In the SCCA classification system there is a hierarchy based on car preparedness level.

1. Stock classes allow minimal changes from the way the car left the factory (not the showroom).
   
2. Street Prepared classes allow a lot more modifications while retaining interior trim.
   
3. Prepared and Modified classes allow full race car preparation.

In recent years, two new options have become popular.

1. Street Modified (SM) and Street Modified 2 (SM2) allows lots of "street tuning" modifications like engine swaps, forced induction, etc. 2004 Street Modified Rules
   
2. Street Touring (STS) and Street Touring 2 (STS2). This class allows many of the popular Street Prepared mods, but requires "street" tires. 2004 STS Rules

If you bought your car new and haven't changed any parts, you can quit reading now as you can run in stock class. If you've made some modifications, you'll need to check the chart for legality. This chart is not complete, but rather intended to address the most common mods. Remember that SCCA rules state that "if the rules don't address it, it's not allowed." If you can't find your modification here, check an SCCA Rulebook (pdf). If the modification is not listed in the rule book, it's not legal.

Permitted Items	Stock	SP	SM SM2	STS STS2
Car issues
Any car as it was delivered from the factory or as it "could have been" delivered from the factory. This means any factory option or complete package is acceptable. For example, if you could only get a sport suspension with cloth seats and a six speed, you can't replace your leather seats with cloth seats unless you remove the sport suspension.	X	X	X	X
Remove air conditioning (although this may be allowable under the rule directly above for Stock)		X	X	X
Use parts/assemblies from other year model cars (see your SCCA rulebook for more details)		X	X
Body/Interior	Stock	SP	SM SM2	STS STS2
Harness/Replacement Belts - Stock class requires retention of the original belts. To use upper body restraints, a roll bar must be installed.	X	X	X	X
Different stereo, additional gauges	X	X	X	X
Roll bar	X	X	X	X
Trailer hitch, additional tow hooks (but not factory tie-down removal)	X	X	X	X
Modify fenders/bumpers for tire clearance		X	X
Strut bars (may connect only to struts)		X	X	X
Spoilers/Trim pieces (Must be cosmetic)		X	X	X
Replace pedal covers		X	X	X
Replace seats with any fully padded, upholstered seat with reclining back		X	X	X
Replace seats with any fully padded, upholstered seat		X	X
Subframe reinforcement			X
Replace hood, fenders, deck lid			X
Replace steering wheel (if no airbag) - must be within 1 inch of original size for Stock	X	X	X	X
Replace steering wheel		X
Wheels	Stock	SP	SM SM2	STS STS2
OEM Size, Offset within .25 inch	X	X	X	X
Any size/offset (STS STS2 is restricted to a maximum 7.5 inch width)		X	X	X
Tires	Stock	SP	SM SM2	STS STS2
Any size/width (Must fit on stock wheels for Stock - Restricted to 225mm section width for STS<)	X	X	X
Non-zero tread depth	X	X	X	X
140 or higher treadwear rating REQUIRED				X
Shock Absorbers	Stock	SP	SM SM2	STS<
Adjustable perches (must be welded to stock height for Stock), bump stops shortened, replaced or removed (they may not be longer than stock), adjustable shocks (only two external adjustments for Stock), external reservoirs (and required holes for routing lines)	X	X	X	X
Coil-overs		X	X	X
Suspension Issues	Stock	SP	SM SM2	STS STS2
Any alignment using stock parts	X	X	X	X
Alternate front sway bar using stock mounting points	X	X	X	X
Any sway bars		X	X	X
Any springs		X	X	X
Any non-metallic bushing		X	X	X
Camber kits		X	X	X
Brakes	Stock	SP	SM SM2	STS STS2
Any pads	X	X	X	X
Alternate brake lines (steel allowed only on pre-'92 for Stock, STS STS2 must be DOT approved)	X	X	X	X
Drilled/slotted rotors			X	X
Cooling Ducts		X	X	X
Electrical	Stock	SP	SM SM2	STS<
Any spark plugs, wires, coil	X	X	X	X
Change timing (outside manufacturer spec)		X	X
Change battery size/location		X	X	X
Engine	Stock	SP	SM SM2	STS STS2
Change fuel injection, intake/exhaust port matching		X	X
Any air filter element	X	X	X	X
Any cat-back exhaust	X	X	X	X
Any oil filter	X	X	X	X
Any thermostat	X	X	X	X
Silicone hoses	X	X	X	X
Any oil or grease	X	X	X	X
Change oil pan, sump, oil cooler, heat shields, intake manifold, intercooler, remove emissions controls, change fuel lines, cooling fan,		X	X
Exhaust headers (must be emissions certified for STS<)		X	X	X
Modified ECU (or devices that modify sensor outputs to trick the ECU)		X	X	X
Water Injection		X	X
Drivetrain	Stock	SP	SM SM2	STS STS2
Any metal clutch		X	X
Any mechanical shift linkage		X	X
Any limited slip differential		X	X
Short throw shift kits		X	X	X

Written by Lauren Dunn, original work at dfwmiata.com