Engine swap? How to properly hang the alternator | From the Archives

Step 1:

The factory Miata alternator fits low down on the right side of the engine block, while the MGB engine bay narrows down to a welded-on subframe–right where the Miata alternator would like to live. Notching the subframe would weaken it too much, and raising the alternator would lead to clearance issues with the intake manifold. We were going to need a smaller alternator for our swapped MG.

Step 2:

While one of our guiding principles with this swap is to use only MGB or Miata parts to make future replacements simpler, we had to compromise on the alternator and find something smaller. Many people like the various compact “one-wire” units available online, but we’ve found that these units' longevity is sometimes suspect. We prefer a traditional alternator with multiple wires and a provision for an idiot light. We also want an alternator that’s relatively easy to buy at a parts store. The answer was a Denso alternator designed for an early-’90s Geo Metro. It has the shape and 82-amp output needed for many of our projects.

Step 3:

While the size of the Geo replacement alternator body worked for us, the pulley did not. The Geo uses a belt with one less rib, so it is too narrow for our Miata belt. To solve this, we just used the Miata pulley, but it has a larger shaft diameter. We had a machinist friend make up a bushing to adapt the pulley.

Steps 4 & 5:

We blocked the Geo alternator into the rough position and took enough measurements to pattern a mount. (Our favorite pattern material comes from file folders.) We then traced our pattern onto a piece of 1/4-inch-thick steel and used a spring-loaded prick punch to locate the three holes it needed. We could then drill our holes.

Step 6:

Next we rough-cut the bracket using a 4-inch grinder fitted with a 1/16-inch cutoff wheel.

Step 7:

We now had a bracket that we could use for some test fitting.

Step 8:

We bolted the alternator to this first part of the bracket, and spaced the bracket with washers until the alternator pulley was in perfect alignment with the water pump and crank pulleys.

Step 9:

The washers that we used for the test fitting wouldn’t cut it for the final product. We fabricated a proper spacer out of 1-inch round stock. Then we drilled out the center.

Step 10:

We tack-welded the spacers to the bracket and then started working on the back part of the piece. Using transfer punches, we marked the two holes that needed to align from the front to the back of the bracket.

Step 11:

After some more drilling and shaping, we added a piece of steel to link the front and back of the bracket. Then it was time for more test fitting.

Step 12:

Each time our bracket got a little closer to its final shape, the tolerances used during the test fitting mattered more. It’s helpful to have a really clear understanding of alternator mounts, and this picture shows an important detail: Most alternators have two ears that form the mount’s pivot point, and a long bolt then goes through these ears. That bolt’s alignment is very important, as bending the ears will cause problems. 

Since manufacturing tolerances exist, most alternators feature a sliding piece on one of the ears–in this case the rear ear. That sliding piece is usually a large roll-pin that is pressed into the ear. While the piece doesn’t slide easily, it does move when needed to create the proper tension on the mounting bolt.

Steps 13 & 14

As we mentioned, the pivot part of the mount has to be designed so it won’t bend or break the alternator’s ears. If one long bolt is used to mount the alternator, then a support must be placed between those ears. Without a support, either the ears will get damaged, or the bolt won’t stay tight. We made our support from a small piece of tubing. We drilled the bracket for an interference fit to the tubing that would let it slide slightly while keeping support between the ears and bracket.

Step 15:

With the pivot side of the mount finished, it was time to address the adjuster bracket. We were able to use the factory Miata piece, but it needed a spacer to bolt to the right position. We started that step by taking a measurement.

Step 16:

Next we made a spacer from the same 1-inch stock that we used for our other spacers. We drilled a 10.5mm hole so its 10mm mounting bolt could easily pass through.

Step 17:

We made sure that we had plenty of adjustment in either direction.

Step 18:

In the end, we made a very sturdy mount that offers decent clearance in all directions. While we made a big compromise in using a Geo alternator, we’re glad that we could at least use the stock-length belt. We figured the belt is more likely to need replacement anyway, so we feel we made the right compromise.

Read The Whole Swap Science Series:

1. Swap Science: 14 Steps Before You Swap That Engine
2. Swap Science: How To Build Custom Engine Mounts From Scratch
3. Swap Science: How To Make Transmission Mounts
4. Swap Science: How To Get Your Clutch Setup Just Right
5. Swap Science: How To Make A Functional Gas Pedal
6. Swap Science: How To Hang An Alternator
7. Swap Science: Engine, Steering, and Exhaust Fitment
8. Swap Science: Keeping Things Cool by Building a Cooling System