"2-Bolt" to "4-Bolt" Short-Block Conversion
for the 3000GT/Stealth

by Jeff Lucius

First a bit of trivia. Mitsubishi made literally millions of the 6G72 engine since its introduction in 1988. This engine in its SOHC, normally-aspirated, 141-HP form was first used by Chrysler in the New Yorker in 1988. Later it was standard equipment in the LeBaron (the most notable version of which was the LeBaron TC in 1990, a combined Chrysler and Maserati effort). The SOHC 6G72 was used in the following cars:
Chrysler and Dodge continue to use the SOHC 6G72 (up to 200 HP and 200 lb-ft torque now) in the Stratus and Sebring coupes. Mitsubishi now uses this successful engine design in the 2000 and newer Eclipse models (SOHC), after having installed the DOHC version in some Diamantes. Unfortunately, the SOHC block is not the same as the DOHC block used in the turbocharged version of the 6G72 in the Mitsubishi 3000GT VR4 (1991-1999) and in the Dodge Stealth R/T Twin Turbo (1991-1996). Of all the vehicles listed above besides the 3000GT/Stealth, only the Diamante used the DOHC version of the engine. Bummer.

The DOHC 6G72 is an extremely strong engine, with an alloy steel block and aluminum alloy heads, and is capable of delivering well over 500 HP with turbocharging. As good as the first versions of the engine were, Mitsubishi decided to make the engine even stronger in 1993 models (starting with June 1992 production-date cars) giving it a 4-bolt main bearing cap girdle (the original used 2-bolt caps with stiffening stays on the middle 4 bolts) and a nitrided-steel crankshaft (the original was cast iron or mild steel). Mitsubishi made other changes, most significant of which was moving the crank and cam angle sensors from the left head (where it was user-adjustable for timing) and into the block (not adjustable). Major lower-engine parts that remained the same include the piston and pin assembly [MD307972 is listed as the standard-size replacement for all years] and the forged-steel connecting rod assembly [MD131329 is the only part number listed for all years]. Because the newer nitrided-steel crankshaft [MD318150] is listed as the replacement part for all years, it is possible to upgrade the "2-bolt" blocks with the better crankshaft (~$600-800 new). Clevite 77 bearings are an excellent choice to upgrade the stock main and rod bearings. As long as you have the engine apart, you might consider upgrading to forged aluminum pistons - 6G72 piston upgrade guide.

I gathered the information below to assist the Z16A (turbocharged) 3000GT/Stealth enthusiast in converting the 1991-1992 "2-bolt" block to the 1993+ "4-bolt" block. This is not an engine swap and so does not involve or require any changes in the wiring harness or ECU. The 1991-1992 heads are retained, including the adjustable crank/cam angle sensor. The parts listed below are required to replace the 1991-1992 "2-bolt" block to a 1993+ "4-bolt" block, while re-using some of the old components. Supposedly, complete "4-bolt" blocks (short block assembly) can be purchased for a bit under $3000 from Mitsubishi dealers.

John Monnin has performed this 2-bolt to 4-bolt conversion and has put together an excellent set of web pages devoted to this topic. If you plan to do this conversion or are just curious, I highly recommend you read through John's web pages. He has a much more complete description than I have here.
http://www.wrenchmonkey.com/Webpages/4bolt.html

Required items (be sure to verify part numbers with your dealer)

Optional items but strongly recommended (be sure to verify part numbers with your dealer)

Acknowledgments

Some of the information presented here was gathered from various email lists, message boards, the Mitsubishi CAPS (parts lookup program), the 3000GT '91-'94 Parts Catalog, and vendor web sites. I would like to particularly thank Joe Gonsowski , Greg Rush, and Jeff VanOrsdal for their contributions.


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Page last updated October 19, 2006.