The SL-C can accept a wide range of seat belts. Most builders choose to use competition belts for several reasons, but it is possible to fit standard 3-point belts if local requirements or personal preferences dictate them. Belts can be acquired from inexpensive sources like RJS or Crow. Other suppliers include Simpson, Impact Racing (oddly apt, isn't it?), and others.
Like so many other parts on the SL-C, parts are not all the same, and there are suppliers willing to make custom setups to meet more specific or stringent requirements. Willans, for example, makes a wide range of belts and supplies F1 teams, as does Schroth.
Whatever belts and seats you choose, follow the manufacturers instructions, especially as they relate to mounting and installation angles. Sometimes belts that are installed or used incorrectly can be more dangerous than no belts at all.
In any case, please do not use any 4-point system, even if the belts are sold that way. This is largely because the recline angle of the seats- all SL-Cs need to have a sub belt of some kind, either a 5 or 6-point harness (if you are not using conventional 3-points). Any 4-point belt installed in the SL-C will likely cause injury in an accident as the belts cannot effectively stop submarining due to the lack of a sub belt, and the lap portion of the belt will ride up and cause injury to internal organs.
While Schroth offers a DOT-compliant 4-point that is very tempting, it is not intended for seats with a recline angle as extreme as our cars, and notably, in any case, the SL-C is not on the official Schroth Vehicle Reference List of approved vehicles in which the 4-point system may be installed, and therefore is not DOT-legal when installed in an SL-C (or any car not on the approved list). Note that 4-point belts are specifically prohibited in most sanctioning bodies for track events or racing, because of the risk of bodily damage from submarining.
In summary, please use either a conventional 3-point system, or competition 5, 6, or 7-point belts in your SL-C.
Five-point vs. six point discussion here.
Belts need to be mounted correctly if they are to work effectively. For shoulder belts in the SL-C, the mounting is easy: just procure wrap-around shoulder belts and wrap them around the part of the roll cage that parallels the fuel tank. For most drivers, and most seat mounting locations, this is a perfect location. However, if you have seats with the shoulder belt openings that are much lower than the norm, as sometimes happens with shorter drivers, you may need to use another mounting method in order for your belts to have the correct angle.
For lap belts, the problem is somewhat more complex. Many builders just use an eyebolt-style mount that bolts to the floor, using the standard 2" washers. This is probably acceptable, but a weldment that ties to the chassis or the floor is better still. The SL-C makes it easy to attach the inboard end of lap belts, as the center aluminum spine can be drilled for a long 7/16" bolt to which the inboard belts can be attached. If you have a welder capable of AC, It's easy to weld in a 1/2" aluminum spacer for more strength there. In any case, when mounting belts, follow the manufacturers instructions.
Of particular importance is the angle of shoulder belts (normally, they should be horizontal with no more than 5 degrees off 90 degrees), and the lap belt. It is critical to follow the manufacturers instructions for these belts, as the optimum performance of the safety gear is obtained only when the belts are installed correctly. Improperly installed belts can be worse than no belts at all, so please do it right the first time here!
There are many examples of incorrectly installed belts on the web, from shoulder belts that are attached to the floor directly behind the seat (sure to cause a compression fracture of the spine in an impact) to more subtle errors like improper lap belt angle. Please read and completely understand the manufacturers instructions for any belt system you are installing.
As mentioned above, no SL-C should use 4-point belts, as the risk of submarining is too great, with the consequent risk of severe injury or death. Thus, crotch or sub belts are needed, either a single mount (part of a 5-point system) or a dual mount as for a 6-point system. There are probably many solutions to the crotch belt- including the obvious one of just bolting it to the floor- , but one other solution is to weld a short piece of 3" diameter aluminum rod to the floor, and drill and tap the center for the belt bolt. Another alternative is to weld or bolt a strong tube across the seat brackets and attach the sub belt to that. Be sure to consult a qualified engineer if you choose not to follow the seat belt manufacturers directions.
Pull Up or Pull Down?
No, we aren't talking about diapers here, but how you adjust the belts. This is more important than you might think, as belts need to be very tight for racing, and if, for example, you can't pull them tight enough because you can't get your hands where they need to be, they won't work correctly. Work with your vendor to help figure out what works for you.
One car with 6-point belts has pull-ups for the lap belts, and pull downs for the shoulder belts. Choose what works for you, but be sure- you generally cannot return belts because you chose the wrong kind of adjuster.
In general, pull-ups for the lap belt, and pull-downs for the shoulder belts are what works best in the SL-C for most drivers and situations.
Latch or Rotary?
Competition belts typically use one of two methods to latch the belts: the cam-lock type which uses a rotary motion to unlock the belts, and a swan neck type of latch that requires the user to move a lever to latch or unlatch.
The camlock style is more expensive, but is the norm in all forms of motorsports, except for offroad driving where the swan neck style is preferred for its resistance to jamming because of dirt in the mechanism.
Choose the one you prefer; for most SL-Cs, the camlock design will be the latch of choice. If you were building a D Type or other 50s or 60s style car, the swan neck latch type would be preferable, as they are period correct. Modern cars all use the camlock style.
The original stock seats are derived from a Porsche 962 seat, and are delivered un-cut in terms of seat belt mounting slots, as that is usually something that is different in every car. The latest seats also need to have the lap belt and sub belt openings made by the builder. When you cut slots, line the opening with a gasket made of split fuel line or other molding to avoid abrading the belts on the sharp fiberglass edges of the slot. Abraded belt webbing will normally result in an ejection from a competitive event, so take this seriously.
Lap belts in the SL-C using the standard seat will require slots to be cut in the seats for the belts to pass through in almost all cases. If you run the belts over the seat sides, the stock seats will not permit the lap belt to center correctly on the hips and safety will be severely compromised.
Likewise, the crotch or sub belt must always come through a slot in the seat bottom and can never be just looped over the seat bottom if it is expected to work correctly.
Use of the stock seats requires the builder to create the appropriate slots for the lap belts and the crotch belts if competition harnesses are used.
The new seats available on the SL-C have a built-in port for the lap and shoulder belts, and only need to be modified for the sub belt (if one is used, as in a race harness).
All racing seats typically come with engineered slots already cut (and upholstered, as needed) for competition belts, so additional slots are generally not needed. Occasionally, as in the case of the Tillet B5, the seat sides are already low enough to permit lap belts to be used without slots (the B5s come with shoulder harness openings, as well as sub belt ports).
Racing seats also have a wider range of fitting options as a rule, and most of them have provision for seatback bracing as that is increasingly required by race sanctioning bodies like NASA and SCCA. Check with your rulebook before you buy a race seat to be sure it can be mounted and used in your class or sanctioning body. Not all racing seats are legal in all cases, and in some or most cases, fabrication (as of mounts and seat back braces, for example) may need to be done to meet local rules.
Interaction with other parts of the car
If you are racing, you need to select belts that meet your sanctioning bodies requirements. One of those requirements is that the belts not be aged out. All competition belts have a manufactured date and or an expiration date. Your sanctioning body may require belts to be no older than a certain number of years. That means it is best to order belts just before you are ready to race the car. Otherwise, the belts will be sitting on a shelf in the garage, aging out before you even mount them in the car.
In addition to requiring that belts be mounted according to the manufacturers instructions, some sanctioning bodies also impose other requirements, like disallowing modifications of street seats, or requiring both seats in a car to have the same kind of restraint system. The point is to read and thoroughly understand the rules where you will be competing, before you show up at the track.
You also have to plan ahead in terms of kind of head and neck restraint you will use. Why is that? Because many head and neck systems now assume the use of 2" shoulder belts, or hybrid belts that are a combination of 2 and 3-inch wide belts. Be sure to order your belts with the exact system you plan to use when on track. If possible, order all parts of the total restraint system at the same time, with consultation from an expert at the store where they are bought, to minimize the risk of incompatible parts. This includes making sure your helmet is able to be fitted with the hardware needed for your selected head and neck restraint system.
Belts are critical for survival in an accident. If you are unsure about how you are mounting them, check with the manufacturer, or a competent race shop for help.
Use 3 point or 5,6,7-point belts only, and mount them properly, according to the manufacturer and your sanctioning bodies rules.
Understand that belts are one part of a total body restraint system and make certain that every part of the system (seat, belts, mounts, attachment to other parts of the restraint system like helmets and HANS devices or the like) all work together. Don't assume that everything works together nicely without being sure.