BP(Builder Prepared)
The intent of this class is to give those who want to make an incremental move up from Box Stock or a change from stock flathead a class of their own with horsepower similar to the flathead "stock" classes that are becoming increasingly expensive. 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) 6.5 OHV – GASOLINE ONLY 87-93 octane – Approved Engine: 6.5 OHV 196cc clone engine modified only according to BP OHV Engine Spec. Tires are track/series option.

ENGINE SPEC SHEET FOR BP 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 (exception Blower Housing and Shrouds).

Cylinder Head Requirements: Stock cylinder head only. Machining of gasket surface is allowed. No addition of material in ports or to cylinder head allowed. Valve seats may have two angles, 45° valve face and 30° top relief. Inside diameter of valve seats must be stock(Int .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. Minimum head gasket thickness is .009”. More than one gasket may be used. Cylinder head guide plate for pushrods must remain stock. 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. (See tech book)

Carburetor/Intake Requirements: Stock Huayi or RUI*ING carb. Venturi .615” NO-GO. Rear carb bore .751” NO-GO. Air Filter adapter of 1.375” max length allowed. No air rams. Pulse-type fuel pump is mandatory. Fuel pump must be pulsed from either the crankcase or the valve cover. Black phenolic carb insulator must be used. Choke assembly may be removed. Jets, air bleed jets, and emulsion tube are non-tech. Throttle shaft, screw, washer, and butterfly must be stock and must be present. Stock intake runner gasket configuration only. One extra gasket may be used with restrictor plates. No other alterations are permitted.

Ignition system: Ignition timing is non tech. Stock ignition module only. No modifications of any type allowed. Sparkplug connector must be stock as from factory.

Piston Requirements: Must be stock dished piston with no modifications. Oversize Clone or Honda ZOT dished piston is allowed up to .035" oversize. Rings must appear stock and all rings must be installed. Piston may not pop out above cylinder deck.

Valve Train: Stock valve-train only in stock configuration except any single valve springs and valve spring shims are allowed. No additional support for rocker studs permitted. Valves must be one angle only, 45°. 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, ez-spin assy must remain as stock. Cam lobe base circle diameter .865" -.005”/+.010” Duration check for Intake and Exhaust lobes (taken off pushrod) +/- 2 degrees for wear. Intake duration of 219 degrees at .050 lift/86 degrees at .200 lift. Exhaust duration of wear and gauge variances.) Max Intake lift on cam .225” – Min .215”lift taken at the pushrod. Max Intake lift at the valve .238” Taken on valve spring retainer with zero lash.
Max Exhaust lift on cam .232” – Min .222” lift taken at the pushrod. Max Exhaust Lift at the valve .242” Taken on valve spring retainer with zero lash.

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 may NOT be welded for repairs. No addition of material to block (i.e. welding, jb weld, etc). All bolt bosses in block may be drilled and tapped for repairs or other uses. Additional side cover 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, but compression release mechanism must remain on camshaft. Current Approved flywheels: ARC 6619, ARC 6618

Header and Muffler Requirements: Header Pipe Length: Minimum 18”- Maximum 22”. Silencer must be a RLV 91_L type with .1285 no-go hole. Header must be securely wrapped from flange to muffler prior to the race.