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The Slant Six, introduced in late 1959 for the Valiant, has a legendary reputation for endurance - not unlike the flat-head engines it replaced. It also has a hidden secret: good performance, given the right conditions. Unfortunately, for most of its life, Chrysler burdened it with a single-barrel carburetor that wasn't known for speed. According to one of its designers, and a developer of the Super Six project, the slant six responded quite well to better carburetion and exhaust.

Even with the standard carb, the slant six (so named because it was a straight six - in other words, an in-line six - slanted over 30 degrees) had a nice mixture of torque and economy, with power better than similar, same-year GM and Ford offerings. The "Super Six" with a two-barrel Carter carb compensated for smog controls, and with the Feather Duster package it was capable of 30 mpg highway. An export version pushed the tilted engine to 160 bhp, and the fabled HyperPak (a Mopar aftermarket package with a four-barrel carb) raised the USA bar to 196 hp (on the 225).

The heavy-duty engines used in fleet and heavy duty truck/bus service used a double-row roller timing chain (but just one chain!), chrome-plated upper piston rings, and other features depending on service duty (polyacrylic valve stem seals, positive valve rotators, stellite-faced exhaust valves, and/or a high-volume oil pump).

The slant six replaced a flat-head design in 1960 models and lasted in US-built cars through 1983, in US-built trucks until 1987, and survived as a marine engine until 1991. After USA production (in Trenton) ended, it was produced at Chrysler's Toluca, Mexico engine plant until the Magnum 3.9 liter V-6 MPI engine was introduced, continuing to serve in Mexico and possibly South America through (at least) 2003. The next new truck six would be a derivative of the 4.7 V8, and would be introduced in 2001; but the slant six would be the last in-line Chrysler engine with more than four cylinders.

Some major changes include a hydraulic cam (1978/1981, see below), and changes to the spark plug wells (their tubes were cast into the heads starting in 1975). Scott Fluke noted that with the later spark plug well design, you must replace lifters (if you plan to do that) before installing the head.

Charrlie S. noted that you can tell the difference between the 170 and 198/225 engines by looking at the rubber hose that goes from the water pump to the underside of the head: if it is about 1 1/2 to 2 inches long, it's a 170 block (G engine). If it's about 3 inches long, it's a 198/225 (LG engine).

This is hardly a complete list, and we would appreciate a chronology of the slant six.

By slanting the engine in the compartment, four distinct advantages are achieved:

• The center of gravity is kept much lower than in upright engines.
• A low, stylish hood is made possible.
• Engine accessories are more accessible.
• The water pump can be moved to the side of the engine to reduce the engine's overall length.

A common problem is manifold cracking - see below for a discussion.

 The slant six gained electronic ignition in 1973 (earlier models could be retrofitted, and many were). The electronic ignition was extremely reliable, and the only component likely to break down was the ballast resistor, which costs about $3 and two minutes. However, most mechanics seem to replace the electronic ignition system instead.

Also unknown to most mechanics is the crankcase ventilation system and the crankcase inlet air cleaner, an important part which must be cleaned once a year (unless you like blue smoke, in which case I suggest you buy a used Chevy). This applies to V-8s as well as sixes.

Wet weather stalling can be fixed by using a high quality distributor cap and rotor (e.g. Mopar or Bluestreak), high-quality wires, a distributor cap gasket (Standard PN AL-483G), and putting weatherstrip foam over the ignition and voltage regular modules. You can get wires and other parts cheaply on-line.

Dan Stern recommended Autolite 56 (nonresistor), 66 (resistor), or AP66 spark plugs for pre-1975 SL6. In the ten or so years since that first recommendation came down, he wrote:

But the Autolite 925 works very much better, I've learned in the meantime. Tough to get it off the shelf, usually has to be specially ordered, but its extra-long electrodes put the spark closer to the center of the combustion chamber and away from a quenchout area. These plugs were originally designed for engines with difficult combustion conditions (stratified charges, extra-lean mixtures, etc.) such as the emission-controlled AMC 232 and 258, and Chrysler's own 1981-83 318 TBI engine in the Imperial.

Dan also advised owners to check for timing chain stretch.

Ben Deutschman of the Slant Six Club of New York/New Jersey wrote that owners should be careful not to put on spark plugs with the "crush washer" (the loose metal washer at the end), but to take off the washer first, if they have a 1960-1974 engine. After 1974, the head was redesigned, the plug tubes were eliminated, and taper seat plugs were used. If you do have a 1974 or earlier slant six, be sure to replace the plug tube seals - they are cheap and easy to replace.

Ben also suggested using a composite rubber/cork gasket: “I've found the rubber ones tend to harden up too quickly, and then leak, and pure cork isn't much better. In so far as additional sealants are concerned, I don't believe the rubber gasket is intended to have any sealer applied to it.”

Firing order is 1-5-3-6-2-4 (thanks, Ed Friedemann) - note the diagram in this section, sent by DJAlDogg.

The standard carb for many slant sixes was a single-barrel. You can probably increase performance and gas mileage by switching to a good dual-barrel carb. Dan Stern provided these standard carburetors used by Chrysler for North American applications:

1960-1971: Carter BBS one-barrel
1962-1972: Holley 1920 one-barrel
1963 only: Stromburg WA-3 one-barrel
1974-1980: Holley 1945 one-barrel
1981-1987: Holley 6145 feedback one-barrel
1976-1981: Carter BBD two-barrel

Another, more detailed (and probably more accurate) chart (courtesy of Joseph Newhouse)

Engine	Year(s) Available	Compression Ratio	Horsepower @ RPM (see note)	Torque  (lb-ft) @ RPM (see note)
170	1963	8.2 to 1	101 @ 4400	155 @ 2400
170	1964-1966	8.5 to 1	101 @ 4400	155 @ 2400
170	1967-1969	8.5 to 1	115 @ 4400	155 @ 2400
198	1970-1971	8.4 to 1	125 @ 4400	180 @ 2000
198	1972	8.4 to 1	100 @ 4400	160 @ 2400
198	1973	8.4 to 1	95 @ 4000	150 @ 1600
198	1974	8.4 to 1	95 @ 4000	145 @ 2000
225	1960-1962	8.5 to 1	145 @ 4000	215 @ 2800
225	1963	8.2 to 1	145 @ 4000	215 @ 2400
225	1964-1971	8.4 to 1	145 @ 4000	215 @ 2400
225	1972	8.4 to 1	110 @ 4000	185 @ 2000
225	1973	8.4 to 1	105 @ 4000	185 @ 1600
225	1974	8.4 to 1	105 @ 3600	180 @ 1600
225	1975	8.4 to 1	95 @ 3600	170 @ 1600
225	1976	8.4 to 1	100 @ 3600	170 @ 1600

The engine had four replaceable steel-backed (aluminum alloy on steel) babbitt crankshaft main bearings. Oil grooves were in the upper half of the bearings, and in the lower half of the #1 bearing. The connecting rod bearings were aluminum on steel. In most years, closed-circuit crankcase ventilation was used; crankcase vapors were routed to the combustion chambers through a PCV valve located in the valve cover. Vapors were burned before being expelled into the air through the exhaust system. Oil pressure ranged from 35-65 p.s.i. @ 2.000 rp.m.  Crankcase oil capacity was four quarts (one more when changing the filter).

From mid-1961 to 1953, 50,000 225-cid Slant Six engines were produced with an aluminum engine block.

A special Hyper-Pak was available for any slant six. A competition engine, the Hyper-Pak featured a much more radical cam; a Carter AFB four-barrel carburetor mounted on an intake manifold with long ram passages; steel tubing exhaust headers; higher compresion pistons and a special tuned exhaust system.

Advertised at 195 hp, Hyper-Pak engines are often said to put out more power. They were the rulers of the lower stock classes at the drag races. The Hyper-Pak was available as a dealer-installed or user-installed option.

Hyper-Pak specs:

• Overhead valve. Cast iron block.
• Compression ratio: 11.5:1 (225), 10.5:1 (170)
• Brake hp: 195 at 5200 rpm (225), 148 (170) *
• Four main bearings.
• Solid valve lifters.
• Carburetor: Carter AFB 3083S four-barrel.
• Larger exhaust system using cast iron hearders.