Diesel Engine Detail

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General Information

Note: The information contained in all of the "Engine Detail" sections should be read before proceeding with modifications, etc., because some information that applies to all engines, or all small blocks or all big blocks, might not be duplicated in every section.

Intake manifold is specific to the diesel. It is cast iron, and fits like any other manifold. Topped off by a crossover aluminum casting which is topped off by an air cleaner which is ducted to an intake box at the front driver side fender/header area. The manifold has a deep valley with a machined approximate 3 1/3" opening at the front for the injector pump. No distributor, but in the place of the distributor (with the same type of clamp) is the vacuum pump. A required item since it drives the oil pump. Non A/C or non cruise control equipped cars may not have the vacuum head.

The V-6 diesel featured "styrofoam" "lost foam process" cast heads. Late style crankshaft bolt pattern.

The diesel mains have all the metal of a solid-web block, plus all the metal that should have gone in those windowed-main gas blocks. Pretty hefty. Good for nitrous and turbo/super charger applications. The crankshafts are supposed to be nodular.

The diesel rod is slightly shorter than the gasoline rod, for the increased ring-land necessary for a diesel, I'm assuming. The diesel has shorter rods. Probably for torsional strength and rigidity, as well as to give the piston more mass for compression ignition. They are thicker at dome, and a tall piston.

Olds diesel engine crankshafts are cast, though of nodular iron. Unfortunately, a diesel crank is the only small-block crank that won't directly fit in your gas 350; the diesel motors have the big-block 3.00" main journals, while the gasoline small-blocks use 2.50" journals.

The roller cam debuted on the diesel in 1980. I think full floating wrist pins did as well (anyone?).

The 350 diesel block with a 425 crank and the stock bore size block (4.057") will yield a bulletproof 411 CID gasoline small block. The 350 diesel block can be safely overbored .125" without sonic testing, to make a 437 CID small block. The 350 diesel block can be bored out to 4.25" when sonic tested. With a shaved down 425 crank and a 3.975 stroke, that works out to 451 cubic inches.

The extra meat in the cylinder walls is necessary for the pressure a diesel generates. When you cut the walls with a major overbore as described, they are still adequate for the lower cylinder pressures that a gas engine generates, but will likely not survive in diesel use (heck, those motors were underdesigned in stock form).

While it's fun to speculate about these oddball diesel conversions, I suspect that when you're done, you'll have a motor which not only has the same displacement as a big block Olds, but also has more weight and significantly more cost. For almost every application, I'd just go with a big block unless the extra inch of height and width really makes a difference. About the only application that I'd consider for one of these motors is to sneak it into a 64 442 and tell everyone it's the original 330. Of course, you could paint your 455 gold and likely fool 99% of the people anyway.

The 350 diesel block cannot be bored to 403 size; you can't bore a 455 to use the 403 sized piston (4.351") either. The CID displacement and the letters "DX" appear in raised letters on both sides of the block. The block heater was installed in the driver side front freeze plug.

An experimental diesel, reported in the pages of "Motor Trend", when Olds was in the thick of diesel development, highlighted a high-speed diesel that Olds developed, using tuned-port air induction, on a 263 CID V-6. They put it in a modified Ciera, and it bested the acceleration of the gas version by a wide margin.

[ Thanks to Thomas Martin, Bob Barry, Joe Padavano for this information. ]


Diesel Maintenence

Simple maintenance:
They either start or don't. Change the oil & filter @ 2000 (2500 max), but usually at 1500. Change the air filter once a year unless dirty. Change the fuel filter twice per year: one change in mid winter. That's it.

Fuel:
Diesel has a cetane rating, usually in the 40 to 45 range. This is comparable to an octane rating. The 350 doesn't like anything under 42. Try different brands to find the one(s) that your Olds diesel likes. Never let the tank get below ½ full unless on a long trip.

The winter blend (25% kerosene) must be used during winter. This is blended at the pump. Even with this, when below 15°F, it is wise to use an additive that prevents fuel gelling, and lowers the pour point.

Diesel fuel contains parafin which will precipitate out at lower temperatures. Yes, it's wax that clogs the fuel filter and everything, and is tough, tough, tough to dissolve and work out. Never use any crap fuel, heating oil or unknowns. It's just not worth it.

Water rusts the injector pump, and besides, diesel fuel is the lubricant for the pump. Due to nature of diesel, firing any appreciable slug of water into head by injector pump, into a rotating engine, will squirt in when piston is at near top of stroke, and will definitly blow the head gasket if you are lucky (be happy that the noncompressible water did not bend a rod, the crank, streath a head bolt, etc.).

Alcohol ruins the governor vane in the injector pump. This was supposedly corrected in later years. Any additive you use must say "for diesel engines".

Timing:
Is critical. To set the original timing, the pump and pump ring are lined up, and a "special tool" (chisel in a guide) is whacked, producing a line on both the pump and pump ring. Supposedly the width of the line is worth 1-2°. Use a razor blade in the chisel groove to assure line up. Reports from the street indicate that only a few degrees off will produce major problems and blown gaskets. There are also reports of increased performance, which is very tempting to some, hence many blown gaskets.

Glow plugs:
Last about forever.

Batteries:
Two- definitly clean terminals 4-6X per year. Use the largest diehard that will fit, but you can usually make two kids or friends happy if you feel better changing them out at 3 years.

Head Gaskets:
Did anyone know there are two different head gaskets? A standard, and a 0.010" over (thicker) for engines that had their decks milled. With an engine at 22.5:1 compression ratio, this makes a big difference. You can imagine the interference on a decked engine with a standard thickness gasket, BUT the final critical compression would be incorrect, so a loss of performance would be experienced with 0.010" thicker head gasket on an un-decked engine.

Block Dowel Pins
Olds sent out many relacement blocks with dowel pins in the deck that stuck up too high! Olds issued a Tech Service Bulletin about the problem. So, you should check this when doing any head work. Put the head on naked (no gasket), and poke around with a 0.001/0.002" feeler guage, and if it slips around, grind the dowel pins a bit and recheck.

Compression Test
Don't "squirt oil into the cylinder to check the rings" on a diesel!! The cylinder is hot enough at ½ compression stroke to ignite the oil - with potentially damaging results. You must use a diesel compression guage to accomodate the higher pressures and threads of the glow plug.

Crankcase Pressure
Olds replaced a number of engines where the crankcase pressure was up to 3½" W.G., which met criteria for new engine courtesy of Oldsmobile. To test this, you need a guage that measures inches of water. These are available at any heating supply store in the tool rack (~$30). To test, run the engine, hook up to dipstick tube, and measure the pressure.

Decent fuel economy:
18.5 around town, 25 on the road, not taking it easy, 4000 lb car loaded with extras and baggage.

Gasoline is ~118,000 BTU/gallon. Diesel is ~140,000 BTU/gallon. With all other items to the side, (efficiency, burning rate, etc), diesel has more power per gallon than gasoline. Reportedly, the precombustion chamber's swirling and mixing properties are the key to the diesel's economy.

When a diesel goes into the compression stroke, and at near TDC, the air is compressed so much that it is hot enough to ignite the diesel fuel. In the timing sequence, the injector squirts (at 1,200-1,800 psi) the diesel fuel into a preignition chamber where it ignites and results in power.

Overall:
Clean fuel, clean oil, clean filter, life is usually OK.


Gas Conversion

Daily Driver / Street Machine
Add 403 heads, a SB carb and intake manifold, a fuel pump and a gas engine camshaft need to be installed, but I doubt it's just that simple. What other work has to be done?

You need to adjust the spacing in the timing chain area, and add some plugs somewhere. Mondello has the whole kit with all these little miscellaneous pieces. Unless you have a machine shop at your disposal, you'd probably be bucks ahead to get that kit with all the right parts, or else your engine can become a very heavy and expensive failed experiment.

As I said above, you need the little pieces to make up the differences between the gas and the diesel block. I think the Mondello kit has the proper roller timing chain and the like. Give him a call, and see what he recommends. The other parts sound like a great starting point.

[ Thanks to Bob Barry for this information. ]

Race Engine
Don't throw away the diesel. If rebuilt the right way it is a very fast, strong built block. I do have some info and pics from a magazine article that would make you think twice about diesels forever. Maybe one of the drag car guys can fill you in. I will be posting these pages for everyone to read as soon as I can.

Quote "Aside from the stock diesels nearly bulletproof bottom-end and deck stuctures, these block's 0.400-thick cylinder walls can handle an 1/8-inch overbore and still retain more wall thickness then a late 350 gas block has to begin with. 3/4-inch thick main bearing webs and 455 V-8 main bearing caps for added rigidity. Most pages are from A.J. Foyt and Smith. Very impressive stuff.

[ Thanks to Randy Hermsen for this information. ]


Part Differences With Gas Engines

In doing this little project I became more aware of the differences between diesel and gas Olds motors. My 455 timing chain cover had corrosion behind the water pump. I decided as long as I have it apart to put in a timing cover that is in better shape. I have a bunch o' covers lying around, but I just took apart a 350 diesel that had a perfect cover. I installed that cover, put on the water pump and when I put on the balancer guess what? I drove the front seal right into the oil slinger! So I learned that the 350 diesel timing cover DOES NOT WORK on gas motors! The seal is mounted about 3/8 of an inch outward. As a matter of fact the offset is exactly opposite of that in a gas motor. So off comes the balancer, water pump and timing cover. now. If I can only get that dam thing back on without removing my oil pan! :) At least the engine was out of the car! Live and learn!

[ Thanks to Tony Waldner for this information. ]


Parts Usage For Gas Engine Cars

Brakes
These cars also used a power steering pump assisted brake booster instead of a vacuum assisted booster. A very nice idea for use with a rumpity cam!

Exhaust Manifolds
The car 350DX motors used the regular dual-outlet gasoline manifolds. It is the diesel trucks that used the true-dual manifolds. I don't believe that they flowed phenomenally well; certainly not better than a W/Z setup.

Diesel trucks did get dual exhaust, but again, capped single exhaust manifold on the passenger side. You small block guys, thats where you get the factory exhaust crossover cap for running true duals on a small block.

[ Thanks to Dorian Yeager, Bob Barry for this information. ]

Oil Pan and Parts
Unfortunately, diesel oil pans are 4 quarts, with one more for the filter, and two more in the oil cooler.

Radiator
The Olds Diesel engines commonly used a remote radiator oil cooler. Their 3 core radiator plus oil cooler is a cheap way to get both. The filter mount is plumbed for ½" steel lines. Very nice, sano, and pretty bulletproof for remote filter installations.

This car will have the large oil cooler in the radiator. You most certainly can use it with your gas engine. I would strongly advise you drop the big buck and install an oil cooler thermostat in the line. The gas engines do not create the high oil temps that the diesel does. You could cause big time condensation problems in your oil by not getting the temp up high enough and soon enough to dry out any condensation.

[ Thanks to Mike Hall for this information. ]

Short Block
Blocks and cranks make nice Mondello big inch small blocks!

Springs
The front springs can likely handle more weight than the regular springs used on the same model, so they would work out nicely to support a big block!

Transmission Cross Members
Those late seventies Eighty Eights w/Diesels! They had dual hump members. They are tubular crossmembers. You will find two sets of attachment points on the drivers side of the car for the crossmember. This is for TH200R4 attachments and TH-350. The forward set is for the TH-350, the rearward for TH200R4.

[ Thanks to Mike Hall for this information. ]
[ Thanks to Charley Buehner for this information ]


Rebuilding


Tuning / Power Boost



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Submit corrections and additions to this information to The Olds FAQ Compiler.