Box Stock Blueprinted Class Engine Rules

Box Stock Blueprinted – Age 15 years & up – 375 lbs. Only gasoline allowed for fuel.  Approved Engines: Box Stock Project, Harbor Freight, Homier. Any style clutch allowed.  Maxxis tires only, any compound allowed.

Block Requirements: Stock engine block as produced with no alterations or modifications other than what is specified here. Blocks may be welded for repairs as long as the repair does not constitute a functional modification to the block or the blocks function. No welding to block from cooling fins up. Governor stub hole may be tapped and plugged or siliconed. All bolt bosses in block must be stock size but may be Heli-coiled. Bore and Stroke: Stock bore is 2.677” and may be over bored to 2.718” (approximately .040” overbore). Stroke is 2.138” +/-.010”. Stock sheet metal components must be in place and secured. Blower housing must remain stock. Pull start is mandatory. Any size fastener is allowed to secure sheet metal.

Gaskets: Gaskets are a non-tech item. Maximum amount of side cover gaskets is 3. Maximum amount of head gaskets is 1. Minimum thickness on head gasket .005”.

Piston Requirements: Must be stock dished piston with no modifications. Any OEM Oversized or standard size pistons may be used including Honda. Wrist pin height to top of piston .570” minimum and .578” maximum.

Piston Rings: Piston rings must be stock piston rings. Over sized rings are allowed. Ring to be checked against know stock parts. Top ring and scraper ring minimum thickness .056”. Minimum width .094”. Ring gap is a non-tech item.

Pop Up: Maximum pop up of .005” checked length and width of the piston with an indicator and square stock.

Camshaft Requirements: After market cams are allowed but must meet stock lift and profile below. Minimum base circle size of .820” is mandatory. Stock cam core is mandatory. No grinding or lightning allowed on stock cam core. Stock cam core will be checked against sample stock cam.

All profile readings are taken directly off the push rod with push rod guides removed at zero lash with a degree wheel set to zero on the compression stroke. Set indicator on push rod at zero while the crankshaft is at TDC Tighten rocker until the indicator moves .001”. Reset indicator to zero. Any indicator is acceptable.

Intake Profile

Exhaust Profile

Lifters: Stock lifters are mandatory. No after market lifters allowed. Lifter to be compared to know stock part. Head of lifter minimum diameter of .922”. Maximum diameter of lifter .940”. Length of lifter minimum 1.360”, maximum length 1.365”. 

Connecting Rod: Stock factory rod with no alterations. 2.345” Minimum length, .2365 Maximum length.Any OEM connecting rods are acceptable including Honda. Billet rods are allowed. ARC and WMS billet rods are acceptable. Stock length billet rods allowed with no alterations.

Wrist Pin: Stock factory wrist pin mandatory with no alterations. Minimum OD of .709” Maximum OD of .710. Maximum ID of .547” Minimum ID of .544”.

Crankshaft Requirements: Stock unaltered factory crankshaft only. Factory unaltered timing gear only. Timing gear must remain in factory location. No modifications to crankshaft allowed. Crankshaft Journal diameter is 1.180”, 1.175” minimum. Stock factory stroke 2.138” +/- .010 is allowed.

Valve cover: Stock unaltered valve cover is mandatory. Valve cover will be compared to know stock part. Only alteration is pulse fitting for fuel pump.

Cylinder Head Requirements: Stock unaltered cylinder head only. Machining of gasket surface allowed. No Porting allowed. Must be a sharp edge on roof of port both intake and exhaust. Valve seats can be replaced. Valve seats 45° only. Stock unaltered head bolts are required. Minimum head gasket thickness is .005”. Push rod guides and plate must remain stock. Push rods must remain stock and stock length. Head specs: Minimum depth from gasket mounting surface to top of combustion chamber surface .312”. Minimum depth from gasket mounting surface to top of valve seat .287”. Intake port specs: .855” maximum width at entrance. .855” maximum height at entrance. Exhaust port specs: .900” maximum width at entrance. .900” maximum height at entrance. Intake valve seat Specs: Maximum inside diameter of .893. Exhaust seat Specs: Maximum inside diameter of .853”. If lip of valve seat is not visible for checking depth, head is illegal.

Valve Train: Valve train must remain stock. Rockers, ball adjusters, rocker arm studs, lock nuts, pushrods and lifters must be stock and unaltered. Max lift at valve check will be enforced. Stock valves only. Valves must be one angle only, 45°. No polishing or lightening of valves. Valves may be reground but no knife edging of valves is allowed. Valve springs are not subject to tech. Any size valve lash allowed. Valve Specs: Intake valve minimum head diameter .977. Intake valve minimum length 2.499”. Exhaust valve minimum head diameter .936”. Exhaust valve minimum length 2.450. Minimum valve stem diameter on both valves is .209”. 

Valve Guides: stock valve guides or after market valve guides are allowed. Lifter bore guides are also allowed.

Valve Springs: Any valve spring allowed.

Flywheel: ARC #6619 #6618 billet flywheel with cooling fins only. Other flywheels will be considered later. No machining or alteration of any kind allowed to flywheels. Flywheels will be checked to known stock part. Minimum weight for flywheel is 3.3 lbs. Timing key is not a tech item and can be removed. Crankshaft nut non tech item.

Carburetor: Stock carb as produced from the factory with no alterations. Carb to be checked with no go gauge and/or other measurement devices. Carb at choke inlet maximum size .942” tall. Maximum venturi size .625”. Carb at intake manifold maximum size .755”. Stock throttle shaft minimum thickness .080”. Minimum butterfly thickness .035. Butterfly screw can be no shorter than .317. Stock air filter box must be removed. Any filter adapter allowed except for off set adapters. AIr filter adapter maximum length 1.375”. Any after market Air Filter allowed. No baffles allowed in air filter. Choke assembly may be removed. Jet sizes are a non tech item. Emulsion tube non-tech item. Any throttle system allowed. Return spring on throttle mandatory.

Intake Manifold: Black phenol carb insulator/intake manifold must be used. No alterations of intake manifold are allowed. Will be checked against known stock part. Minimum thickness of .268”, maximum thickness of .282”.

Fuel Tank: Stock fuel tank must be removed. Floor pan mounted fuel tank is mandatory. After market fuel pump of any kind is allowed. Fuel pump must be pulsed from the valve cover. Fuel line pulse hose length can be no longer than 14”. 

Ignition system: Stock ignition system only. No modifications of any type allowed. Subject to known stock part on shape, looks, voltage and ohms. Spark plug connector is a non-tech item. Spark plug is a non tech item. Maximum ohms of 2000. Minimum ohms of 500. Kill switch may be removed. Spark plug boot non tech item. Spark plug wire must be stock and unaltered.

Header and Muffler Requirements: Any single stage, one-piece header allowed. Pipe OD must remain round. No oval pipes allowed. Maximum I.D. of tubing is .1.00”. Stock Briggs or similar 5 HP “hot dog” style muffler required with no alterations. Muffler must be clamped to pipe end. Maximum outside longest length of the header from flange to muffler is 24”. Header may be bent in any shape to keep it away from the driver. Muffler must not extend past rear bumper. Header must be wired from one bolt to the other. Header straps are mandatory. Chain guard with heat shield protection is mandatory.