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Around the Block,Don Fedak :The ‘Autostrology’ of Auroras and Cadillac Northstars

By Don Fedak

The ‘Autostrology’ of Auroras and Cadillac Northstars

by Don Fedak

In 1915, Cadillac replaced its six-year-old four-cylinder engine with a revolutionary new 3-1/8˝ bore V8. Over the past eight-and-a-half decades, the relatively compact V8 configuration has continued to evolve and remain popular because advances in materials and processing have allowed engine designers to reduce weight and increase power density. Today, some V8s are small and light enough to fit sideways in FWD vehicles.

For six-and-a-half decades, Cadillac and most other domestic V8 engines were mainly built of cast iron and steel. Then, in 1982, Cadillac introduced the milestone HT4100 (High Technology 4.1L) push rod V8, one of the first domestic designs to utilize replaceable cast iron sleeves in an aluminum block.

The cylinder heads, exhaust manifolds and camshafts were also cast iron, while the crankshaft was nodular iron and the oil pan and front cover were steel. At the time, the trade press speculated that it would "probably be the last eight cylinder engine to come out of Detroit." The manufacturer indicated that "no exotic materials" were used and that "no chances were taken" that might jeopardize the reputation of the engine.

But the new design failed to live up to its billing. It survived for 14 years only because a few independent-minded aftermarket engine remanufacturers were able to reverse engineer solutions to correct some inherent engine bearing and seal problems. Blocks with stripped threads were reclaimed and strengthened to sustain proper clamping loads by installing stainless steel inserts. Ultimately, the difficulty of maintaining the separation of coolant and oil in this basic design was recognized.

The HT4100, along with its improved siblings, 4.5L and 4.9L versions, that all married OHV cast iron cylinder heads and aluminum blocks with replaceable sleeves were mercifully discontinued.

The current nine-year-old 4.6L DOHC Northstar Cadillac V8 and its little sister, the 4.0L Aurora, utilize aluminum cylinder heads, engine blocks, intakes, oil pans, oil pumps, pistons and valve covers. Cast iron, sintered steel powder alloys and steel forgings are still preferred for components that require strength and/or wear resistance such as cast-in-place integral cylinder liners, crankshafts, camshafts, connecting rods, chains, gears, piston pins, valves, valve guides and valve springs.

These new engines were installed in production vehicles after minimal testing. For at least three years, dealers were not allowed to get inside the Northstars to service them. During this extended test period, all necessary repairs were performed under warranty by OE factory technicians. Cadillac owners joined the growing list of consumers who also serve as part time unpaid test drivers.

These engines incorporate a number of significant design and material changes. If you sell starters, advise your parts people that, since this DOHC V8 design requires no balance shaft, the starter is hidden in the available V-cavity underneath the intake. Also advise them that the intake gasket is a dealer item. In fact, no aftermarket manufacturer catalogs currently list any gaskets for these engines.

The majority of the critical gaskets for the 4.6L Northstar and its little sister 4.0L Aurora engines, except for the graphite head gaskets, are expensive-to-produce, long molded rubber noodles that fit in precisely machined channels. Either the OEM suppliers have been constrained from supplying their products to the aftermarket, or all the traditional aftermarket suppliers have determined that, given the problems associated with sealing these engines, they don’t need the hassle.

The huge DOHC 16-valve aluminum cylinder heads are a typical example of current practice and the advances that have been made in aluminum casting technology in the last few decades. With replaceable cast iron guides, valves with 6 mm stems and bucket style hydraulic lifters pushed by chain driven cast iron camshafts, these heads have proven to be reliable throughout the engine warranty limits when they receive sufficient lubrication and are properly cooled. And, so far, no problems have materialized with the later versions that use a low friction lifter plus roller-follower arrangement.

Because both the heads and blocks are aluminum, loss of combustion seal due to differential expansion is not an issue. Most of the head gasket problems with these engines are related to either cooling system failure or a decrease in gasket clamping force. Oil in the coolant is most likely the result of head gasket failure. Major coolant or oil leaks occur when the threaded holes in the aluminum blocks weaken and allow the cylinder head bolts to loosen.

Each head has ten 11 mm x 140 mm head bolts that extend 65 mm below the deck surface. Sometimes, loose bolts, damaged threads and gasket failures are traceable to casting porosity. Although Aurora and Northstar blocks are less likely to dump coolant than their HT series predecessors, they do share the necessity of frequent thread repair and dealers are supplied with the necessary tooling.

Some low mileage 2000 4.6L Northstar and 2001 4.0L Aurora engine blocks have shown a tendency to slowly lose coolant via a small stress crack at the bottom of the water jacket of cylinder #7, adjacent to the starter. GM appears to have identified and rectified the source of the problem.

The way these engine blocks support the crankshaft is really different. Extra thick steel-backed main bearings are clamped between two large castings, the upper and lower block, that are joined at the main bearing parting line.

Except for drilled passages that channel oil to the cylinder heads and the chain tensioners, the upper block contains no oil galleys. Oil is fed to the crankshaft via oil passages that are machined into the lower block and capped with a steel plate that is sandwiched between the lower block and the cast aluminum oil pan. Many owner complaints of oil leaks are associated with gasket failure at this interface. An appropriate fix has been difficult to achieve.

A compact gearotor oil pump is located under a flat steel front cover and driven by the crankshaft. A short chain drives an idler gear that drives two long timing chains. Three hydraulic chain tensioners are used. An integral trigger wheel next to the crankshaft center main bearing journal sets ignition timing.

Short skirt hypereutectic pistons with narrow rings push on sintered steel alloy rods with bronze pin bushings and free floating pins. Although a few suppliers list pistons, no one seems to know when they might be produced. Since the pistons protrude .018˝ above the block deck, “carbon knock” is a common complaint. Some aftermarket suppliers list rod bearings.

These engines, though different than most traditional designs, are just two examples of what the future holds in store. Sooner or later, we all must change, adopt new techniques and procedures and become familiar with current designs. At the same time, we will also have to invest in the tooling that will ultimately be required to repair and remanufacture these aluminum assemblies.

Certainly change is inevitable and we all should be ready to adapt to changes in engine design. But we also need parts. With the current fashion of short production runs, it frequently seems like aftermarket engine builders are under embargo. More often than not, the engines in our shop are either too old, too new or too expensive to repair simply because the OE manufacturers are the sole source of critical replacement parts.

Every machinist and engine builder must seize every opportunity to urge their traditional parts suppliers and manufacturers to pick up the slack and provide us with all the missing pieces of these new lightweight puzzles.