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I’ve noticed working on both Olds and Buick’s over the past several years there is a problem with replacement carriage head bolts offered in the aftermarket or hardware store. They are used on front and rear bumper brackets to frame for at least 70-72. I know some of the assembly plants used ordinary bolts instead. I like to use the carriage as that is what was used on the Michigan built cars I always work on.
The big problem is replacements from aftermarket sources have a much bigger square section under the head. This was the case with the last ones I ordered a few years ago and I’m not sure it’s changed? The bolt is meant for the square section to sit in the frame only to hold in place. The square section should not extend into the main bumper bracket at all. If the bigger square ones are used they lock the frame and bracket together and it does not allow up and down adjustability of the bumpers. It can be real frustrating if you don’t catch that. I suspect that’s why you see so many badly aligned bumpers at shows. I have resorted to getting out my grinder and cutting down the bigger square sections especially on the corners. This works fine but I prefer to use originals after having them phosphate plated. Originals can be tough to come by as you often have to cut them off. Anyone know if a source for correct ones? My guess is they have the big squares?
black is original. Silver are regular carriage bolts. I have found three different head types on originals. Three hash marks which identifies a grade 5 bolt. Also three hash marks and RWB manufacturer. And others with no marks at all.
"Carriage bolts" are standardized manufactured items. The black bolts you show are different, yes. They are specially made for auto manufacturers and for a special purpose. There is "someone" who has "specialty bolts" and fasteners for the auto industry. I am sure someone will chime in with a name.
You definitely want Grade 5 or better for bumper bolts.
None of the AMK are correct. The stainless cap ones looks like they have small square but they are not correct for bracket bolts. The correct size is 7/16” by under head length 1 1/8”
Would try someone such as “azchevs” who parts a lot of southwestern cars. For originals. Am sure I have a few but most of the cars I took parts off in the early 00’s in the new mexico yards were Chevrolet.
Last edited by newmexguy; Feb 28, 2021 at 07:31 PM.
Would try someone such as “azchevs” who parts a lot of southwestern cars. For originals. Am sure I have a few but most of the cars I took parts off in the early 00’s in the new mexico yards were Chevrolet.
This is in lines offering. Note the big squares. These will not work properly unless you get the grinder out and knock down about half the squares. Clearly they never tried these. https://www.inlinetube.com/products/...waAroYEALw_wcB
