Can You Change a 2.8 Transmission to a 4.3?


GM used three basic cam-in-block V6 engine designs during the 1980s and 1990s: the Buick 90-degree V6 (which includes the turbo 3.8-liter and later 3800 series), the GM 60-degree V6 (which includes the 2.8-liter and second-generation 3.1-liter) and the 90-degree V6, which was essentially a Chevrolet small-block V8 with two cylinders lopped off. Some engines will bolt to the others' transmission, some will interchange using adapter plates and others won't interchange at all.

Transmission Bellhousing Patterns

  • GM upgraded or replaced all of its three-speed automatic transmissions during the 1970s, leading to a line of four-speed automatic overdrives. The earlier lighter-duty TH350 and heavy-duty TH400 three speeds had evolved to become the light-duty TH200R4 (a.k.a., 200-4R, 200-R4 and 200R4) and heavy-duty TH700R4 (a.k.a., 7004R, 700-4R and 700-R4) four-speed overdrives. The TH200R4 used a mullti-case bellhousing with bolt patterns to match almost all contemporary GM V8 and V6 engines. The stronger TH700R4 used one of two different bellhousings; the "small-pattern" housing fit the GM 60-degree V6/Buick V6 and the "large-pattern" housing fit GM 90-degree V6 and V8 engines (including the Chevrolet small-block and big-block).

Bellhousing Size

  • Only GM 60-degree V6 and Buick V6 engines will bolt to a transmission using the small-pattern housing. The reason for this becomes immediately obvious when you look at the engines. 60-degree and 90-degree V6 and V8 engines are so named because of the angle formed by the "V" shape of the block; 60-degree engines have a much narrower "V" than 90-degree engines, making the block itself narrower at its extremes. The small-pattern housing isn't really much physically smaller than the large-pattern housing; it's just narrower and a bit taller.


  • Bolt pattern itself isn't the issue with small/large-pattern interchanges; an adapter plate will allow you to physically bolt a large-pattern engine to a small-pattern case, or vice versa. The problem lay in the fact that large-pattern engines tend to have larger flex plates and torque converters that won't fit inside the small-pattern's narrower bellhousing. It is for this reason that you often find adapter plates to affix a small-pattern engine to a large-pattern transmission, but not the other way around. If you're fortunate enough to have a 200R4, it will readily adapt to either engine. The same holds true for front-drive transaxles.


  • Identifying which transmission you have is a matter of crawling under the car and looking at the transmission oil pan. All TH700R4 transmissions use an almost perfectly square oil pan and TH200R4 transmissions use a long, triangular oil pan. There is no way to tell which transmission bellhousing pattern you have without referencing the engine or looking at the housing itself if the engine isn't present.


  • If you have a small-pattern, 60-degree engine transmission and want to install a 90-degree V6 (like the 4.3-liter), the only viable option is to get rid of your old transmission and install a different one. The good news is that the 90-degree V6 uses the most common bolt pattern GM ever, so it will bolt up to the 4L60 (which is what GM called 700R4s after 1989), the electronically-controlled 4L60, the stronger 4L65E, the dual-pattern TH200R4 and several older three-speeds, including the TH350 and TH400.

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