I need the 8 bolts that attach the front seat of my 1950 Roadmaster to the floor. The previous owner lost all but 2 of them. They are 5/16" by 20 tpi which is not a ASME standard size. I suppose I need to get someone to pull them out of a parts car somewhere but does anybody a good place to start asking? I am guessing that this size bolt was used in other places on the car and in other GM cars of the period as well. Any ideas? Any help would be greatly appreciated. Jim in San Jose
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Non standard body bolts needed
- Thread starter Jim in San Jose
- Start date
Jim,
The Buick Parts Catalogue shows the bolts attaching the seat adjuster to the floor pan are GM part no 4584867. This is a 5/16-24 x 5/8 hex bolt with captive external-star lock washer. It was used on the body in some other locations. It appears in Chevy catalogs as well. Attached is a factory service bulletin showing the repair of a ‘54 Estate Wagon tailgate using these bolts.
I tried Googling ‘5/16-20 bolt’. Some guys in one thread were trying to find 5/16-20 hardware and a tap. One noted that a metric M8-1.25 thread is extremely close to what a 5/16-20 English thread would be.
Are there any marks or printing on the head of the remaining bolts? That would indicate a vintage later than 1950. It would be interesting to see if a M8-1.25 nut will thread onto the bolts easily. Another idea would be to try threading a 5/16-24 bolt into the floor pan nuts in spots where these two bolts were not installed.
The Buick Parts Catalogue shows the bolts attaching the seat adjuster to the floor pan are GM part no 4584867. This is a 5/16-24 x 5/8 hex bolt with captive external-star lock washer. It was used on the body in some other locations. It appears in Chevy catalogs as well. Attached is a factory service bulletin showing the repair of a ‘54 Estate Wagon tailgate using these bolts.
I tried Googling ‘5/16-20 bolt’. Some guys in one thread were trying to find 5/16-20 hardware and a tap. One noted that a metric M8-1.25 thread is extremely close to what a 5/16-20 English thread would be.
Are there any marks or printing on the head of the remaining bolts? That would indicate a vintage later than 1950. It would be interesting to see if a M8-1.25 nut will thread onto the bolts easily. Another idea would be to try threading a 5/16-24 bolt into the floor pan nuts in spots where these two bolts were not installed.
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Thanks for the suggestions. I will try the metric bolt idea as that would be by far the easiest to do if it works. If they will screw into the caged nuts in the floor my problem is solved since there are 8 of them to hold the front seat down. I will report the result.
Many thanks to the suggestions above. I tried a metric R8 bolt and it is an almost perfect match for the 5/16" by 20 tpi body bolts in my 1950 Buick. The fit great. One question, "How did you know" I would never have thought to try it. Jim in San Jose
suntreemcanic
Experienced Member
My 52 Roadmaster has 5/15 fine thread bolts attaching the seats to the floor. I think there is a misprint or some confusion in the bolt size. I bet if you got a 5/16X24 thread tap and chased those threads you would discover they are 5/16 fine thread.
There were some educated guesses involved. Parts books showed Buick used 5/16-24 bolts-not 5/16-20 or metric.
The Google search showed there is such a thing as 5/16-20 hardware, but it is rare. That one message thread introduced the idea of the metric bolt being so similar to a 5/16-20. The dimensions check very closely. A 5/16 bolt shank dia is 0.3125”. An 8 mm bolt shank is 0.3150”. A metric thread pitch of 1.25 mm per thread works out to 20.32 threads per inch. It would be hard to tell the two apart unless maybe there are some metric markings on the head.
Whatever those two remaining bolts are, the nuts in the floor pan now are threaded to fit the metric threads. I think what happened is someone in the last 30-40 years removed/reinstalled the seat and decided to replace the bolts in the process. They grabbed whatever size looked right and wrenched ‘em on in. This re-threaded the nuts for metric threads. 5/16-20 bolts are too special and hard to obtain to be considered as the replacements back then. It might be best to just use the new metric bolts if they thread in easily rather than re-cut the threads again back to 5/16-24.
The Google search showed there is such a thing as 5/16-20 hardware, but it is rare. That one message thread introduced the idea of the metric bolt being so similar to a 5/16-20. The dimensions check very closely. A 5/16 bolt shank dia is 0.3125”. An 8 mm bolt shank is 0.3150”. A metric thread pitch of 1.25 mm per thread works out to 20.32 threads per inch. It would be hard to tell the two apart unless maybe there are some metric markings on the head.
Whatever those two remaining bolts are, the nuts in the floor pan now are threaded to fit the metric threads. I think what happened is someone in the last 30-40 years removed/reinstalled the seat and decided to replace the bolts in the process. They grabbed whatever size looked right and wrenched ‘em on in. This re-threaded the nuts for metric threads. 5/16-20 bolts are too special and hard to obtain to be considered as the replacements back then. It might be best to just use the new metric bolts if they thread in easily rather than re-cut the threads again back to 5/16-24.
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I have attached a picture of one of the original Buick body bolts that were holding the front seat down in my '50 Roadmaster. It doesn't show in the picture but the head of the bolt is recessed and shaped with a captured washer as is common with all the other body bolts on the car. The picture also shows a thread gage clearly showing the 20 tpi thread pitch. I believe this bolt is original to the car. Luckly for me a metric R8 bolt is nearly identical in diameter and thread pitch.
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When push comes to shove, I do not believe that any American made cars, and extremely few products in 1950 used any metric fasteners. The whole metric !@#$% thing came in, in the '70's and slowly. There "may" have been a few manufacturers tooling up specifically for exports, but I would be extremely surprised if it was a car manufacturer. A non standard bolt would be an unnecessary cost increase and a confusion on the assembly line.
I believe whatever fits in there now, it started out as a standard coarse or fine thread bolt of common threads. There is no reason to deviate for a seat!
IMHO
I believe whatever fits in there now, it started out as a standard coarse or fine thread bolt of common threads. There is no reason to deviate for a seat!
IMHO
I am not suggesting that any bolt on my Buick was ever metric. They appear to be 5/16" 20 tpi which is not a standard bolt size of course. It just happens that the metric R8 fits.