From the NHRA Rulebook 2016:
"CYLINDER HEADS
Must be correct casting number for year and horsepower claimed,
per NHRA Technical Bulletins or NHRA accepted. Porting,
polishing, welding, epoxying and acid-porting prohibited.
Combustion-chamber modifications prohibited. Cylinder heads are
additionally restricted in that they must retain original-size valves at
original angles +/- 1 degree and must be able to hold original
cylinder-head volume per NHRA Specifications. Runner volumes
may not exceed the current Super Stock cylinder-head volumes as
listed on
www.NHRARacer.com. Regardless of the poured volume
measurement, any modifications to intake or exhaust runners
prohibited. Any evidence of modifications from the original castings
will be grounds for disqualifications as determined by NHRA in
NHRA’s sole and absolute discretion. Any aftermarket steel valve
permitted, must retain stock head and stem diameters. Only
engines OEM-equipped with sodium-filled valves may use sodiumfilled
replacement valves. Titanium prohibited. Hardened keepers
permitted. Lash caps prohibited. Valve-diameter tolerance: +.005-
inch or -.015-inch from NHRA Specs. The following are prohibited:
spark-plug adapters; any grinding in ports or combustion
chambers; removal of any flashings; sandblasting or any other
modification to cylinder head; any film coating of intake and
exhaust runners; any film coating of combustion chamber. Runners
and combustion chamber must retain OEM appearance. Final
acceptance as determined by NHRA in NHRA’s sole and absolute
discretion. External modifications prohibited. Intake side of head
may not be cut into any part of valve cover bolt holes. Valve-cover
bolt holes must remain unaltered and in their original location.
Heat riser passage may be blocked from intake manifold side of
cylinder head. Blocking passage down in valve pocket prohibited.
The following are permitted: polylocks, jam nuts, screw-in largerdiameter
rocker studs or pinned studs, bronze-wall valve guides,
cylinder head studs. Valve spring umbrellas optional. Cylinder
head may have all of the seats replaced. Any valve job
permitted,O-ringing prohibited. Exhaust plates prohibited."
I took the +/-1° to mean the valve angle relative to the deck. Sort of limiting how much you can angle mill the head.
