I'm interested in others' thoughts on this. First my theory:

As the proud owner of 3 apparently good #14 heads, I have leaned toward heads from properly maintained cars, with minimal boost. So far I'm batting 100 (whatever that means), as it appears that all three engines have good heads.

The reason I look for minimal boost, is that I suspect metal fatigue as the failure, and not necessarily from thermal cycling (but it can be a contributor).

The more boost, the more bang (Alda regulates fuel enrichment based on boost pressure). The more bang, the more stress/strain on the areas surrounding the bang. More heat also, so I'm not sure how to involve them both in the matrix, other than they are in direct proportion to each other, or more boost = more enrichment = more heat & more pressure.

The later head castings were reinforced around the water jacket according to the FSM. Whether this area fails due to being too weak overall, or merely being the weakest point and therefore the stress concentration of the part I'm not speculating, either could be true.

When in the casting business, one of our clients made Police cars, and we were quoting intake manifolds for that engine line. The Engineers at that automaker specified a PA6/6 (nylon) with glass fill, worked pretty well for the regular engines. For the Police car, the extended full-throttle runs caused the intake manifold to fail in the water jacket crossover, we found water pressures around 130psi (pressure pulses) at full-throttle were common. Needed to go to a cast intake manifold.

So, in a diesel engine, pressure pulses are even higher, and a weak point in a water jacket, especially sharing a wall with a combustion chamber, would be subjected to a great deal of shock and stress both thermal and mechanical.

In addition, I'm wondering about how cavitation pitting might affect the head combustion-water jacket interface. I know how it affects cylinder liners, and how important nitrates and molybdates are to that surface, could it also lead to thinning of the water jacket wall in the head? If so, would owners' and mechanics' almost universal lack of understanding of the coolant needs of a diesel engine have led to many of the failures?

I'm interested in others' expert opinions and discussion on this.

__________________

Gone to the dark side

- Jeff

Jeff,

(as you know no expert here.)

Cavitation is a very powerful force (as you've reminded).
I wonder if any forensic inspections have been done with specifically that in mind?

I assume that's what you're getting at?

1.When you discuss it's affects on cylinder liners are you referring to "Wet"liners?
2.Nitrates and Molybdates necessity to the liners surface? ("Lucy", as in, "You've got some S'plaining to do".)
3.What are the universally mis-understood requirements of Diesel cooling systems?

Thanks
(Can't discuss, if I don't understand)

__________________
'84 300SD sold
124.128

All the above is true, however, if it was the primary cause of head cracking, you'd see a slew of heads that crack when they reach 200K or 250K or so. The fact that there is no definitive mileage level where a crack occurs will tend to point you to another theory.

The generally accepted theory is that the original head will survive perfectly well, indefinitely, if it's not overheated. But, one overheat will run the risk of a crack developing and propagating over time.

This theory fits well with the failure rates. There is no mileage level that is characteristic of the failure.

The effect of cavitation pitting is usually most critical on wet liner engines, although I've read that it is an issue on dry-sleeved engines also. Most of the discussions & papers are regarding larger engines such as the big Detroit 71/92/60 series engines, big CAT 3406 to C-14 et al. It is always on an iron cylinder casting however, so I don't really know if it affects the head in the same way, and whether it would affect cast aluminum alloys. Further, cavitation pitting is typically concentrated on one side of the cylinder liner, not all of the way around, I believe it is toward the non-drive side of the engine/cylinder. Again this implies that location/orientation is a major factor in the metal loss, so in the head?

It might be that no data has ever been collected retarding any cavitation pitting similarities in a head, especially an aluminum head, as there really aren't many aluminum-head diesel engines out there in the commercial world pulling loads for millions of miles. Further, I'm not a Mettallugical Engineer, so I don't understand why Molybdates and Nitrites are the additives / SCAs that prevent cavitation pitting in cast iron, or whether there is an equivalent need and prevention for cast aluminum.

Agreed, it would be best to have a failed head and the equipment & knowledge to do a forensic analysis on the head to see if there is any metal missing in critical areas, and whether it is due to cavitation pitting or some other means be it electrical or mechanical erosion.

I'm just looking for clues here, hoping that someone has torn into a carcked head. I know there was someone here looking for bad #14 heads to attempt a repair, didn't know if anyone had ever sectioned a bad head or observed anything.

__________________

Gone to the dark side

- Jeff

I don't agree Brian with the contention that all engines would fail at the same approximate mileage. Some engines have low pressure loading with long highway trips running sub-20hp cruising, some with high-power / high-boost launches in city driving, some run lots of short trips with the associated thermal-cycling, some maintained better, some in the south with higher ambient temps and regular A/C use.

Of the 4 I've encountered so far:
245,000miles w/ good boost & low ALDA
188,000miles w/ switchover valve broken - no boost enrichment
248,000miles w/ switchover valve clogged - no boost enrichment
186,000miles w/ wastegate stuck open - no boost

No real repeatability on this, and it is difficult to tell how long each has been in such a state.

__________________

Gone to the dark side

- Jeff

Yes, I agree that engines in various states of disrepair will skew the results somewhat.

But, there are folks on the forum with engines in proper condition that have survived with 300K or more on the #14 head. That bit of data is hard to reconcile if cylinder pressures are to be considered the primary culprit for failure.

Driving style, maintenance actions, climate, lot of variables to take into account. Maybe it was just a natural progression to product improvement. (Hmmm.. Why aren't more vehicles getting more than 35+ MPG) I digress...

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92 300D 2.5L OM602 OBK #59