Hidden front bumper bolts on an S4

[1owner69Elan]

I am replacing my original factory Federal bumper on my S4 for two reasons:

1. Wanted to get rid of the integral side flatted reflectors existent only on Federal cars. These are not the lighted Sprint versions but simply screwed on reflectors.
2. My bumper came with a wedge carved out in the front to accommodate a US license plate. I had this filled immediately as it destroyed the smooth lines of the car. The fill "repair" has not aged well and is distorted.

I purchased a new bumper from TTR. It came with screw-in bobbins on both ends. The original bumper had holes for a through bolt with the head on the outside and a nut inside. With the new bumper presumably one only needs to screw a bolt into the bobbin from the inside. Then, fill and smooth over the bobbin on the exterior resulting in a "bolt-less" look. Maybe this is how the 26r (shorter bumpers) are set up as I don't recall seeing external bolts on those?

Also, the new (and old) bumper do not have the central bolt / bobbin indicated as being present in S4's and later. My original body has no hole either for a central bolt. I suspect the Federal S4 bumper, being different already and "cobbled together" to satisfy new Federal regulations, may not have had this central bolt. Wondering whether I should put a central bobbin in the new bumper. Anyone have an opinion whether this is needed? Note: I am fitting the new bumper with no plastic chrome strip in the gap, so the gap needs to be tight. The body shop thinks the bumper may be secured OK with just the two end bolts and a body form fitting foam filling.

[gjz30075]

My Federal Sprint ('72) has the central bolt and I'm thinking of adding it to my S3. It really adds
stability.

[nmauduit]

I like also to avoid the plastic filler if I can : my Federal S4 has the central bolt, and when I fitted a new bumper on the S4 project I'm currently working on it turned out quite helpful to get the alignment right. I had 2 new bumpers to chose from and some trimming margin, and still it is difficult to get a good fit on such a long piece, not a lot one can do to cheat for getting good gaps there, and without the central hold the bumper may have a way to rotate around the axis corresponding to the 2 bolts at the ends.

But if the bodyshop think they can do without, then add lightness indeed... and please post photos of the result.

[1owner69Elan]

So, TTR confirmed that end bolts on their standard road bumper are screwed into the integrated bobbins from the inside. The bobbins are covered over on the exterior. Thus, no bolts show. I think this must be how the 26r bumpers are also fixed.
The bobbin thread size is smaller than the original chrome bolt and nut. Needs a smaller bolt, not provided.

[1owner69Elan]

Following up on this earlier post on fitting a new bumper. Nmaudit had asked for photos on the result.

Also, this is related to a recent post by another member on bumper fit.

In the end, a new bumper was fitted with no central bolt. The bumper foam was formed to fill the central cavity evident on my S4 body (perhaps where the central bolt is supposed to be present). This cavity shown in the picture below:

With the two side bolts and the form fitting foam the new bumper is very stable and solid. No movement whatsoever.

Also, the bumper was fitted with no filler strip and minimal gap: