Mark V Front Bumper Question

Taking apart my front bumper, I found an anomaly or peculiarity.

I refer to the RS.31-38 round headed bolts with the big flat washers holding the O.1679 mainspring or upper cross beam to the O.1680/1681 LH/RH chassis attachment inner brackets.

These round head bolts are 5/16" diameter x 1" long and have a round section with two flat sides under the head, measuring 5/16" across the flats. They are intended to fit reasonably snugly in oval slot holes, and in other places on the bumper they do fit.

Not these four, these bolts do not fit snug, they turned in the slot when I put a wrench on the nuts.

I had quite a time getting them out. Vise-grips couldn’t grip the round heads. I used my air grinder and 1/16" cutting wheel to grind away part of the lock washers, then broke the lock washers apart with a hammer and punch. Then I could get the vise-grips on the round heads.

I find the slots in the upper mainspring measure 3/8" wide, and the flat washers (not in the SPC) were apparently to take up the space so the bolt heads would not pull through the slots.

This looks to me like a manufacturing error, somebody punched 3/8" slots in this upper mainspring where they should have punched 5/16" slots.
Or they should have used 3/8" diameter bolts.

Is this a commonly found British bolt, round head with the two flats?
In America a similar bolt has four flats under the head and we call them carriage bolts.

There are 7 of these on the bumper, and 7 of the RS.31-35 bolt which is the same round head 5/16" diameter with flats but 7/8" long.

Hi Rob, some of the bumper bits have 0.320" slots and some have 0.375". I just pulled one bolts on a short offset bracket piece 3.5" long and found the slot to be 0.320" while holding the bolt with 0.3125" across the flat under the round head and one inch long.

I’ve not sorted the specs for RS.31-38 or RS.31-35.

Yes, that’s exactly what I found, the 3 short bars supporting the license plate panel fit the bolts correctly, and so do the slots in the upper chrome bars.

I couldn’t find any reference to a British standard carriage bolt like these.

So I decided to use American 3/8" carriage bolts. All I could find was 3/8-16 UNC thread. I had to file out the holes a tiny bit to get the bolts to fit, but they worked fine, fit nice and tight.

This seems like what they should have had from the Wilmot Breeden factory.
If I get ambitious over the winter I’ll make some with 3/8 BSF thread.