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How Do I Clock An Alternator?

Alternators

If your new alternator bolts up fine but the charging lug points the wrong way, nothing is wrong with the alternator. You just need to clock it.

Overview

Short Answer

GM 10SI and 12SI and Ford 1G and 2G cases can be rotated in 90 degree increments. Hold one case half, remove the rear case screws, separate the halves only slightly, rotate to the orientation you want, and put the screws back. Pull the halves too far apart and the brushes fall out.

Why these alternators are not application specific

The GM 10SI and 12SI case and the Ford 1G and 2G cases both had very long production runs, starting in the 1960s and running into the early 1990s. The case mounting locations never changed through that whole life cycle, which makes them incredibly versatile.

That versatility is why these units get mounted in so many different locations and orientations, and it is why replacement and upgraded alternators are almost never application specific. The trade off is that the charging lug will not always land where you want it on your particular build. Clocking is how you fix that, and it takes a few minutes.

An alternator power lug on the rear of the case contacting the engine, showing why the case needs clocking

This is what clocking fixes. The power lug on the back of the case is hard against the engine, so there is nowhere for the cable to go.

Look at the case screws first

Look at the case attaching screws on the back of the alternator. They are equally spaced, and that spacing is what lets the rear of the case attach to the front case in 90 degree increments. So you have four possible positions for the lug, not an infinite range.

How to clock it

Work slowly. The whole job is about not letting the case halves come apart.

Hold one case half securely. Get a solid grip before anything comes loose.

Remove the case screws. Be very careful not to separate the two case halves any more than necessary.

Separate the case slightly. Only enough to let the rear housing turn. This is the step that goes wrong.

Rotate to the orientation you want. Remember it moves in 90 degree steps.

Reinstall the screws. Check that the lug ended up where you actually wanted it before you put the alternator back on the engine.

The one thing that will ruin your afternoon

If you are not careful and slide the case halves apart too far, the brushes can fall out. They are not fun to put back in. That is the entire risk in this job, so if you take nothing else from this, take that: separate the halves as little as you possibly can.