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Mount Mix-'n'-Match?My father took a '70 Chevelle originally equipped with a small-block, built a bored-out 402 big-block for it, and converted it into a race car. Then the Chevelle sat in the garage for a good number of years. Since then I've rebuilt the engine with new everything, slapped on a set of Hooker Competition headers, and dropped it into the car, bolting it up to a Muncie M20 four-speed. The passenger-side motor mount went together with some convincing, but on the driver side, the header hit the rearmost engine tower bolt. I inverted it, placing the bolt down through the hole instead of up, but this didn't give me the clearance I needed. The question is, did small-block Chevelles and big-block Chevelles have different engine towers that year? If so, where can I get the appropriate ones?JustinVia the Internet
According to the GM parts books, the '70 Chevelle used the same frame brackets and engine mounts for both small- and big-block engines, though earlier Chevelles ('65-'67) are shown to have different mounts for small-blocks and big-blocks. However, there is another variable that may be playing a part in your clearance issue. In the mid-to-late '60s, Chevrolet was dealing with motor mount failure issues. It seems that when the older-style engine mounts would tear, they would completely separate, allowing the engine to rise on the driver side under load. This in turn would sometimes cause the throttle to stick open, since the cars of that era had solid lever-and-rod type linkage. You may have noticed that some Chevys of this era have a small cable looped around the upper control arm's cross-shaft and pinned to a bracket bolted to the engine. This torque strap, referred to as an "engine stop lift," was intended to restrain the engine in the event of a motor mount failure. Later, new motor mounts were designed that were less prone to tear, and still later, another engine mount was developed that was larger, with visible interlocking tabs to prevent the mount from separating if the rubber portion became severed. This mount seems to have been used first during the '68 model year, but only on higher-output engines. It's important to know the difference between the two because each mount must be used with the corresponding frame bracket-they can't be mixed and matched. This is because the heavy-duty mount is thicker than the standard type, so the frame bracket for heavy-duty mounts is shorter to maintain the proper engine location.
We suspected this might be part of the problem with your Chevelle. You said the car originally had a small-block, but you didn't specify which one. As mentioned previously, the GM parts books we sourced show the same frame brackets and mounts for 350, 402, and 454, but our first attempt to find a 307 frame bracket came up empty; our parts books simply didn't list one. We suspected strongly that there was a different frame bracket for the 307, since the parts books showed the smaller V-8 used a different engine mount than all the others. Fortunately, thanks to GM parts guru Jim Losee of Original Parts Group, we were able to find that there was, in fact, a different frame bracket for the '68-'71 Chevelle with 307 engine (GM PN 3980707, left; 3980708 right), and the 307 used the smaller standard-duty engine mount (PN 3990914). The 350, 396/402, and 454 used frame bracket PN 3980711 (left) and 3980712 (right) with heavy-duty engine mount PN 3990918.