The Stovebolt Site Forums The Shop Area Driveline Grade 8 or Grade 5 3/8 x 31/2" Carriage Bolts

Hi all,

I've had my '46 for almost 30 years and have had my rear spring shackles fail 3 times, most recently last Sunday. The part that fails is always the carriage bolt. Seems like the very slight flexing it does fatigues the metal after several years and they finally break. The bolts that are supplied in the shackle kits are Chinese 307A, which apparently isn't very hard.

What I want to do is to replace all four carriage bolts with Grade 8, or at least Grade 5. Haven't been able to find them at any local hardware type stores. Seems like it would be tough to get a hardware supplier to sell you 4 of something like this - more trouble to them than it's worth. What's everyone's opinion on this. They obviously don't fail very often, but if you're on the road it's a real PITA. Anyone else run into this?

Thanks,

David

If it's that regular carriage bolt they call the "draw bolt" and it can be a regular hex bolt instead, many Gr 8 available. If it has to be a carriage, post the size and length. That is probably available at McMaster Carr and others (maybe not Gr 8 but SS). You can buy bolts in small quantities many places on the web. If you want help finding, need size and length. The only reason it would have to be a "stovebolt" is head clearance or the square under head is fitted into a square hole. All it does is pull the plates together and hold. Not an AD guy, but don't see why it can't be a hex bolt. Easier to find in Gr 8.

Carbon steel won't fatigue under the endurance limit, which is about half the tensile strength. The fit of the tapered pins into the shackles is usually really tight so the bolt really does almost nothing. Have you replaced the entire shackle assembly? Are the spring eyes in good condition?

I agree Gr 5 is plenty good but if you want Gr 8, I understand. If they break, it's time to get stronger. Be sure, as Sput is saying, that something is not wrong with assembly or another part which is over-stressing this bolt. It does not seam like it is under much load, but I'm not sure. If the plates are not bent after bolt breaks and they are not shifting, one up and one down, then I don't see any stress. Where exactly do the bolts break? Middle? head? nut end? strip? Are the holes in the plates for this draw bolt oblonged? If it's 3/8 bolt it has 1000's of lbs strength. Somethings up. 307A= 60KSI= 60,000PSI. Gr 5= 120,000PSI, Gr 8= 150,000psi as a comparison. Caution: Bolts must be torqued to specs or they may be subject to early failure in some cases. Many online bolts will be Chinese also. It is possible that off shore fasteners do NOT comply with the specs and/or are not made/tempered properly. Are you jumping ramps at car shows? Are you a moonshiner?

Imperial Supply sells a grade 5 carriage bolt.

https://www.imperialsupplies.com/item/0241600


However, if I need to replace a shackle bolt, I drill them out to where they take a regular shackle bolt.

http://www.stengelbros.net/Leaf-Spring-Bolts_c_67.html

It seems like what David says, Imperial sells in 50 and 100 Qty?

Try a farm store. We have Tractor Supply that sells bolts, nuts and washers by the pound. It is quite reasonable. If not try Fastenal. They have a great selection. Have fun and good luck
Dan

In my opinion, you are shopping for a band-aid for a compound fracture.
The only way that the carriage bolt will bend or break is if your spring shackle threaded bearings/pins are severely worn. You will need to separate the shackles from the leaf springs to check them properly. (They will seem tighter than they actually are until you remove the weight of the truck from them.) You will need to check where the bearing ends meet the shackles for a tight fit, and the threaded area of the bearings and pins for excessive play.
Carl

You have a problem with a bushing, probably be a good idea to check it out before something bad happens.

Thanks for all the suggestions. Yes, I've replaced the entire assembly, bushings and all.

As to where the bolt fails, I'm double nutting it on either side of the outside shackle plate. Looks like the bolt failure is on the inside of the inside nut, if that makes any sense.

The bolts are 3/8" x 3 1/2" coarse thread. I will try some of your suggestions. Seems to me that the only reason a carriage bolt was used is that the shackle plates have square holes in them that (I suppose) keep the bolt from turning at all.

David

Could be a bushing is to tight and causing it to bind up.

Try not double nutting. Try stronger bolt. Make sure there are no special instructions for that bolt like: don't tighten too tight, etc. Is it only one specific place???? (that would be a sure sign). The square hole is to hold the bolt while tightening nut. Why are you double nutting? 3 times in 30 years is not frequent but odd. No others have reported it that I know of. Don't know if it happened on old parts AND on new parts?? On a 3/8 307A, the shear strength is about 4000 lbs!!! The proof load is about 3300 lbs. A quality Gr 5 will not break.

I decided to try bartamos suggestion about not double nutting. When I thought about the failures it occurs to me that the problem is that the double nutting is keeping the bolt from having any flex to it (although this would be very minimal). I'm starting to think that over a period of years and thousands of miles the repeated slight stress at the inside nut is causing metal fatigue eventually. Since nobody else is running into this problem, and lots of folks are using the Chinese replacement shackle kits the problem must be how I'm assembling it. Thanks for all the input. Only problem is that it'll be 5-10 years before I know if this worked.

David

keep everone posted,we will be here.

That's long enough to consider it fixed. I think you are doing the best you can do right now.

Yeah, I'm going to consider it good. Sure is a pain when one of those bolts breaks when you're on the road, though. I carry an extra bolt in my toolbox, but you still have to find the two shackle plates, which can be a bit of a trick. Thanks for your suggestion.

David

Only thing I can think is putting that much pressure on the shackles would be a bad bushing at the fixed end. It might allow enough movement to shift the shackle end. If that is not the issue then I would look close at the "U" bolts for sure.