tl:dr Your roof rack bolts are engineered to prevent leaks, don't ignore them.
I'm in the process of reassembling and remounting my factory roof rack after restoring the finish, which had begun to peel after 26 years of exposure. I've been reading here about how to avoid leaks and I realized that there does not seem to be a good documented understanding of how Toyota designed the mounting hardware, so I thought I'd share what I discovered. After all, engineering kept the water out of the car for a quarter century, why start letting it in now? So the below are just my observations, not something I have verified, but hopefully food for thought if anyone is doing the same. It's a deep dive into something that seems trivial but can lead to a lot of problems if ignored.
First, like the sunroof, mounting holes in the roof are managed mostly through controlled leakage, and only a little sealing. That's important to understand. What that means with the rack is that Toyota envisioned a place for water to go and minimized where you actually have to seal. Second, the 12 bolts (or 8, if you have a pair of 2-pillar mounts) are specifically engineered and should not be replaced with just M8 bolts. I'd bet it's 50/50 you'll get a leak if you do that (with a factory rack).
This is one of the 12 bolts on my rack. It's one of the 4 shorter bolts for the center mounts. The forward and aft mounts use a slightly longer version, but the design is similar. Two features: 1) The distance between the head and the thread (the shoulder) is critical - it ensures that when you tighten the bolt, you do not damage the plastic rail mounts. The shoulder bears down on the metal mounting pad and the plastic mount gets snugly trapped with the bolt but not crushed. This is also why you don't seal with a rubber washer, since that would effectively lengthen the shoulder and loosen the mount.
2) the white elastomeric tube at the bottom of the shoulder extends about 1mm past the shoulder. When you add some sealant around the threads and tighten the bolt, this tube seals off against the threaded hole in the roof and appears to 'dam' the sealant just around the hole. You actually don't want sealant all over the place because...
Remember the controlled leak? The white tube's job is not to plug the hole in the plastic mount. In fact, look at your fore and aft mounts - they are oblong.
Water can flow right through and into the rain gutter, where it's supposed to go. You only need to seal immediately around where the threads enter the threaded hole in the roof. If you overdo it on the sealant, you may end up sealing the drain, and water will collect in the plastic mounts, rusting your bolts and causing long-term problems. This is especially true with the center mounts.
They are open facing up and their holes are not oblong. On mine, the bolts had rusted anyway, making them swell under the white tubes, which sealed them even tighter. When I removed my rack, the center mounts were full of water. Don't hasten that process by sealing them shut.
...and speaking of sealing, Uncle T engineered a second seal for those threaded holes. If you look closely at the little pads on the roof where the M8 threaded holes are, you'll see that they're stuck on a piece of butyl tape.
The screw hole in the tape is slightly smaller than the diameter of the threaded portion of the bolt, so that means a) there's a ring seal on the bolt threads, b) your threads get wiped clean on their way down, so anything wet on the threads will bunch up near the top of the hole, as you can see from my first attempt with red sealant. This is why you should not use wet Loctite, it will never make it down the hole. The dry kind may be ok. And speaking of sealant, use urethane or solvent-based 3M automotive body sealant. Silicone/RTV flows too easily and will end up sneaking its way out and up into the mount.
So there you have it - everything you thought you did not need to know about roof rack hardware. I appreciate corrections if I got anything wrong.
I'm in the process of reassembling and remounting my factory roof rack after restoring the finish, which had begun to peel after 26 years of exposure. I've been reading here about how to avoid leaks and I realized that there does not seem to be a good documented understanding of how Toyota designed the mounting hardware, so I thought I'd share what I discovered. After all, engineering kept the water out of the car for a quarter century, why start letting it in now? So the below are just my observations, not something I have verified, but hopefully food for thought if anyone is doing the same. It's a deep dive into something that seems trivial but can lead to a lot of problems if ignored.
First, like the sunroof, mounting holes in the roof are managed mostly through controlled leakage, and only a little sealing. That's important to understand. What that means with the rack is that Toyota envisioned a place for water to go and minimized where you actually have to seal. Second, the 12 bolts (or 8, if you have a pair of 2-pillar mounts) are specifically engineered and should not be replaced with just M8 bolts. I'd bet it's 50/50 you'll get a leak if you do that (with a factory rack).
This is one of the 12 bolts on my rack. It's one of the 4 shorter bolts for the center mounts. The forward and aft mounts use a slightly longer version, but the design is similar. Two features: 1) The distance between the head and the thread (the shoulder) is critical - it ensures that when you tighten the bolt, you do not damage the plastic rail mounts. The shoulder bears down on the metal mounting pad and the plastic mount gets snugly trapped with the bolt but not crushed. This is also why you don't seal with a rubber washer, since that would effectively lengthen the shoulder and loosen the mount.
2) the white elastomeric tube at the bottom of the shoulder extends about 1mm past the shoulder. When you add some sealant around the threads and tighten the bolt, this tube seals off against the threaded hole in the roof and appears to 'dam' the sealant just around the hole. You actually don't want sealant all over the place because...
Remember the controlled leak? The white tube's job is not to plug the hole in the plastic mount. In fact, look at your fore and aft mounts - they are oblong.
Water can flow right through and into the rain gutter, where it's supposed to go. You only need to seal immediately around where the threads enter the threaded hole in the roof. If you overdo it on the sealant, you may end up sealing the drain, and water will collect in the plastic mounts, rusting your bolts and causing long-term problems. This is especially true with the center mounts.
They are open facing up and their holes are not oblong. On mine, the bolts had rusted anyway, making them swell under the white tubes, which sealed them even tighter. When I removed my rack, the center mounts were full of water. Don't hasten that process by sealing them shut.
...and speaking of sealing, Uncle T engineered a second seal for those threaded holes. If you look closely at the little pads on the roof where the M8 threaded holes are, you'll see that they're stuck on a piece of butyl tape.
The screw hole in the tape is slightly smaller than the diameter of the threaded portion of the bolt, so that means a) there's a ring seal on the bolt threads, b) your threads get wiped clean on their way down, so anything wet on the threads will bunch up near the top of the hole, as you can see from my first attempt with red sealant. This is why you should not use wet Loctite, it will never make it down the hole. The dry kind may be ok. And speaking of sealant, use urethane or solvent-based 3M automotive body sealant. Silicone/RTV flows too easily and will end up sneaking its way out and up into the mount.
So there you have it - everything you thought you did not need to know about roof rack hardware. I appreciate corrections if I got anything wrong.
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