Tech Talk: Shifty Secrets

How to blueprint your bellhousing

Words and Photos: Jeff Smith

It’s a warm summer night where sound can carry for miles. Off in the distance, you hear the strains of a performance engine quickly rowing through the gears. The 1-2 and 2-3 shifts are crisp and well-executed, and in your mind’s eye, you see the driver pulling back on the shift handle into fourth gear, anticipating another quick gear change. Instead, you hear the unloaded engine scream as the gears fail to mesh. Whiffed it!

It’s the classic missed shift. If you are a manual transmission fan, you’ve heard it and probably experienced it. Most enthusiasts of the art will quickly attribute the failure to driver error — a lack of hand-foot coordination. But, there might be another explanation. It could be the bellhousing is not doing its intended job. Let’s look at how this works.

Besides giving the transmission a solid location and perhaps providing protection from an exploded clutch, the bellhousing has only one important job. That’s to make sure the transmission input shaft is perfectly aligned with the centerline of the crankshaft. But, production tolerances being what they are, this doesn’t always happen. If the input shaft is sufficiently misaligned, it makes it much more difficult for the transmission to shift gears cleanly at higher engine speeds.

The most obvious example is the scenario we offered in the beginning of this story. In a four-speed manual trans, fourth gear (or any transmission with a 1:1 ratio) is achieved by connecting the input and output shafts together. Completing this gear change should be — and usually is — a simple process when all the variables are within spec. But, place the input shaft at a slight offset to the crankshaft at 7,000 rpm and the result will be a missed shift every time, because the conflicting angles will not allow the slider to make that connection.

The spec for this centerline is very precise — that large input shaft hole in the bellhousing must be within 0.006-inch of centerline. This is known as total indicated runout (TIR). There’s also a second spec, squareness, that is often overlooked, but just as important. This is where we check to make sure the face where the transmission bolts to the bellhousing is perpendicular to the engine — or at least again within 0.006-inch.

In our experience, most engines and OE bellhousings are usually within spec; although the combination we tested was 0.025-inch out. Some older steel scattershields we’ve tested had a tendency to be off center by almost 0.020-inch. If after testing the housing exceeds the limit, there is an easy way to dial it back into spec. The easiest way is to use offset dowel pins sold by Lakewood, Moroso, and a small company called McRobb Performance. The pins come in three offsets — 0.007, 0.014, and 0.021-inch. The least offset at 0.007-inch can effectively dial a wayward bellhousing that is as much as 0.020-inch out of spec. By doubling the 0.007-inch offset, this moves the bellhousing as much as 0.014-inch in one direction, which would bring a 0.020-inch offset back within the 0.006-inch limit.

Any of the offset bushings can accomplish this task, but ergonomics also play a part. The Moroso dowel pins are intentionally designed to fit loose in the block. This makes turning them in the block easier, but requires drilling a small lateral hole in the block to drill and tap for a small locking set screw that is supplied with the bushings. That’s okay if the engine is on the shop floor, but near impossible in the car. McRobb Performance offers an intelligent alternative where the offset dowel slides into place, making it easy to adjust, and then is tightened into place with an internal expanding set screw. These offset pins are not that expensive and make this job much easier.

When installing the offset dowels, be careful to install the offsets so they are oriented in the same direction. Otherwise, it will be difficult or impossible to slide the bellhousing over the dowel pins, since they will be offset in relation to each other. It’s easy to do and can make this job frustrating until you figure it out.

You will need a dial indicator and a magnetic base to perform this test, and you’ll find it takes longer to set everything up than to do the actual test. But, the effort is well worth it. Dial in that bellhousing, and those high rpm shifts will go smooth as 7,000 rpm silk.

Sources: Holley Performance Products (Lakewood),; McRobb Performance Products,; Moroso Performance Products,; Quarter Master,; QuickTime,

About Jeff Smith

A clue into how long Jeff Smith has been writing technical automotive stories might be his following of second generation readers. Writing continuously for nearly 40 years, his focus with Xceleration covers all things technical. His collection of cars includes a bevy of Chevelles and El Caminos. When not writing about cars, he likes to spend time with his wife Valerye, children Amber and Graham, and granddaughter Celeste.