What I'd like to know is who makes an extender for the SSII style buckle...I'm a big guy and my seatbelt fits with just a few inches of slack, the new cars, any make, seem to have miles of seatbelt. If I had a passenger larger than me there would be no way to buckle. Oh well I guess.
My rear seat belts became seized from lack of use and I was about to replace them but after fiddling with them for a while I got them unstuck and they now work perfectly again. Every now and then I give them a pull to keep them operational so they shouldn't seize again in the future. If that's what you are experiencing I suggest that manipulating the reels might restore them to normal.
Appologies for late response as promised but I have been tied up for the last few days with a friends SS11.
Probably best to start this with 'DON'T TRY THIS AT HOME'.
I have often restored damaged webbing on MoT test failures seatbelts by renewing webbing, extended the length and changed webbing colours for people on seatbelts. I do have an industrial sewing machine and use the correct twine/cotton following the exact original stitching pattern with an additional picture frame stitch around the edges.
However 'add on' commercially produced extensions can be bought and ideal for temporary use as/if required. I am not sure what or even if the SS11 US spec seatbelts were anything different but my SS1 has inertia ones and the stalks and buckles are fairly standard type.
Static belts are much simpler if needing to be extended obviously but generally I have found inertia belts can easily take an additional couple of turns within the internal space which actually can give up to an extra 12 inches in length. Also some of the webbing used on earlier cars can be much thicker than the modern webbing available today. The inner roll ends up approx 2 inch diameter when retracted so circumference increase with 2 additional turns works out around 12 inches.
All my stitching is then given a 1 ton pull test which I would consider adequate for a car seat belt test.
I would rather know an inertia seat belt has at least a couple of turns left inside at the centre bar as if fully extended sometimes it can lock out and stops the pull action to initiate the return action. As the centre point retention shaft for the webbing is only a small diameter a couple of turns here is less than a few inches of webbing length required.
Seat belts definitely are a must and can save lives and injury and rear seat belts too are essential especially if the front seats have no headrests.
One of the known major injuries in the past was rear passengers being thrown forward and hitting the heads of the front passengers causing severe head injury to both. So good idea to fit seatbelt in the rear if you regularly carry rear passengers.
Having been involved with producing cars for crash testing I do have some personal reservations though on seat belt performance. The crash dummies used are fairly standard and the amount of crash tests done fairly limited. That is to say you can't crash lots of cars with different size dummies simulating every seat adjustment position and every varying body mass of a passenger. Albeit today modern graphics and computer simulation does now happen.
The position of the seat belt around a person and the type of crash incident can totally vary how the body will be thrown about.
Sometimes a body might actually be forced downwards into the footwell hence many modern cars now also concentrate too on 'lower' knee crash rolls/bolsters under the dash/facia.
I was asked once to do a long extension for one person who's passenger had a thyroid problem so was quite large. To achieve this and prevent too much webbing being allowed into the retractor mechanism I added a stud into the webbing. These can be seen again as a fairly standard fitting on seat belts to hold the buckle up from dropping to the base at the lower fixing. This prevents having to grovel around to find the buckle and the buckle ending up being trapped and spoiling the door trim/seat. See pictures below.
Again it is hard to say how any of this would perform in a crash so I obtain a disclaimer on any alterations from the owner, and say it is up to them if they want to advise their insurance company. No one really knows how all the seat belt aftermarket 'gizmo's' from those that attach as an aftermarket fit actually effect safety or operation. However my simple view is anything is probably better than nothing, but where airbags are fitted to a car I would now decline any modification other than a like for like webbing replacement repair.
On my MGB the car came with factory structural seat belt fitting points, but back then they were simply an option and generally across Europe it is accepted as many older cars do not have seat belt points built into the structure the cars can be driven free from fines without wearing a seatbelt. I decided though to fit a set of Securon 'after market' inertia belts to the MGB.
These are quite complex as they come with universal vertical and horizontal adjusters so the retraction can be set accordingly depending on how/where the inertia mechanism has to be mounted. When we were in France the road down to the village was quite a steep hill and my wife had asked me to put a waste bag into one of the bins the French have on the roadside. I stopped halfway down the hill and offloaded the trash bag, then due to the slope could not get the seatbelt to release to put it back on.
Knowing flat ground was ahead I travelled a few hundred metres down the road straight into two Gendarmes at the corner.
When I tried to explain what had happened and turned to point to the rear seatbelt mechanisms as I turned back I had a pistol in my face as the 'nutter' had drawn his gun. No argument or explanation I just paid the 90 euros..........Since then I have found some original equipment ones and fitted these as they are a little less sensitive on the mechanism angle operation...
Another common problem I see so often on Shadows is damage to the door metal inner and the door card trim where the seat belt buckle has been trapped when the door closed as mentioned above. This can also damage the webbing as it gets trapped in the door striker plate . You can get a little plastic stud as I have fitted on my cars that can be fitted through the webbing which stops the buckle falling down and also prevents having to grovel to find it down between the seat and 'B' post base.