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Old 02-01-2020, 01:46 PM   #1
AGBill
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Default Body Mounting Bolts

The previous owners of my 1930 Sport Coupe have stripped out the square holes in the body sheet metal when removing the rusted body mounting carriage bolts. The holes are now all round holes.

Has anyone tried to use new knurled shaft carriage bolts that engage the frame mounting hole instead of new modern hex head bolts? I like the looks of the smooth carriage bolt heads where they are visible after the body is mounted.

Any other solutions other than welding up the old holes and filing new square holes for the carriage bolts?
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Old 02-01-2020, 02:23 PM   #2
AGBill
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Default Re: Body Mounting Bolts

I realize that the commercially available knurled carriage head bolts are knurled right under the carriage head, (there is no square portion of the shaft). The length of the knurled shaft is typically about 1/2" long. This would engage the rounded out body sheet metal hole and the top portion of the wood mounting block, which would be of limited strength. I am talking about the possibility of custom knurling the bolt right above the threads to engage the frame mounting hole.....
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Old 02-01-2020, 03:16 PM   #3
redmodelt
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Default Re: Body Mounting Bolts

Unless you are going for points, I would just use the hex bolts with washes underneath the head. You could grind the heads a little thinner and remove any markings. They will be under the floor mat anyway. The knurled part would have to be larger then the hole in the metal to grab and the wood probable might not grip the knurled part anyway depending on the age and hole sizes in it.
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Old 02-02-2020, 08:39 AM   #4
Stew Masche
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Default Re: Body Mounting Bolts

You may possibly find a machine screw with a slotted head the right size, or cut slots in the heads of the carriage bolts to hold them with a screwdriver. Check with Fastenal for the machine screws.
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Old 02-02-2020, 10:12 AM   #5
Badpuppy
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Default Re: Body Mounting Bolts

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I assume you're referring to the four "carriage" bolts next to the floorboard. Mick Isbell's hardware reference shows A-20922 as a 3/8-24 x 2-1/4 carriage bolt, which is a contradiction, since carriage bolts, like step or elevator bolts, are coarse thread by definition. From the fuzzy pictures I've managed to find of these bolts, the head appears to be wider and flatter than a carriage bolt, like a step bolt. On my 30 standard coupe I found the PO had installed standard 3/8-16 x 2-1/2 carriage bolts in the round holes with the heads sticking up above the square shoulders - also unnoticeable under the carpet.

Since several body bolts were completely missing I ordered a set from one of the vendors only to find they provided the same standard carriage bolts, but at least of the correct length. I couldn't find step bolts anywhere in either fine thread or 2-1/4 length, so I decided to simply grind off the square shoulders of the ones I had received and install them as is. There is sufficient friction on the heads to prevent them from spinning.

If anyone has these original bolts, it would be good to know their actual dimensions.

On a side note, the flat washers in the kit have no use that I can find; you can't start a nut with both flat and lock washers in place.
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