Flathead Rules

Wondering how one can obtain the 2006 Flathead Limited Modified rules?

I've seen posted on this board the basic rules, but I'd like to have the in-depth rules, such a head depth measurements, valve seat heights...etc - the'real' full rules
 
Go to ikf web site . I remeber seeing a link to old rules .
Seat height should be as stock .
Old tech manual .
 
Go to ikf web site . I remeber seeing a link to old rules .
Seat height should be as stock .
Old tech manual .

Thanks flattop1,

There is a tech tool that sits in the cam bosses then a rod runs down through the valve guides with a washer type device that sits on top that they measure the seat height with. They have measurements for that, depth in the spark plug area of the head...etc., decking rules...
 
rules ltd 001 (2).jpg

everything else is following stock rules.
Seat height dimension are on nka rules .
 
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Burris Rules


SECTION 14 - Briggs & Stratton Flat Head Limited Modified

14.1 Block; Must be stock Briggs & Stratton 5hp FH (or approved aftermarket) item. Maximum
bore allowed is 2.643” (+.080” over stock.) Must be as produced with no alterations or
reworking except minor grinding for rod clearance permitted. Block may be welded or
repaired as long as no performance enhancement is involved. No bushings of any kind allowed
except bushings approved in this tech manual. DU bearing may be installed location on the
flywheel side. Sleeving of the block with ferrous sleeves is permitted. Repair of coil posts
permitted. Valve guides that meet stock physical dimensions may be replaced, guide material
optional. Must have flywheel shroud but head and cylinder shroud non-tech.

14.1.1 Ports – Porting allowed. Intake Port .880” No-Go, exhaust port 1.005” No-Go. Finish is
non-tech, no addition of material allowed. The block cannot be machined on the intake or
exhaust port gasket surfaces. No grinding is allowed on the underside of the intake
valve seat.

14.1.2 Valves – Stock valves only. Must be one angle and polishing and/or lightening prohibited.
Intake valve maximum diameter is 1.115” and seat angle is 30°. Exhaust valve maximum
diameter is .990” and seat angle to be 45°. Margin on both valves to be .035” minimum

14.1.3 Valve Springs – Valve springs and retainers non-tech.

14.1.4 Valve Seats – Must meet stock specs. One angle only, intake angle is 30° and exhaust
seat angle is 45°. Intake seat I.D. is 1.004” maximum no-go with a minimum/maximum
thickness of .199”/.215”. Exhaust seat I.D. is .880” maximum with a minimum/maximum
thickness of .199”/.215”.

14.2 Side Cover; Must be stock Briggs & Stratton 5hp FH (or approved aftermarket) item. Pin
punching gasket surface and additional breathers allowed.

14.3 Cylinder Head; Stock Briggs & Stratton 5hp cylinder head (or approved aftermarket) only.
Head cannot be machined below flat cylinder portion of the head. Machining of two pockets
for valve clearance or a single continuous relief are from side to side across valve area is
permitted. In no instance may the relief area touch either the back or sidewalls of the head
and should be done in a manner as to not affect airflow. No O rings allowed.

14.4 Crank Shaft; Must be stock Briggs & Stratton 5hp FH (or approved aftermarket) item.
Stroke is 2.437 +/- .010”. Crank to be stock appearing in shape and finish (except minor
grinding for cam clearance.) Heat treating allowed. No polishing allowed. Minimum journal
diameter is .990”

14.5 Camshaft; Cam and lifters non tech. No roller cams allowed.

14.6 Carburetor: Any commercially available butterfly type Tillotson HL series allowed.
Minimum/Maximum venturi = .850”/.900”. Minimum/Maximum throttle bore = .990”/.1.010”.
Metering holes are non tech. Filter cups, adapters and air filters non-tech items. Pulse type
fuel pumps only permitted.

14.6.1 Fuel-Methanol only, no other additives.

14.7 Intake Manifold; aluminum only and must be one piece.

14.8 Ignition System; Stock unaltered Briggs & Stratton coils are required. Spark plug wire and
connector non-tech. Resistance from plug wire to be from 2,000 to 5,000 ohms.

14.9 Flywheel; Approved flywheels only. 4lb 6oz minimum weight. Must be ran as supplied, no
alterations allowed. The following flywheels are approved. ARC #6608, Clements #BAFWL &
#FWSLTD, J.R. Racecar #555-3284 and United Machine and Fabrication #1001.

14.10 Piston; Piston, wrist pin and rings non-tech. Flat top pistons only. Deck height non-tech.

14.11 Connecting Rod; Non-tech, aluminum only. Bolts non-tech.

14.12 Starter; Non-tech, auxiliary or pull starters allowed.

14.13 Miscellaneous Non-Tech Items; Gaskets, fasteners and breathers.
 
Burris Rules


SECTION 14 - Briggs & Stratton Flat Head Limited Modified

14.1 Block; Must be stock Briggs & Stratton 5hp FH (or approved aftermarket) item. Maximum
bore allowed is 2.643” (+.080” over stock.) Must be as produced with no alterations or
reworking except minor grinding for rod clearance permitted. Block may be welded or
repaired as long as no performance enhancement is involved. No bushings of any kind allowed
except bushings approved in this tech manual. DU bearing may be installed location on the
flywheel side. Sleeving of the block with ferrous sleeves is permitted. Repair of coil posts
permitted. Valve guides that meet stock physical dimensions may be replaced, guide material
optional. Must have flywheel shroud but head and cylinder shroud non-tech.

14.1.1 Ports – Porting allowed. Intake Port .880” No-Go, exhaust port 1.005” No-Go. Finish is
non-tech, no addition of material allowed. The block cannot be machined on the intake or
exhaust port gasket surfaces. No grinding is allowed on the underside of the intake
valve seat.

14.1.2 Valves – Stock valves only. Must be one angle and polishing and/or lightening prohibited.
Intake valve maximum diameter is 1.115” and seat angle is 30°. Exhaust valve maximum
diameter is .990” and seat angle to be 45°. Margin on both valves to be .035” minimum

14.1.3 Valve Springs – Valve springs and retainers non-tech.

14.1.4 Valve Seats – Must meet stock specs. One angle only, intake angle is 30° and exhaust
seat angle is 45°. Intake seat I.D. is 1.004” maximum no-go with a minimum/maximum
thickness of .199”/.215”. Exhaust seat I.D. is .880” maximum with a minimum/maximum
thickness of .199”/.215”.

14.2 Side Cover; Must be stock Briggs & Stratton 5hp FH (or approved aftermarket) item. Pin
punching gasket surface and additional breathers allowed.

14.3 Cylinder Head; Stock Briggs & Stratton 5hp cylinder head (or approved aftermarket) only.
Head cannot be machined below flat cylinder portion of the head. Machining of two pockets
for valve clearance or a single continuous relief are from side to side across valve area is
permitted. In no instance may the relief area touch either the back or sidewalls of the head
and should be done in a manner as to not affect airflow. No O rings allowed.

14.4 Crank Shaft; Must be stock Briggs & Stratton 5hp FH (or approved aftermarket) item.
Stroke is 2.437 +/- .010”. Crank to be stock appearing in shape and finish (except minor
grinding for cam clearance.) Heat treating allowed. No polishing allowed. Minimum journal
diameter is .990”

14.5 Camshaft; Cam and lifters non tech. No roller cams allowed.

14.6 Carburetor: Any commercially available butterfly type Tillotson HL series allowed.
Minimum/Maximum venturi = .850”/.900”. Minimum/Maximum throttle bore = .990”/.1.010”.
Metering holes are non tech. Filter cups, adapters and air filters non-tech items. Pulse type
fuel pumps only permitted.

14.6.1 Fuel-Methanol only, no other additives.

14.7 Intake Manifold; aluminum only and must be one piece.

14.8 Ignition System; Stock unaltered Briggs & Stratton coils are required. Spark plug wire and
connector non-tech. Resistance from plug wire to be from 2,000 to 5,000 ohms.

14.9 Flywheel; Approved flywheels only. 4lb 6oz minimum weight. Must be ran as supplied, no
alterations allowed. The following flywheels are approved. ARC #6608, Clements #BAFWL &
#FWSLTD, J.R. Racecar #555-3284 and United Machine and Fabrication #1001.

14.10 Piston; Piston, wrist pin and rings non-tech. Flat top pistons only. Deck height non-tech.

14.11 Connecting Rod; Non-tech, aluminum only. Bolts non-tech.

14.12 Starter; Non-tech, auxiliary or pull starters allowed.

14.13 Miscellaneous Non-Tech Items; Gaskets, fasteners and breathers.


THANK YOU!
 
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