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[PegLeg45]

GM ended up buying all, or at least part of, Isuzu.  Their diesels are all Isuzu.  The old Oldsmobile was a good diesel for what it was designed for, but people were looking for what it wasn't.  Translation:  They wanted a 454 big block, but they bought a Vega 120 four banger.  That, plus the fact that they made it a 350 ci, so, of course, the know-it-alls claimed it was just a gas engine with diesel injectors in it.

Yeah....that always got me when my gearhead friends were bench-racing and someone brought up 350 diesels. I was like, "Yeah...they're practically the same engine....except for the thimble-sized combustion chambered heads and 2.5 times the compression ratio of a gas engine, etc." 

[tombogan03884]

Didn't they just drill and bore a different set of holes in the existing casting ?

[Majer]

As I recall, the heads were the main difference, I think you could swap the heads, Intake system and install a distributor and convert it back to use gas.

[PegLeg45]

The block used similar dimensions because using the same bore and stroke meant Olds could build the engines using the same tooling as the gas engines. The block was a beefed-up design intended to deal with the engine’s 22.5:1 compression ratio, which was nearly 3 times the CR of the gas engines.

Biggest differences were inside the cylinder heads and the head bolts. To get that high of combustion ratio, the chamber was tiny. One thing Oldsmobile didn’t change was the head bolts — type, pattern and number. And that would prove to be the engine’s undoing … or at least part of it.

There were two key problems with the Olds diesels. First, the head bolts simply weren’t numerous or strong enough for the diesel’s high compression ratio, so they started blowing head gaskets.

And, one of the consequences of a blown head gasket is that coolant can enter the cylinder, and unlike air, coolant does not compress. If a given cylinder took on enough water, a piston on its upward compression stroke would literally run into the immovable object. The piston would stop, but the crankshaft wouldn’t; the connecting rod would bend and mucho mechanical destruction would ensue. High compression engine using low compression parts and it didn't take long for the engines to start grinding their own internals to bits.
In most cases, this would render the engine irreparable — but in the event it didn’t, repairing the engine using the same type of head bolts would simply give it another fine opportunity to destroy itself. 

Second big problem: cost-cutters decided not to add a water separator.
Unlike gas, diesel is subject to water condensation — hence the need for a water separator.
Without one, water in the fuel becomes water in the engine, where it can rust either the cylinders or the very complicated mechanical fuel injection pump.
The former could destroy the engine, while the latter would screw up the engine’s running characteristics and possibly crap-out the pump — which, in a mechanically-injected diesel, is an incredibly intricate and complicated device that is expensive to replace.

I attached the two different heads showing the combustion chamber. The gas is first and the diesel second....note the diesel looks like a flat-head. 

[MikeBjerum]

Didn't they just drill and bore a different set of holes in the existing casting ?

No, all castings were of new design.  The concept of the 350 ci was that they were able to use existing tooling with minimal change, and the only needed changes in the metal work line was holding fixtures.

The three main failures that I recall from being in a dealership in that era were, first, the glow plugs wouldn't hold shape.  The tip would mushroom, and couldn't be pulled back through the hole when changing.  For this reason a bad original glow plug meant the head came off.  After a couple years the replacements were better.  The second failure, and this was also fixed, was that this was GM's first venture into torque to yield bolts.  However, the head bolts were not engineered heavy enough.  Simple fix - New bolts when you replaced the glow plugs, or new glow plugs when you replaced the head bolts due to failed head gasket.  The third failure, and I don't think there was a fix for this until they redesigned the block, was the bottom end.  The main bearing cap bolts were not long enough to hold the diesel torque in place.

This was one of those areas where GM's strange structure showed up.  As they attempted to do something new, rather than letting Detroit and their track record handle it for the larger group, they went to Olds.  The idea was that this was to be a car/light truck engine, and they did not want it influenced by Detroit's Class 8 and industrial focus.  Of course, the ever loving, Detroit engineers were full of compassion and understanding as Oldsmobile fought their way through.

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[tombogan03884]

Didn't they also try an aluminum head that warped like hell ?
They started that before I went in the Marines and I've never been a car guy, but I remember people who were car guys talking about it.

[MikeBjerum]

I think the aluminum head issue for GM was in the new V6.  The Big 3 all suffered with aluminum.  If you overheated the engine, the head would warp right away.  You couldn't shut them down fast enough if you saw the temp gauge rising.  We had a K car, and I replaced a head during Christmas break in 93.

Before the diesel, GM fought the Vega and Monza.  These cars were built to replace the Corvair, and to compete with Japan.  Great little cars, that failure to read MOM (Manufacturer's Owner's Manual) led to an issue.  The 120 ci four banger was intended to be a throw away engine.  No tune ups, no timing belt changes.  They were inexpensive, and they were intended to be a new long block at about 40,000 miles.

[PegLeg45]

No, all castings were of new design.  The concept of the 350 ci was that they were able to use existing tooling with minimal change, and the only needed changes in the metal work line was holding fixtures.

The three main failures that I recall from being in a dealership in that era were, first, the glow plugs wouldn't hold shape.  The tip would mushroom, and couldn't be pulled back through the hole when changing.  For this reason a bad original glow plug meant the head came off.  After a couple years the replacements were better.  The second failure, and this was also fixed, was that this was GM's first venture into torque to yield bolts.  However, the head bolts were not engineered heavy enough.  Simple fix - New bolts when you replaced the glow plugs, or new glow plugs when you replaced the head bolts due to failed head gasket.  The third failure, and I don't think there was a fix for this until they redesigned the block, was the bottom end.  The main bearing cap bolts were not long enough to hold the diesel torque in place.

This was one of those areas where GM's strange structure showed up.  As they attempted to do something new, rather than letting Detroit and their track record handle it for the larger group, they went to Olds.  The idea was that this was to be a car/light truck engine, and they did not want it influenced by Detroit's Class 8 and industrial focus.  Of course, the ever loving, Detroit engineers were full of compassion and understanding as Oldsmobile fought their way through.

Yeah....because they used the regular bolts from gassers. You can't use something designed for 8.5:1 compression with something that makes 22.5:1 compression. 

Some of the cost-cutters I used to work with also found that out in the hydraulic sector of the joint I used to work at. Can't use grade 2 bolts on 5000 psi pressure lines either.