Builder Prepared Rules

DynoDon

Moderator
I get many calls and e-mails about changing the rules for the builder Prepared class. Here is something I thought some of you might like.

ENGINE SPECIFIC TECH FOR: BUILDERS REPARED-STOCK/Florida bp rules 2021
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 or a change from stock flathead a class of their own with horsepower similar to the Briggs animal “stock” classes that are becoming increasingly expensive. The rules allow a builder into the engine but with the intent of making it possible for the average guy to build his own engine.
BP (Builder Prepared) 6.5 OHV – Fuel Methanol ONLY – Approved Engine: 6.5 OHV 196cc clone engine, tillotson thick block 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.
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, 45-degree valve face and 30 degree top relief and 60 degree 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 may be 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, no roller tips 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 2.725”.040 over bore Stroke is 2.123” + .010”-.005”.
Connecting Rod: Billet aluminum rods recomended, 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. The CC check is the official check.
Carburetor/Intake Requirements
: Stock Huayi, Rui*ING, Tillotson clone carb. NO TILLOTSON PUMPER CARBS. Venturi .625” NO-GO. Air filter adapter 1.375 max length air filter cannot be used as a ram air . 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 bleeds jets, and emulsion tubes are non-tech. Throttle shaft, washer and butterfly is non-tech. 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. Spark plug connector must be stock as from factory. Spark Plug non-tech.
Piston Requirements: Any oversized Stock and Honda ZOT dished piston or Wisco flat top piston up to .040” oversize. Min gram on piston without the rings is 120 grams 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 and stock size valves (Stainless Valves Allowed). Any single valve springs and valve spring shims are allowed. No additional support for rocker studs permitted. Valves must be one angle only, 45 degree. No polishing, lightening or knife edging of valves. 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). Push Rod Guide Plate may be removed. Any 3- piece pushrod that meets standard length 5.285 must go 5.230 no go Valve cover gasket is non-tech valve cover spacer allowed for champion rocker assembly
(Champion Rocker Arm assembly, 1:1 ratio only no roller tips allowed).
Camshaft Requirements
: Stock camshaft cores ONLY. Duration check non-tech. Max lift at the valve retainer intake .238” and exhaust .242” taken on valve spring retainer. This will be checked at running lift. (Taking off pushrod, Max intake lift .225”. Max exhaust lift .232”. Compression release may be removed.
CRANKSHAFT Requirements: Stock factory crankshaft only with stock, factory timing gear in factory location. No modifications to crankshaft allowed. After-market 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 covers gaskets as requited for crankshaft thrust are permitted. All parts associated with the governor and the low oil sensor may be removed, plugging any associated holes. .375 solid dial pins in side cover allowed. Side cover bolts may be replaced with stud and nut.
Flywheel:
Approved SFI certified billet aluminum flywheel only. No machining or alteration of any kind allowed. Minimum weight for flywheel is 3.3lbs. 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 NF-S1 & RSP-13-077, Rewheel F-S1, Speed Karts Aluminum Billet.
Header and Muffler Requirements: Header Pipe Length: Minimum 16 ½”-Maximum 20 ½”. (measured on inside of pipe with ¼” steel tape). Silencer must be a RLV 91_L type with .1285 no-go hole. All 4 baffles must be in place. Header must be securely wrapped from flange to muffler prior to the race. Exhaust Pipe must be doubled nutted or safety wired and silencer must be supported by clamped on brace to secure it in place.

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K12

Member
Great job Don. Solid "dowel" pins, not "dial" pins. Also, I would like to be able to use crankshaft shims to adjust end play. Thanks Don
 

Chipg56

Member
Power wise, how does something like this compare to a blue printed animal? When built by a builder what is the cost difference between this and a BS Clone?
 
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DynoDon

Moderator
Ya, I saw that written error on dial pins. Should have been fowl. I didn’t write the rules. They came from Florida. When I built my builder prepared by the original AKRA rules that are in the current book my builder prepared would put a woopin on a stock blue printed animal.
 

DynoDon

Moderator
I can’t tell you what the cost difference is because I myself only build engines for my family and friends. I don’t build for other people unless personally requested. But I will say I had less than $500 in my own project. Keep in mind that was back 5 or 6 years ago
 

GClary

Member
Looks good . Are the tins considered as being part of the engine ? I know a lot of people are now powder coating or painting their tins to be unique and fancy , silly question I know , but , as being in the rules of no coatings allowed , that would fall under that grey area unless otherwise noted . And also allowing painting or powder coating of the valve covers if they wish .
 

Skidude

Member
Sounds like a great step up class. I really like the rules on the head, cam and rod. A great engine that will live in the semi-upper RPM range.
 

DynoDon

Moderator
The rules were given to me by Chops. He is an AKRA tech in Florida. He told me it is taking off right now. As for powder coating tins, I see no performance gain
 

flattop1

Dawg 89
I always felt the camshaft rule of max lift was too conservative .
I was surprised it didn't take off in the beginning .
Billet rod , open porting perfect for diy racers .
 

flattop1

Dawg 89
Yea whats a crank shim gonna do . Much like powdercoat , no preformance advantage .
 

DynoDon

Moderator
You can always call AKRA and request it
 
The rules were given to me by Chops. He is an AKRA tech in Florida. He told me it is taking off right now. As for powder coating tins, I see no performance gain
I know Chop and he knows me as well. We talk quite often when im back in Fl.
I dont see where its taking off, at The Super 8 Series at Jasper this past weekend there was none, Volusia zero, FDCS Series none as well. Maybe the Valdosta area in Ga.
 

DynoDon

Moderator
How about the word “growing”?
 

Chipg56

Member
Since "box stock" stopped being box stock years ago, as little as I know about china clone rules, it seems like these rules are a better alternative for adult classes. If indeed they could run heads up against animals it could be a viable alternative that would lead to fewer classes and larger fields.
 
Since "box stock" stopped being box stock years ago, as little as I know about china clone rules, it seems like these rules are a better alternative for adult classes. If indeed they could run heads up against animals it could be a viable alternative that would lead to fewer classes and larger fields.
They outrun Animals.
 
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