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.