Engine bore measured = 94.615 mm
Engine bore spec = 94.6 mm
Pistons diameter measured = 94.49 mm (skirt)
Piston diameter spec = 94.555 mm
Piston size is stamped on the top of the piston is 94.555 mm. This is supposed to be the stock size of the 3.8 liter engine. There is another number on the top which is 10.5. The .5 represents the cylinder wall clearance. To determine the clearance, multiply the .5 times 4 and that is the wall clearance in thousands, or .002 inches. Converting this to mm = 0.0508 mm. Adding this number to the piston size of 94.555 mm gives 94.6058 mm. The machine shop measured the bore at 94.615 mm or .00236 inches larger. This is acceptable with wear.
Conclusions:
Stock piston size 94.555 mm
Stock bore size 94.6058 mm
20 over pistons 94.75 mm
20 over bore 94.8008 mm
Metric Mechanic recommends .0005 inches more or ½ a thousandth more overbore for the front and rear cylinders. This will aid in heat expansions problems with these two cylinders.
The stock bore size along with the 90 mm stroke yields 3876.6 cm3 or over 3.8 liters. This equates out to 231.6 cubic inches. With a horsepower rating of 340 hp for this engine, the HP per cubic inch is 1.46. That’s a serious engine. If this was a Chevy 350, the power output would be over 513 HP! Now that is my kind of engine.