A Stock Allowed Modifications:

Driver harness
Roll bar or roll cage (SCCA specs)
Shock absorbers with the same mounting type and height
Trailer hitch
Wheels with the same diameter and width as stock
Front swaybar
Brake pads
Air filter (not intake)
Tires (anything D.O.T. legal including R compounds)
Cat-back exhaust

STU Allowed Modifications:

All modifications from the Stock rules are allowed, except for:
Tires only up to 245 width, and at least 140 tread wear rating (requirement for AWD vehicles).
Wheels of any width.
In addition, the following modifications are allowed:
Remove air condition.
Pedal covers.
Seat (fully upholstered seating surface, Seat + brackets & sliders must weight at least 25 lbs).
Removal of factory emblems (debadging).
Rolling of inside fender lip.
Addition of body kits.
Replace rear wing and bumper covers (wings up to 8 square feet).
Cross drilled and/or slotted brake rotors rotors (diameter equal or larger than factory, ferrous or aluminum alloy).
Brake calipers (original attachment point).
Brake dust shield modifications to accommodate alternate rotors and calipers.
Brake lines, e. g. stainless steel.
Air ducts for brakes.
Disabled ABS (not removed).
Shocks (fewer restrictions than stock). No electronic adjustment, like Tein EDFC!
Springs (can be height adjustable, e. g. coilovers).
Any suspension bushings, non-metallic.
Any differential mount bushings, non-metallic.
Any subframe mount bushings, non-metallic.
Adjustable camber plates (caster adjustment allowed).
Camber bolts.
Adjustable front control arms (only if no front camber plates or camber bolts are used). Must have non-metallic bushings, spherical bearings are not allowed.
Adjustable rear lateral links (only if no rear camber plates or camber bolts are used). Must have non-metallic bushings, spherical bearings are not allowed.
Rear sway bar, mounts, end links
Strut bars (not triangulated).
Battery, relocation of battery (to trunk).
Air Intake.
Headers.
Reprogrammed ECU or piggyback that controls ignition and fuel by modifying inputs to ECU.
Short-throw shifter.
Crankshaft and accessory pulleys.
Engine mounts (non-metallic).
Uppipe (can be catless).
LSD may be added.
Turbo-back exhaust. Must contain at least one (high-flow) cat placed not more than 6" after the original exit of the last factory cat.
ECU reprogramming. Map must have unaltered factory boost maps.
Piggyback, see STS. No changes to boost control are permitted.

B Street Prepared Allowed Modifications:

The allowed modifications in SP are extensive, the following is a general summary.
Anything allowed in Stock
Update/Backdate rules for any cars on the same line in the rulebook
Any wheel and tire width
Shocks, springs, swaybars, strut bars/braces
Camber plates
Any fully padded seats, steering wheels, pedals, roll cages up to 8 points of attachment
Fuel cells allowed with certain qualifications
Suspension bushings
Limited Slip Differentials
Carburetion, fuel injection, intake manifolds, intake, exhaust manifolds, exhaust
Motor mounts
Clutch/flywheel
Overbore to .0472" with oversize pistons
Balancing
Port matching, milling
Pulleys
Ignition system

Street Modified Allowed Modifications:

Anything in Stock, Street Touring, or Street Prepared
Drivetrain is unlimited except: engine from same company as manufacturer
Max displacements:
FI: 3L OHC, 4L pushrod
Suspension is unlimited except: Must use original suspension attachment points
Brakes unlimited
Rear passenger seat can be removed
Hood/front fenders can be changed
Firewall, roof, doors, rear quarter panels, floor pans, trunk lid must be stock
Minimum weights:
AWD: 2600 lbs FI

Beyond Street Modified:

Consult the rule book for Prepared and Modified categories. The following list is just a reminder of a few popular mods that are not allowed in Stock, Street Touring, Street Prepared or Street Modified, they will put you into Prepared/Modified classes where you will be competing with race cars:
Replaced body parts not allowed in SM
Weight reduction not explicitly allowed in SM: e. g. gutted interior, removed heater, wipers, lights.
Tires that are not DOT approved (racing slicks).
Braces other than strut tower braces: triangulated braces (V-braces), lower arm braces.

Not Allowed In Any Class:

The following are not allowed in any class:
Nitrous (bottles not allowed on event sites for safety reasons).

Uncertain Mods

No clear rules are currently available for the following popular mods:
Ground wire mod.

Common Questions
Why is my XYZ mod illegal? I ran with it all year, and tech never said anything!
Tech inspection is only responsible for checking if your car is safe. Properly classing your car is your own responsibility. Unless somebody files a protest, nobody will check if you have illegal mods for your class. Even if nobody ever protests, many people think that running with illegal mods is unfair and shows a lack of respect for your fellow competitors.

Ok, so once you’ve done your research, and still don’t know the answer to a question let’s ask it here. This way, we can keep a running document so that others in the future can reference this. I may have left things out, and there probably are errors, so please let me know if you see anything and I will change it.

Again, if a modification is not explicitly authorized by the rules, IT IS NOT ALLOWED, no matter how trivial it seems. Now there are “creative” interpretations to some rules, but you should try and follow the “spirit” of the rule.

Additional Resources

Official rule book (available for download)
FasTrack (official SCCA publication containing latest rule changes)
Rules on Moutons.org (partly out-dated)
Solo II information on SCCA web site
SCCA Forums (message boards)