RAM has found in almost every case, the air/oil separators on all 421C models and late 421B models (after serial number 500) have never been cleaned since installation when the airplane was assembled. Can you imagine what the inside of these units look like after 30-40 years worth of oil, carbon, sludge and grime passing through them?
GTSIO-520-H, L & N models have an air/oil separator canister between the turbo charger and the engine scavenge pump. (Figure 1 & 2).

Oil passes from the turbo charger into the air/oil separator canister, then is drawn out by the engine scavenge pump which passes the oil back into the oil pan. The air/oil separator is considered an airframe part and is almost always neglected during engine overhaul or exchange.
When your newly overhauled engine is installed the contamination that has built up in the air/oil separator is still there. Fresh oil from the new engine passes through the air/oil separator mixes with sludge, oil and grime left from the previous engine, going directly into your fresh engine. Even when an engine is removed for metal contamination in the oil, the new or repaired engine is usually installed without consideration for what contaminates are left in the air/oil separator. On the very first start of the fresh engine, whatever is left in the air/oil separator is drawn directly into the engine.

The gunk and crud that builds up in the bottom of the air/oil separator can also effect the ability of the scavenge pump to work properly. The build up of contamination restricts the outlet of the air/oil separator and can inhibit the ability of the scavenge pump to draw oil out of the separator canister
(Figure 3) causing oil to dump overboard and affect the proper circulation of oil to your engine.

The clearance between the bottom of the air/oil separator canister and the outlet pipe is only about 1/16th of an inch (Figure 4). The build up on the bottom of the canister further reduces the clearance. This restriction can cause the oil to dump over board from the air/oil separator overflow outlet during engine start up and reduce circulation of oil to the engine. Figure 4 shows the inside of a properly cleaned air/oil separator.

1. Remove air/oil separator in accordance with the Aircraft Service Manual.
2. Flush the air/oil separator with mineral spirits to remove oil.
3. Utilizing compressed air, remove mineral spirits from air/oil separator.
4. Place entire air/oil separator in container of Berryman’s Chem-Dip (Figure 5), making sure unit is submerged, and all air pockets have been removed from internal surfaces of the air/oil separator.
5. Soak air/oil separator overnight for best results.
6. Remove the air/oil separator from the Berryman’s container and rinse with water.
7. Utilizing compressed air, remove any water from both inside and outside of the
   air/oil separator.
8. Reinstall the air/oil separator in accordance with the Aircraft Service Manual.
9. Check for oil leaks and verify proper operation.