How To Polish Aluminum Engine Parts
Here's an example of why chroming an aluminum engine part is a bad idea. Not only does it hold in engine heat, over time it pits and flakes.
Polishing aluminum is always the better choice.
There are generally three steps to polishing aluminum. First, there's filing to get out any deep pits or grooves. Next is the sanding to remove all the scratches and make the finish uniform. The last step is polishing (buffing), which brings out the shine. A hand polishing may follow if desired.
The pitting and corrosion seen on these old Sportster parts is typical for a 50 year-old unrestored motorcycle.
There really is no easy or safe way to remove old chrome in your home workshop. I sent the primary cover, cam cover, and valve covers to a plating shop to have the chrome chemically removed.
Six weeks later the engine parts returned. I started with the valve cover that had the heaviest corrosion. Because the depth of the pitting, I began with a 12" metal file.
Aluminum is a soft metal and is easy to over-file. Work the file slowly and evenly - you only want to remove enough metal to get to a flat, level surface.
Notice the diagonal (45 degree/135 degree) swipes left by the metal file. This is to offset the left-right filing, which helps keep the surface flat and even.
Next step is the sanding. Starting with a folded-over piece of course Emory cloth, sand over the file marks in a left-right direction.
Take long, even strokes both ways. If the base metal isn't dead-flat, it will not produce a shine when polished.
Stop periodically and hold the work up to the light at different angles. This gives you a better perspective of the entire work-piece.
After the diagonal swipes were no longer visible, switch to fine Emory cloth. This brings the aluminum to a flat and dull but uniform finish.
DIY Bench Sander/Polisher
The final step to polishing aluminum is the buffing process. Of course you can buy a bench sander, but you can make one fairly cheaply. The one shown is an old electric motor fitted with an arbor that was purchased separately.
The motor is pretty lightweight and when not in use, I store it in a plastic tote under my workbench. I have also used this motor with a wire-wheel attachment to remove rust of old metal. Drawbacks to a DIY bench sander like this are, it only has one-speed and one arbor, so you'll be switching wheels quite often.
There are many different styles and sizes of buffing wheels. For aluminum polishing I have three, sort of like coarse, medium, and fine.
Buffing wheels are available in different diameters. Since my DIY sander is small, I use 7" buffing wheels.
There are three main buffing compounds (Tripoli, Stainless, and Color) and each have a specific color so you can identify them. Tripoli is a reddish brown, Stainless is grey, and Color is white.
Each of the buffing compounds will be available in different grades for a specific type of metal (or plastic or wood). For aluminum, Tripoli is all you need.
TIP: If you need more buffing wheel surface, mount two of the same wheels next to each other.
Polishing aluminum engine parts is time consuming, but I find it to be very relaxing. It's a nice change of pace from other restoration tasks, such as rebuilding a bottom end or finding an electrical short.
I used the same sanding/polishing techniques on these Sportster front forks. The "before and after" difference is not as noticeable, as they were in pretty good shape before the buffing.
Here's a Harley 45 cam cover I polished following the same procedures; filing, sanding, then polishing with a buffing wheel.
After several months, the lustre of the polished aluminum does fade. To bring the shine back up, grab a scotchbrite pad (fine, not coarse) and it will bring the shine back up.
On aluminum forks and other parts that aren't exposed to engine heat, you can spray clear coat paint after the buffing is done. Personally, I've never had good results with clear coats looking good or lasting for any length of time. I recommend leaving them bare and cleaning and polishing them when needed.