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Tech Article

BMW E28 and E30 M20B27 Engine Valve Spring Size Differences

Last Updated: 03/14/2020
   
Parts Group: Valve spring
On overhead-cam engines such as the BMW M20 engine, the sizes have to match as to each valve spring and the two pieces of metal, at the top and bottom respectively, against which the spring directly exerts its force. BMW refers to these pieces of metal as, respectively, the upper spring plate and the spring pocket.

On the M20B27 engine for the 1984 and 1985 production years of the E28 and E30 (focusing on the US market) the springs have a diameter of 30.8 mm. Then, starting from the 1986 production year, the springs have a diameter of 30.2 mm. Accordingly, the upper spring plate and the spring pocket are also both sized differently.

The larger vs. smaller springs, upper spring plates and spring pockets have different part numbers and it seems prudent to assume they are not safely interchangeable. Problem is, we could discern no part numbers, so we used a micrometer to measure the parts we had, when we weren’t sure whether they came off an early or late M20B27 engine. We make a point of keeping larger vs. smaller springs, upper spring plates and spring pockets separate, and we treat them as completely different parts types, each with their own part numbers.

The 30.2 mm valve springs were also used subsequently in the M20B25 engine as used in the E30 325i, and later in the E34 525i. Perhaps this was BMW's way of simplifying their inventory management, so that it doesn't have two similar-looking yet incompatible types of valve springs being used concurrently on its production lines.
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