Forged Alusil Pistons With Vented Skirts

Forged Alusil Pistons
Metric Mechanic has always used nothing but Forged Alusil Pistons in our Performance Engines from the beginning because of the following advantages over cast pistons;

• Forged Pistons are much stronger.
• One benefit of this increased strength is a longer wear life of the upper compression ring groove - an important factor in the extension of the engine life itself.
• Forged pistons can be made much lighter.
• Forged pistons, due to their greater density allow for faster heat transfer which in turn accommodates higher-compression without detonation.
• Alusil (aluminum high silicon content) pistons have a much lower expansion rate than traditional forged pistons - more like cast pistons.
  

Forged Alusil Piston Upgrades
We have, however, "upgraded" the design of our favorite pistons over the years.

• A tougher Alusil alloy is now used.
• The piston skirts are better supported and stronger.
• A slipper skirt design is used in order to narrow the bridge between the piston pin bosses for better pin support and the use of a light piston pin.
• An oil feed system was added to lubricate the piston pin bores.
• Piston pins are now offset to the exhaust side.
• Vents were added to the piston skirt to reduce expansion.

Vented Skirt Update
Metric Mechanic developed a simple matching process that reduces skirt scuffing and thermal piston expansion (growth) to a minimum.

Scuffing
Any BMW engine builder has probably noticed that there is a scuff area (dime or quarter sized) just below the oil ring on the load side of the piston. This occurs because the factory running clearance of the stock piston is less than .002". This means that the piston to wall clearance on only one side is half that - just .001". This makes it very difficult for the oil to migrate to the top of the skirt, so, the area just below the oil ring tends to run dry.

By machining a slit just below the oil ring groove along the width of the piston slipper skirt, oil thrown from the rod bearing journals enters the slit, lubricating the piston skirt from the top down and thereby greatly diminishing the "scuff zone".

Piston Expansion
This slit not only vents the heat of the piston crown but it also prevents heat from taking a direct path to the piston skirt. Instead the heat travels around the pin boss - a veritable "heat sink" area - where some heat dissipation occurs before reaching the skirt. The net affect is that less thermal expansion occurs at the piston skirt.

1. Temperature is Degrees F measured at the top of the pin boss on the inside of the piston.
2. Growth is the measure in 1/10,000 of growth over it's original size.
3. % is the percentage of growth change over a stock cast 535i piston.