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Simone also told me that Diamond Racing now offers a very cool epoxy chamber mold kit that can absolutely save the day in terms of what your application demands. The kit (an A-B pair of epoxy cans, a mixing stick, nitrile gloves and a spark plug hole plug) it sells for a measly $42 or so, and eliminates the need to ship one of your heads to the piston manufacturer for scanning. The kit is easy to use. With the loaded head installed onto the otherwise bare block, turn the block upside-down and level the deck surface front-to-rear and side-to-side. Install the cylinder head, and apply a release agent (WD40, etc.) to the bore and the chamber. Pour the mixed two-part epoxy into the bore, from the bottom of the cylinder, allowing it to puddle and fill the combustion chamber, and the top of the cylinder. The epoxy takes a hard set in about 30 minutes. Scribe a line on the back side of your new mold puck parallel to the piston pin center line, and pop the mold out. As the epoxy hardens, it also shrinks about 2%; so removing the mold is easy after it dries. With this mold in hand, the manufacturer now doesn’t need to guess or wonder about chamber shape, valve placement, etc. This is especially handy if the cylinder head is an oddball, unknown or highly modified head.
INFORMATION THE CUSTOM SHOP NEEDS TO MAKE THE PISTONS
• Engine type (example: BBC)
• Application (street, drag race, oval track, road race, marine, etc.)
• Desired compression ratio
• Estimated desired engine horsepower
• Maximum RPM
• Block deck height (main bore center to deck surface)
• Finished bore size (target)
• Cylinder bore length
• Distance between cylinder bores (center-to-center)
• Crank stroke
• Crankshaft counterweight radius
The piston manufacturer will consider deck height, bore length, crank stroke, rod length and crank counterweight radius in part to make sure that the piston compression distance will provide the desired compression ratio and deck clearance on the up-stroke; and that the piston skirt height will clear the crank’s counterweights on the down-stroke.
• Cylinder head (make, type, model/part number/casting number)
• Cylinder head chamber volume (cc)
• Valve diameters (int/exh)
• Compressed head gasket thickness
• Head gasket bore diameter (assume a round gasket bore)
• Head gasket volume (cc)
The head information is needed to help determine compression ratio and valve pocket placement, pocket radius, depth and angles.
• Connecting rod length (center-to-center)
• Connecting rod material (steel, aluminum, titanium)
• Rod small end width and thickness over the top of the pin hole
Rod information is needed to help determine compression height and distance between pin bosses. Information regarding rod material may be considered as well, due to the theoretical elongation of the rod length when using steel, aluminum or titanium.
• Camshaft type (hydraulic or solid/ flat tappet or roller)
• Gross valve lift (int/exh)
• Lobe separation
• Duration @ 0.050″
• Valve lift at TDC (int/exh)
• Piston valve pocket depth (intake and exhaust, from sample piston, if available)
• Intake valve angle
• Valve spacing
The piston manufacturer will consider actual valve movement off the seat to calculate the needed valve-to-piston clearance. Rod ratio plays a major part in piston-to-valve requirements, due to varying piston speeds @ TDC for a given rod/stroke combination.
• Piston pin support (floating or fixed)
• Piston skirt shape (scalloped, full round, inboard, etc.)
• Desired piston-to-deck clearance (-above or +below)
• Compression height (distance from pin center to theoretical “flat” of piston)
• Piston type (flat top, conical dish, dome, reverse image dish, round dish, or full 3D max dome)
• Piston dome volume (total effective, including valve pockets)
• Axial ring height (top, second, oil)
• Radial ring width (top, second, oil)
• Desired ring back clearance, if not standard (top, second, oil)
• Wrist pin diameter
• Wrist pin length
• Pin wall thickness
POWER ENHANCEMENTS
• Maximum boost (turbo or supercharger)
• Nitrous HP amount
• Number of nitrous stages
• Fuel type/octane planned
This information will help the piston manufacturer determine piston material, crown and cross-sectional thickness required for the power level desired, and will dictate required piston-to-bore clearance.
OPTIONAL PISTON FEATURES
• Double pin oilers with slots (if not Std.)
• Gas ports (side/top)
• Internal weight-removal milling
• Chamfer pins (round wire lock)
• Offset pins (OEM noise control)
• Ceramic top coat
• Moly skirt coat
• Casidium coated pins (Race, Dry sump, Hi-pan vac. applications)
INFORMATION THAT YOU NEED TO SUPPLY
Don’t freak out at the challenge of obtaining all of the above data. Custom piston manufacturers are smart folks. They are able to help you make the final determinations for your engine build, but they can only do so much. Here’s the bare minimum info that you need to supply to them, which tells them what related components and dimensions you plan to use for the build:
• Engine type (BBC, SBC, SBF, BBF, etc.)
• Intended application (street/race, drag racing, marine, etc.)
• Desired compression ratio
• Planned horsepower
• Maximum RPM
• Block make/model (OE, aftermarket, etc.)
• Block deck height
• Finished bore size
• Stroke
• Connecting rod length
• Connecting rod small end width
• Connecting rod material
• Cylinder head (make/model)
• Head chamber volume
• Intake valve diameter
• Exhaust valve diameter
• Valve angles (if non-OE head)
• Head gasket bore diameter
• Head gasket compressed thickness
• Desired minimum valve-to-piston clearance
• Ring dimensions (axial height and radial width)
• Camshaft type
• Valve gross lift
(Note: for head specs, if you don’t have the detailed info, the piston manufacturer can likely find what’s needed if the head is an un-altered OE or aftermarket head)
CALCULATING CYLINDER HEAD GASKET VOLUME
Gasket bore diameter x gasket bore diameter x 12.87 x head gasket compressed thickness.
(Example: If gasket bore diameter is 4.600″ and compressed gasket thickness is 0.038″, then:
4.600″ x 4.600″ x 12.87 x 0.038″ = 10.3485cc)
A custom piston order is not something you want to put off for the last minute, and the job cannot be rushed. The piston is the one part in the engine that has to work in conjunction with all your other internal components, so detailed info is key to getting everything to work together properly. Although order times can vary depending on the time of year (race engine builder demands), typical lead times can range from a minimum of 3 to 5 weeks, while a wait of 6 to 8 weeks (or longer) is not uncommon. In other words, plan ahead. Naturally, don’t final-hone the cylinder bores until you have the new pistons in hand to measure actual skirt diameter. Just make sure that your cylinders will clean up straight at your new target bore size.
Tags: CUSTOM PISTONS, DIAMOND, GASKET VOLUME, J&E, REQUIRED DATA
