Learn How to Clean a Motorcycle Engine Case

Close Up Of Shiny Engine

Noel Albeza / Getty Images

There are few more satisfying tasks when restoring a classic motorcycle than polishing the engine cases. In most instances, the cases will look better than new. However, the owner must be sure that the value of the bike will not be diminished by polishing the cases -- the original bike may not have had polished cases and a collector will not be impressed with the update.

For many buyers of motorcycles, spending time polishing their bike is a pleasure. In the 60s, when polishing the cases on café racers, became popular, many of the owners would periodically apply various aluminum polishing compounds to the clutch covers on their Triumphs, Nortons, and BSAs.

Today more modern classic owners will have their engine cases chrome-plated -- a process that was difficult and too expensive in the 60s.

Polishing and Buffing Motorcycle Engine Cases

Although not strictly original, most restorers of classic motorcycles will typically polish their machines cases. For the most part, polishing aluminum cases is relatively simple, requiring more time than money to accomplish.

For someone intending to restore more than one motorcycle or who has a well-equipped workshop, a buffing wheel is essential. These machines often mounted on a pedestal for easy all round access to the wheels, are relatively inexpensive and cost around $120 for the machine and pedestal. However, it is possible to use a regular hand-held drill with a buffing wheel attachment to get a reasonable finish on a case.

Avoiding Scratches

Before the polishing process can begin, the mechanic must remove the cases from the bike and thoroughly clean them inside and out (it is important to clean the inside too as doing this after the cases have been polished can result in scratches from movement inside a wash tank).

Remove Deep Scratches and Marks

The first stage of polishing (after cleaning) is to remove any deep scratches or marks on a case. The ideal tool for this purpose is an air-powered angle grinder with a soft Scotch-Brite® type pad installed. The mechanic must blend away a scratch by applying the Scotch-Brite pad to the area around the scratch (concentrating on one point will tend to put a flat spot on the case -- most cases are of a double curvature shape).

Note: When grinding a scratch out of a case, the mechanic is grinding the hills away and not the valleys of a scratch, hence the need to blend.

After any large or deep scratches have been blended out using the Scotch-Brite pad, the case should be washed in warm soapy water (dishwasher liquid is ideal) to remove any dirt or large particles that may cause further scratches during the next phase: wet/dry sanding.

Wet/Dry Sanding

Next, start the sanding process using a relatively coarse grade of wet/dry such as 220 and concentrate on areas with any major flaws. The paper should be used with warm soapy water for best results, with periodic cleaning or wiping over of the case to remove any dirt particles. The mechanic should move on to 400 wet/dry next, and use it to sand the entire case. Using the 400 w/d in this manner will ensure a uniform finish all over the case.

The final grade of wet/dry should be 800 or 1,000 grade. Again, the mechanic should sand the entire case to give a uniform finish with periodic wiping to remove any large particles.

After sanding, the entire case should be thoroughly cleaned ready for buffing.

Buffing and Polishing

Before buffing motorcycle cases, it is important to ensure that they are free of grit or dirt as these will scratch the newly prepared surface.


The buffing machine operator must wear suitable eye protection and a face shield because particles will be emitted at high speed from the spinning wheel. In addition, the mechanic must hold the case firmly before applying it to the spinning wheel. The mechanic should avoid buffing across an edge as the spinning wheel will often try to snatch the case from the mechanic’s hand.

The buffing wheel should be coated with a fine rouge buffing compound before the case is slowly brought into contact with the wheel. The mechanic should move the case slowly but continually over the wheel. The case will soon start to get hot due to the friction between the wheel and the case’s surface. At this point, the operator should polish off any black residue (surface oxides) with a clean/dry cloth and allow the case to cool. Alternatively, the case can be held under a tap with cold running water.

When the entire case has been buffed, the mechanic should apply a quality polishing compound, which is readily available at auto parts stores.