Let's assume we're all in agreement that seatbelts are a good thing. Not to put too fine a point on it, but a small, light car designed half a century ago is just not going to afford its occupants the kind of crash protection found in a modern car. Knowing that, there are still some things we can do to these little beasties to tip the odds in our favor. Short of installing a roll cage, one of the most effective self-preservation tools we can bring along for the ride is a set of seat belts. Better than lap belts only, are three-point belts, and (if you remember the late sixties and those stationary shoulder belts) it's most convenient if they're retractable.
There may be some compelling arguments for not installing belts in a 356, but the only one I can think of is authenticity, or keeping a "look." Consider then, that Porsche did offer seat belts as an option, or a retrofit beginning in about 1957 [see photo at right showing the factory seat belt option].
Installation of these requires drilling holes, according to the Porsche shop manual, "behind the front seats between the floor and the vertical sheathing." Beginning around the first of the T-6 B models, the mounts were provided: four threaded holes tapped for a 7/16 SAE fine thread eyebolt. With the introduction of the C series, another mount was installed for a shoulder harness, near the lower rear part of the rear side (inner fender) panel. Owners of these late models can easily install modern or period-looking belts using these attachment points and still consider the car reasonably "stock."
Of course, period belts would be stationary [not retractable], and according to Al Zim, who has done quite a few installations, that's the safest type. Al makes the point that if you're cornering hard or roll over, a retractable belt system may not keep you in the seat. It only takes a few seconds more to put on and properly adjust he says, adding, "If you're in a rush to go somewhere you're probably not going in the 356." [Diest Safety sells period-looking non-retractable lap and 3-point belts]
Moving along from "original look" to convenience, we will consider retractable (inertia reel) three-point belts. Since these weren't offered when our cars were new, it's a matter of retrofitting. Precision Engineered Products (PEP) has been selling kits for years, specially designed for 356s. There are two kits that differ in the mounting point of the shoulder belt. One is for early cars which will mount the reel near the top of the rear side panel (requiring fabrication of a mount point). The other PEP unit mounts the shoulder inertia reel lower, in the factory position on C cars. We'll discuss these after covering lap belts.
The first order of business is choosing the belts you want. Stationary lap belts are available from many sources and are relatively easy to install. Grade 8 eyebolts with 7/16 inch fine thread will screw into the existing holes in T-6 cars (this is the only SAE fastener on a 356. Porsche acknowleged the American specification of the time rather than create problems in their largest market.) These eyebolts are specially made for seat belts and are available idividually or in kit form with a large fender washer and nut (see photo of Bob Barnhart's car above left). On earlier cars, holes can be drilled. A washer or nut/washer on the inside of the car, along with a larger washer and a locknut or pair of nuts should be used on the outside. While this area has just about the thickest metal on the car, Al Zim recommends buttressing the bolt area with a piece of 2 inch or wider angle iron, 3/16 inch thick, placed outside of the pan and drilled for the bolts through the angle. If used, the length of the piece should extend between the two holes for each side.
Lap belt angle
One other consideration should be given to lap belts: position of the belt on the lower torso. You will note that modern cars have the seatbelt anchors mounted near the back part of the seat. Attached to the seat or mounted, the anchor is adjustable to fit everyone from little old ladies to basketball players, maintaining a downward angle which prevents the belt from riding up in an impact. To be effective, the lap belt must restrain the body across the pelvic area. If you slide under the belt, there will be tremendous strain on the abdomen with serious consequences. The Porsche shop manual shows the angle of the belt using the standard mounting points (below), which is adequate (not optimum) for most average- to-larger people. The red arrow shows a safer angle, approximating a modern car's setup. If the driver of your car is small, short or usually has the seat far forward, you might consider mounting the lap belts on the floor, with adequate bracing. Remember, however, the floor will be somewhat fllexible without some serious structural help.
Installation of a three-point belt can be done with a stationary or retactable system. C cars have a factory mounting point (see diagram above under "Installation"). Wyatt Blankinship did some research and noted: "The existing screw is replaced with an 8 mm bolt. A Grade 8 bolt should be used because of its higher tensile and shear strength. Unfortunately, the diameter of an 8 mm bolt can only take a shear load of about 7,000 lbs. This may sound like a lot, but we are dealing with kinetic energy (energy of motion) when we are trying to restrain a body in motion. In a severe head-on collision where the car comes to an instant stop the bolt is likely to fail and shear if the car had been traveling in excess of 40 mph. The eye bolts that restrain the lap belts are much larger (7/16" diameter) and should be good for a load of about 22,500 lbs. each. They should hold under most circumstances."
Whether using the factory mount or making your own anchorpoint, be sure to use fasteners of adequate strength. Inertia reels come with grade 8 bolts, and for a stationary shoulder belt like Al Zim is wearing at left, a seatbelt eyebolt or a proper U-bolt can be used, but be sure the sheet metal of the inner fender is braced properly (see factory drawings at the bottom of this page). Modern cars have a pivot, usually on the B pillar, to keep the shoulder belt high (higher is better) but a 356 has no beefy metal there to allow this.
Bruce Baker notes that Cabriolets can be problematic: the mount for a shoulder belt may interfere with the top frame if mounted in the wrongplace (photo at right).
However you decide to install your seat belts, keep in mind the large forces these belts are designed to withstand, and make sure the anchors are strong enough to deal with them. You must also design and install the belts so they are convenient to use and function properly, especially the inertia reel locks, which by design are sensitive to position. And of course, drive safely!