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Making Combustion Chamber Molds
There are several different methods used for producing combustion chamber molds of cylinder heads. Engine builders who need to digitize a chamber or determine piston to valve clearance often use one of these methods.
Ross Pistons recommends the following process to make a combustion chamber mold:
Clean both the chamber and head surface; chamber must be free of all dirt, oil and grease, and valves must be installed.
Place cylinder head properly on the block using no head gaskets. Snug cylinder head bolts or studs with dowel pins installed. Turn block over until cylinder head is facing downward toward the floor. Cylinder head deck must be level before pouring mold.
Insert plastic plug, or other stopper, into spark plug hole on the chamber side. Tap in if needed.
Spray both the chamber and cylinder bore with WD40 or wipe down with a light coat of oil or silicone spray.
Mix epoxy or "Bondo" as per manufacturer’s instructions.
Pour mixture into combustion chamber from the bottom of block until epoxy covers at least .250” (6.5 mm) up of the cylinder bore from the head surface.
Let epoxy or Bondo cure as per instructions that came with product. Once epoxy or Bondo has hardened, flip engine right side up and remove cylinder head.
Before removing chamber mold from cylinder head, install dowel pins into cylinder head. Place a straight edge parallel with the dowel pins; using a razor blade, scribe a line on the back side of the mold. Scribe line must be on the chamber mold to be scanned. Scribe line is required to be parallel to the piston pin only. It is not required to be on the pin center line (see photograph).
Note: Care must be taken to ensure that the scribed line is absolutely parallel with the dowel pins.
Remove mold from chamber. If mold sticks, lightly tap with a plastic hammer or lightly pry with a screwdriver. Use alcohol to remove any WD40 or silicone spray left on chamber mold and cylinder head.
Tech Tip courtesy of Ross Pistons