How To Polish Aluminum Engine Parts
Here's an example of why chroming an aluminum engine part is not a good idea. Not only does it hold in engine heat, it pits and flakes over time.
Polishing aluminum is always the better choice.
The pitting and corrosion seen on these old Sportster parts is typical for a 50 year-old unrestored motorcycle.
Removing Chrome From Aluminum
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 began working on the valve cover that had the heaviest corrosion.
Before going to the buffing wheel, they'll be three to five steps of filing and sanding the 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.
Steps Before Polishing
- 50 Grit Emory cloth
- 80 Grit Emory cloth
- Fine Emory cloth
- 240 Grit wet sand
Because the depth of the pitting, I began with a 12" metal file. Aluminum is a soft metal and is easy to over-file.
Don't apply too much pressure - let the file do the work. Work 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 coarse Emory cloth (50 or 80 grit), sand over the file marks in a left-right direction.
TIP: Stop periodically and hold the work up to the light at different angles. This gives you a better perspective of the entire work-piece.
Take long, even strokes both ways. If the base metal isn't dead-flat, it will not produce a shine when polished.
TIP: If you're getting blisters from sanding, you're probably pressing down too hard. If the paper you're using is not cutting, switch to a coarser grit.
After the diagonal swipes are no longer visible, switch to fine Emory cloth.
Wet sanding with 240 grit paper should bring the aluminum to a flat, dull finish. The aluminum is now ready for buffing.
Polishing can be done by hand, but a buffing wheel is much faster. You can buy a bench sander, or you can make one fairly cheaply.
This DIY polisher is an old electric motor fitted with an arbor that was purchased separately.
read DIY Motorcycle Tools
Drawbacks to a polisher like this are, it only has one-speed and one arbor, so you'll be stopping to switch 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.
There are three main buffing compounds; they are called Tripoli, Stainless, and Color. Each has 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 are available in different grades for metal, plastic, or wood. For aluminum, Tripoli and Color are the two that I use.
Buffing wheels are available in different diameters. Since my DIY sander is small, I use 6" buffing wheels.
TIP: If you need more buffing wheel surface, mount two of the same wheels next to each other.
Here's a pair of Corvette valve covers that were in a barn for 20 years.
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 polishing.
On motorcycle forks and other parts that aren't exposed to engine heat, you can spray clear coat paint after the polishing 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.
After several months, the lustre of the polished aluminum fades. To bring the shine back up; clean off any dust or dirt, scuff with a scotchbrite pad (fine, not coarse), then apply a small amount of polish.
Here's a Harley 45 cam cover I polished following the same procedures; filing, sanding, then polishing with a buffing wheel.
Polishing aluminum engine parts is time consuming, but I find it to be relaxing. It's a nice change of pace from other restoration tasks, such as rebuilding a bottom end or finding an electrical short.