and jammed, hence the miss ! I Imagine it is extremely rare (especially Cat ) but happens !

wow you would think they would have noticed either the accessories/crank pully not turning or the tractor not movin!! (wherever #6 is)

The engine still ran,the crank fractured, turned slightly,until the peaks of the fracture jammed together "welded" thus placing the crank throw of #6 cylinder out of time a few degrees, thus the miss when the engine was running ! It was not a clean 90 degree cut but a fracture at somewhat of an angle.

I remember, many years ago, my father pulled in with to the works garage with his truck, a 1948 AEC 'Matador' complaining that the engine was running 'out of step'. He was told that there was nothing wrong with it and to carry on, after 30 miles he called the mechanics out because it was getting worse, so they towed the truck back to the garage and dropped the sump to find the crank was broken diagonally across the centre main bearing!
In recent years we rebuilt two 3208 engines which had damged crankshafts in the same boat, after overhaul, the owner claimed they vibrated, after many checks, I went to take a look and found the vibration dampers were both marked, and on checking their history, I reckon they were damaged on the truck that delivered them ,when the engines hit a block of wood behind thhe headboard under heavy braking. The lesson is, if you have a crankshaft failure or the damper has any visible damage, change it!

I read an article in an industry newspaper some time back concerning harmonic balancers. It basically said the same thing regarding any damage to the balancer, replace it. It also went into detail explaining the wear that takes place in the older balancers that had the "ball bearings" in oil, and how they, as any metal part, over time will wear. This , in turn, will cause the balancer to become less effective in it's performance. If memory serves me, the mileage that was given to change the balancer was in the 800K range. I have 2 trucks , both with 3406E's. I changed both balancers. The new ones from Cat were an updated version. After installation, there was a noticable difference in the amount of vibration that I could feel with the engines at an idle. I can only guess that I may have saved myself a larger repair down the road.

so whats involved in changing it? can it be done without pulling a bunch of components?