So, let's say I'm upgrading the seats in my FR-S, and the stock inboard seat belt attaches to the stock seat, on a stud/nut type of arrangement.
No biggie, both the mounting bracket and the seat sliders have integrated (and tested/approved) captive nut anchor points with the DOT-mandated 7/16-20 thread.
So what I have are swank seats, two possible holes for a bolt, and a pair of 7/16-20 nuts from the stock seats. So I need a couple bolts, but everything I own uses metric fasteners, my home toolbox plus on-the-road kit is stocked with metric tools, and every bolt I can find to buy has a standard head!
I'm failing with Google searches, and not coming up with anything via McMaster, Fastenal, Summit, etc.
Anyone have a source? I'm thinking a short 7/16-20 stud into the captive nut and my 14mm hex nut on the other end probably isn't as safe as the proper bolt torqued, so I'm leery of that option, but the thought crossed my mind.
I am pretty sure my spare VW stuff is 17mm. I will check in the morning. If they are not, they are 19m, which is basically 3/4. 19mm = .748"
I don't understand - the problem is that you don't have a wrench? If not, try a 16mm. That's an almost perfect match to the 5/8" head on the bolts.
dps214
Dork
3/22/22 11:17 p.m.
Buy a wrench/socket in the appropriate size. Probably cheaper than sourcing some likely custom hardware anyway. Toss it in your travel box if you feel the need to, but if your seat belt bolts need to be tightened after install you have some kind of other problem to sort out. Or like Keith said, standard head size for a 7/16 bolt looks to be 5/8" which should interact with 16mm tools just fine.
In reply to Keith Tanner :
The problem is that it bothers me to put a mismatched fastener on the car. I'm anal, I'm a perfectionist, the tiny aesthetic details matter to me, and I strive for a cohesive look on my projects. I don't necessarily leave it stock, but I want it to be believable that it is something a factory or thorough tuner could have built, and strive really hard to avoid the "mishmash of whatever was at the FLAPS that day" approach. Japanese stuff is great because you know it will be 8mm, 10mm, 12mm, 14mm, 17mm, etc. 5/8" on a Japanese car...what am I?...a carpenter? 
If I'm reading it right (who knows at this time of day), you want to keep things appearing stock and intentional by choosing not to reuse the stock fastener?
So find whatever OEM bolt in the junk yard that looks the part. They ALL have 7/16-UNF threads, just find the bolt head that amuses you. :)
In reply to classicJackets (FS) :
The seat bolts are made out of hot butter. If you get them out in one go, I would never risk using them again. I bought 10.9 grade shouldered metric bolts off the internet. Correct diameter and length. The bolt strength was more important to me than aesthetics.
SkinnyG (Forum Supporter) said:
So find whatever OEM bolt in the junk yard that looks the part. They ALL have 7/16-UNF threads, just find the bolt head that amuses you. :)
Yep, I've had some Subaru and Toyota seat belt bolts with JIS 14mm hex flange-head.
In reply to ae86andkp61 (Forum Supporter) :
If you want it how the factory would have built it, use the SAE bolts :) If you want it so that you can never use harness eyebolts or any other fastener designed for seatbelts, toss in something non-standard. Don't think of it as 5/8", think of it as 16mm. It was years before I actually realized it was an SAE bolt on the Miatas!
The problem with sourcing a special weird bolt with SAE threads and a metric head is that you'll forget it's SAE and end up cross threading some other metric bolt in its place.
What do actual JDM or Euro models use?
In reply to classicJackets (FS) :
I can't reuse the stock fastener, as it is a stud integrated into the seat. The aftermarket seats provide a nice captive nut in the correct location, hence the need for a bolt.
Here is what I dug up with a quick troll through my bucket. Several with 17mm head, some shouldered; one of the classic 5/8" head; and one with the JIS favorite 14mm flange head. All from Japanese cars, except for the guest appearance by the awful T50 torx versions sourced from an '80s Ford pickup and 20teens Mopar minivan.
Note: you're not going to accidentally ramrod the wrong hardware into the seat belt nut on a Japanese car. JIS 14mm head indicates M10 which slides right past the 7/16-20 threads; while a 17mm headed JIS bolt is M12 which is too large to catch in the 7/16-20 nut.
A trip to a wrecker with lots of Japanese cars will find you the bolt you need, but I'd suggest therapy for the other problem.
