AKRA 2017 Rule Changes/Updates
“…ENGINE SPECIFIC TECH FOR: BUILDERS PREPARED / LIMITED MODIFIED
the intent of this class is to give those that want to make an incremental move up from Box Stock. The rules allow a builder into the engine, but are written with the intent of making it possible for the average guy to build his own engine.
BP (Builder Prepared)/Limited Modified 6.5 OHV – Fuel Methanol ONLY – Approved Engine: 6.5 OHV 196cc clone engine modified only according to BP/Limited Mod OHV Engine Spec. Tires are track/series option.
ENGINE SPEC SHEET FOR BP/LIMITED MODIFIED 6.5 OHV CLASS
Description: Single cylinder, 2-valve overhead 4-cycle engine. No aftermarket coatings of any type are permitted on any part of the engine.
Cylinder Head Requirements: Stock cylinder head only. Machining of gasket surface is allowed. No addition of material in ports or to cylinder head allowed. Porting and / or grinding permitted. Valve seats may have three angles, 45Â° valve face and 30Â° top relief and 60Â° bottom relief. Inside diameter of valve seats must be stock(Into .897” max ID and Ex .862” max ID). Valve seats must appear stock and must be in stock position and depth. Stock head bolts are required and all four are required. Head gasket/s maybe after market, must be of stock configuration, and gasket thickness non-tech. Depth check between the valves, front to back and side to side may not vary by more than .005” max. (Champion Rocker Arm assembly and valve cover allowed.) No other alterations to the stock head are permitted.
Bore and Stroke: Stock bore is 2.685” and may be over bored to 2.718” Stroke is 2.123” +.010”-.005”.
Connecting Rod: Stock or Billet aluminum rods, with or without bearing inserts allowed. No titanium rods allowed. Rod length is a non-tech item.
Combustion Chamber Volume: 25 cubic centimeter minimum with piston at TDC using
procedure posted below. The CC check is the official check.
Carburetor/Intake Requirements: Any Tillotson HL series, with butterfly throttle assembly, and with .850 in minimum and .900 inch maximum venturi diameter. Auxiliary vacuum fuel pump, pulsed from the intake or crankcase is permitted. Pulse-type fuel pump is mandatory. Carburetor manifold is of one piece design only
Ignition system: Ignition timing is non-tech. Stock ignition module only. No modifications of any type allowed. Sparkplug connector NON TECH.
Piston Requirements: Any flat top piston assy up to .035″ oversize. Piston may not pop out above cylinder deck.
Valve Train: Stock valve-train only in stock configuration (Stainless Valves allowed). Any dual valve springs with aluminum retainers are allowed. No additional support for rocker studs permitted. Valves must be one angle only, 45Â°. No polishing, lightening or knife edging of valves (1mm min. margin). Valve length is non-tech. Outside face of valve head may not be below the combustion chamber floor. (i.e. don’t sink the valves) Valve cover may be drilled for fuel pump pulse fitting, otherwise, it must remain unaltered. Valve cover gasket is non-tech.
Camshaft Requirements: Stock camshaft cores only, Duration check for Intake and Exhaust lobes (taken off pushrod). Maximum Duration at .050” lift of 248 degrees and Maximum Duration at .200” lift of 153 degrees. Maximum lift of .310” at the pushrod.
Crankshaft Requirements: Stock, factory crankshaft only with stock, factory timing gear in factory location. No modifications to crankshaft allowed. Aftermarket steel main bearings of non self-aligning type, with or without seal are allowed. No ceramic bearings. Crankshaft Journal diameter is 1.180”, 1.168” minimum.
Block Requirements: Stock Block, as cast and produced with no alterations or modifications other than those specifically permitted. Block head matting surface may be machined, however, no piston pop out is allowed. Blocks repaired from broken rods are permitted as long as the repair does not constitute a functional modification to block. No bushings allowed except for those approved in this tech manual. All bolt bosses in block may be drilled and tapped for repairs or other uses. Additional side covers gaskets as required for crankshaft thrust are permitted. All parts associated with the governor and the low oil sensor may be removed, plugging any associated holes.
Flywheel: Approved SFI certified billet aluminum flywheel only. No machining or alteration of any kind allowed. Minimum weight for flywheel is 3.3 lbs. Any timing key or no key at all may be used. A flat washer or spacer may be used, and is recommended, between the flywheel and the nut. Handheld electric starter may be used. Current Approved flywheels: ARC 6619, ARC 6618, Raceseng Flywheels RSP-13-075 Rewheel NFS1 & RSP-13-077 Rewheel F-S1, Speed Karts Aluminum Billet.
Header and Muffler Requirements: Header Pipe Length: Minimum 16 Â½”- Maximum 20 Â½” . (measured on inside of pipe with Â¼” steel tape). Silencer must be a RLV B91MO type with .1285 no-go hole. Header must be securely wrapped from flange to muffler prior to the race. Exhaust Pipe must be double nutted or safety wired and silencer must be supported by clamped on brace to secure it in place. …”