dkleather

64...is there a general accepted bolt length spec when replacing undercar bolts such as for bumper brackets, gas tank crossmember, etc. Specifically how much of the bolt end should be showing after a washer and nut are installed. My P.O. liked using 2-3” bolts in a bunch of places and it looks nasty. If I knew how much bolt thread should generally be left exposed I could figure out what lengths to buy. I’d like to correct it.
Weird question, I know. Dave

domenic tallarita

Dave,
I can say there is a guide to that in aircraft that I also use when locating the proper bolts for my cars.
It reads at least 1 1/2 threads MUST show. For me no more than 1/8" on average. Some larger 3/16".

Dom

Vettrocious

In general, the last 1.5 threads of a bolt are less than full height and therefore, not structurally sound (this varies, depending on the type of bolt point being used). Additionally, the first internal nut thread usually is not full height, so is unusable. So, if the last trace of bolt thread is 1.5 threads outside the edge of the nut, you have a good safety margin.

If you have a modern Corvette, with modern bolts, this does not hold true. PM me if you need an explanation

MikeM

The standard where I worked was two/three threads past the end of the fastener or half the diameter of the (threaded) bolt sticking through the nut.

That was 25-30 years ago.

asolde

I'm in the aviation business and the standard is 1 1/2 threads should show after the fastener is installed. Makes for a nice safe installation that looks good.

SWCDuke

The Corvette Illustrated Parts Breakdown used by dealers shows both bolt part numbers and dimensions. This document changed over the years and does not cover everything for every year.

I don't know if they are reproduced, but it's a handy reference to have around.

Duke

wmf62

this doesn't specifically answer the question, BUT per AISC, the full strength of a bolted connection is one bolt diameter thread engagement. in practice you want approx. 1 thread projection beyond the nut to visually insure the requirement is met.

anything else is a waste of material and $.



Bill

fla-buckeye

Dave,
I know the weather is cold, and the day light is short..............but!

Hope you get this situation straightened out before the grass starts growing!

dkleather

Well, Norm, I’d be happy to have the garage floor above 32* before I lay on it. That way I don’t freeze to it and have to stay there a day or two. This will be more of a Spring project. Wish the grass would grow but not likely at 10*. I’ve already sent you some of our weather. Dave

Pilot Dan

Dave, there are a couple of ways to get the correct lengths on the bumper bolts. If you want the correct heads, just get the bumper bolt kit from one of the vendors. Another way if you don't care about the markings is to get a hold of one of the old GM parts manuals and look in the back (green pages 7-13 & 7-14 from the 1982 book) and they have pictures which list the bolt sizes and lengths for the locations.

PM me if you need a copy of those pages and I'll get them for you.

dkleather

Thanks Dan,
I do have the unmarked head bumper bolt kits to install. There are other bolts underneath I’d like to correct as well.
Thanks for your offer to copy the pages. I’ve sent you a PM. Dave