I've got this old hitch on my truck, basically 1/2'' steel plate that sits between the wheel wells. It's about 3 feet deep. There's a 12''X12'' section of 1/2'' welded to the middle for a gooseneck ball as well. There are two brackets that come up along the wheel wells and they receieve the 5th wheel hitch. My question is, how many and what grade bolts do I use to secure this beast to my frame. Right now, there are three bolts on either side. They appear to be grade 8...I've heard grade 5 is better because they give more under stress...thoughts? The reason I'm getting serious with this hitch is because of my other thread--camper conversion. If I'm yanking this thing around regularly, I want to be safe! Thanks!
greg

 

Most install kits come with grade 5 bolts, at least the ones I have helped install. On the over the wheel well fifth wheel hitch there shoud be a total of six, two in front of the wheel well and two in back and most have a small 4x4 plate that bolts thru the wheel well on top. On the goose plate is that seperate from you fifth hitch?? It should be secured thru the bed to the frame, most rv parts places sell the L shaped brackets to secure to the frame, and again I would use grade 5 on the goose plate also,one in each corner going thru to the frame itself or to the L shaped brackets. Goodluck,,Rick

 

Grade 5 is NOT better than Grade 8.

You can tell what grade they are by the number of ticks on the head.

You can find the strengths of the fasenters here:

http://www.rockcrawler.com/techrepor...ners/index.asp

 

It is not a question of grade 8 being better than grade 5 it is the stretch and shear force that is used, and grade 5 is what is recommended for hitch installations as they will not shear off like grade 8.

 

A grade 5 will fail before an 8 will stretch in many situations. The 8 is superior overall and the reason for using 5 is cost savings.

 

Its just as stated above. Grade 5 for hitches and what not. A stronger tensile bolt, grade 8, will shear quicker with a load put on it from a trailer hitch. A grade 5 bolt will give a little more befor a shear failure accurs.
I was taught grade 8 when you load the bolt length ways, and grade 5 when the bolt is to be loaded from the sides.

 

Thanks for the replies, fellas! I took a closer look and they're grade 8. Now I'm not sure if I should change them out or not. The trailer is 10k gross, and I'll likely only be hauling my Farmall SuperA or a car here and there. Still, 10k gross is not peanuts...

 

Semi tractors use 5/8" Gr. 8 bolts to attach the fifthwheel hitch to the frame of the truck. That is what I used when I built my g/n hitch.

 

1/2 plate is not very heavy for the ball mount. Mine is 5/8 and 5/8 on top and still flexes too much. The three bolts through the frame is adequite. You are talking about 10,000 pounds, not 40,000. The strength of what you discribe is much stronger than the head mounting of the trailer. Promase you, hit something in the back and the head will bend over, but the bolts will stay where they are. 1/4 inch tubing of four inch diameter, either square or round is the weak point.

 

Thanks HID!

 

Not to get into the middle of this, but to take advantage of the ductility of a Grade 5 bolt, many times the engineer will increase the size of the fastener to obtain the required clamping force (with reserve) while still using a Grade 5 bolt installed at a lower prestress than would be required for the smaller Grade 8 bolt.

Rusty

 

Sooo, step up a size and down a grade and get the best of both worlds, in theory?

 

What sort of safety margin should there be. If a grade 5 or 8 bolt will shear at X lbs, and you are towing Y lbs.... what should the collective shear rating be on all of your bolts. I have seen 5 times as a common safety factor but not sure if that applies for hitches as well. So for example if you tow 10,000 lbs the bolts should shear at no less than 50,000 lbs... (?)

 

Put three 1/2 or 9/16 bolts in each side of the frame of good quality. It ain't goin no where. For an example, the plate laying across the frame. A piece of angle iron with the flange out, three bolts to the plate, three to the frame on each side. Standard practice.

 

Thanks again, HID! FWIW, I was really just guessing on the 1/2'' plate. It could very well be 5/8''. I'll have to check if spring ever comes