When you’re repairing a trailer or truck body, you can count on having to replace fasteners that are old or have been drilled out in the process of removing metal panels or structural members. Common sense says to use the same type of bolt or rivet that was there before, though when things are rushed, it may be tempting to use something else.

Fight that urge, because the body’s integrity is at stake.

“Same” means that replacement fasteners should be of the same type, size and grade as what was there. And they should be installed correctly. With nuts and bolts, this means torquing to the manufacturers’ specifications. For instance, an SAE Grade-2 bolt with fine threads should be torqued from 56 inch-pounds to 979 foot-pounds, depending on size and whether it’s plated or plain.

That’s among the information listed in charts in the Technology and Maintenance Council of the American Trucking Associations’ Recommended Practice 729, Trailer Fastener Specification. The RP came from a survey of 44 trailer builders. Their preferences for using specific fasteners in various applications are also listed collectively (not by name) in the RP, and we can infer that they choose what they do for reasons of function and economy.

Squeezed solid rivets, for instance, are a favorite method of attaching roof sheeting to roof bows, according to the RP. Solid rivets install quickly and are strong enough to hold the pieces together. You know enough not to bang in hollow rivets instead just because that’s what you have at hand; hollow rivets won’t be as strong, and this job can come back to haunt you.

Those old rivets are likely to have been surrounded by an anti-moisture sealant, which should also be replaced when you pop in the new rivets. Ideally, you’ll use the same product the builder did when assembling the trailer or body. Or maybe your shop uses a product that experience shows works well everywhere.

Just be sure it’ll work on what you now have at hand.

Likewise, graded bolts or cap screws should not be replaced by something different. Using the same roof example, substituting a cap screw for a rivet might or might not work well. If it means getting the trailer back on the road where it’s badly needed, and for some reason a cap screw is all you have, then “it works.” But if the cap screw is stronger than the old rivet, twisting it in could pucker up the bow and roof sheet, leading to other problems. So it works, but not well. Substituting a Grade-5 bolt for a Grade-8 ought to get you burned at the stake (or an old iron pole). You could substitute a Grade-8 for a Grade-5 if that’s all you have in your hand is a Grade-8 and it fits. However, you might add a lock washer where there was none, especially if experience shows that bolts in some applications tend to turn out.

You’re probably aware that more and more metric fasteners are showing up on the automotive scene, especially in power units. You also know that there’s little correlation between SAE and metric grades, so you have to be careful to use the correct thing, and especially careful if you’re substituting an SAE for a metric or vice versa. That’s another story.