Rear-subframe mount - hidden Rust area that's not readily Visible
02-16-17, 07:40 PM
#1
Rotary Enthusiast
Thread Starter
Rear-subframe mount - hidden Rust area that's not readily Visible
This is a general heads-up that FC owners should be aware of.
I'm going through overhauling my car's suspension after 25+ years and 160+ thousand miles. I ended up pulling off my rear subframe, suspension and differential as a unit and I found rust around the rear subframe mounting points, the kind that has rusted through.
Now, before anyone panics about theirs, I want to point out that the large studs that attach the rear subframe to the body shell should still be solid. The studs themselves attach to a larger round assembly that's welded to the length of the chassis framerails.
What makes this a magnet for rust is inside the rear crossmember. For whatever reason, Mazda's primer coating doesn't quite reach these areas, leaving bare steel. It's thick steel, and the rust starts at the bottom and slowly works its way up.
To check for rust in this area you:
A. Remove the rear storage tray framework and shine a light down both sides where the rear crossmember meets the frame fails, the corners. There will be two openings that you can shine a light down or a camera.
B. Either pull off the rear subframe or use a mirror or a camerra to look around the area where the subframe bushings mount to the body.
It can appear to be surface rust but if you see rust on both the bottom and the top, poke hard with a screw driver. If you're lucky and it's only surface rust, TREAT it and coat it with a rust inhibitor IMMEDIATELY. If you don't, you will get a rust hole that may turn out to be more extensive than you think.
So far, I've had to cut quite a bit of rotted metal out and I still have a ways to go, but only because I need a Dremel or tool with a smaller cutting wheel to get into the tight spaces.. Also, there are two layers of steel and moisture will get in between them and rot both layers.
On the plus side, your subframe mounting bolts should be otherwise shiny and solidly in place. They should not move at all. If there is any wiggle at all, then you've got a structural problem that will require cutting and welding.
As for repair, depending on how extensive the rot is, it can range from getting away with wire-wheeling to cutting and either welding in or (in my case) gluing new steel in with structural adhesive.
At this writing, I have only started to cut out the rot and I still have hours of work to do. Go carefully. Most of the rot is inward toward the center of the car, and the studs themselves are mounted to the frame rails going outward.
I'm going through overhauling my car's suspension after 25+ years and 160+ thousand miles. I ended up pulling off my rear subframe, suspension and differential as a unit and I found rust around the rear subframe mounting points, the kind that has rusted through.
Now, before anyone panics about theirs, I want to point out that the large studs that attach the rear subframe to the body shell should still be solid. The studs themselves attach to a larger round assembly that's welded to the length of the chassis framerails.
What makes this a magnet for rust is inside the rear crossmember. For whatever reason, Mazda's primer coating doesn't quite reach these areas, leaving bare steel. It's thick steel, and the rust starts at the bottom and slowly works its way up.
To check for rust in this area you:
A. Remove the rear storage tray framework and shine a light down both sides where the rear crossmember meets the frame fails, the corners. There will be two openings that you can shine a light down or a camera.
B. Either pull off the rear subframe or use a mirror or a camerra to look around the area where the subframe bushings mount to the body.
It can appear to be surface rust but if you see rust on both the bottom and the top, poke hard with a screw driver. If you're lucky and it's only surface rust, TREAT it and coat it with a rust inhibitor IMMEDIATELY. If you don't, you will get a rust hole that may turn out to be more extensive than you think.
So far, I've had to cut quite a bit of rotted metal out and I still have a ways to go, but only because I need a Dremel or tool with a smaller cutting wheel to get into the tight spaces.. Also, there are two layers of steel and moisture will get in between them and rot both layers.
On the plus side, your subframe mounting bolts should be otherwise shiny and solidly in place. They should not move at all. If there is any wiggle at all, then you've got a structural problem that will require cutting and welding.
As for repair, depending on how extensive the rot is, it can range from getting away with wire-wheeling to cutting and either welding in or (in my case) gluing new steel in with structural adhesive.
At this writing, I have only started to cut out the rot and I still have hours of work to do. Go carefully. Most of the rot is inward toward the center of the car, and the studs themselves are mounted to the frame rails going outward.
Last edited by cluosborne; 02-16-17 at 08:03 PM.
02-16-17, 07:58 PM
#2
Rotary Enthusiast
Thread Starter
Here are some pictures of the rot. In my case, so far, the driver side mount area seems worse than the passenger side, but there is some rust-though on the passenger side as well.
Just poking with a screwdriver had opened a hole into the rear crossmember. In the last picture, you can see the unpainted steel inside. I could also feel around with my finger and there is clearly rust on the bottom surface.
Just poking with a screwdriver had opened a hole into the rear crossmember. In the last picture, you can see the unpainted steel inside. I could also feel around with my finger and there is clearly rust on the bottom surface.
Last edited by cluosborne; 02-16-17 at 08:04 PM.
02-19-17, 04:26 PM
#4
Rotary Enthusiast
Thread Starter
"Yep" indeed. I've made pgoress getting all the rot out, leaving solid steel. I did find that the subframe stud is not integrated into the frame entirely. There's actually a structure that surrounds the stud and it's thick steel that should hold up, but there is a void inside the outer part of this structre where the length of the stud inside the frame can rust. I'm planning to spray Fluid film into the areas that can't be properly prepped for POR15.
While those of you who have this issue might find some rust holes in the thicker parts, there should still be plenty of material lft. Most of the thinner (relatively) material that will rust through first will definitely be inward.
While those of you who have this issue might find some rust holes in the thicker parts, there should still be plenty of material lft. Most of the thinner (relatively) material that will rust through first will definitely be inward.
02-19-17, 09:08 PM
#5
Rotary Enthusiast
Thread Starter
Here is an updated picture after some more cutting, grinding, and smoothing out. I should also mention that this is the very first time that I've actually cut metal out of my car.
There are some areas that I won't be able to effectively treat without getting acid and water inside the crevices. All metal not exposed to the elements are going to get coated with Fluid Film while parts that are directly exposed will get treated and coated in POR15.
In the picture, you also see that I've covered the stud in two layers of Gorilla tape to prevent damaging it. The metal that surrounds the stud has turned out to be solid; as long as this area is solid, you should be in good shape. What you can't see is the surface rust on the stud inside this structure. It'll get Fluid Film sprayed inside it to neutralize (I hope) rust.
Besides making the sheet metal patch, the difficulty will be drilling the holes for the tapping screws to go in. When using structural adhesive, you must have the parts clamped together while the adhesive cures. In this case, it's going to be self-tapping sheetmetal screws that can stand up to corrosion.
By the way, I think I goofed on the title of this thread. "Hidden area that is not readily visible" is one and the same don't you think?
There are some areas that I won't be able to effectively treat without getting acid and water inside the crevices. All metal not exposed to the elements are going to get coated with Fluid Film while parts that are directly exposed will get treated and coated in POR15.
In the picture, you also see that I've covered the stud in two layers of Gorilla tape to prevent damaging it. The metal that surrounds the stud has turned out to be solid; as long as this area is solid, you should be in good shape. What you can't see is the surface rust on the stud inside this structure. It'll get Fluid Film sprayed inside it to neutralize (I hope) rust.
Besides making the sheet metal patch, the difficulty will be drilling the holes for the tapping screws to go in. When using structural adhesive, you must have the parts clamped together while the adhesive cures. In this case, it's going to be self-tapping sheetmetal screws that can stand up to corrosion.
By the way, I think I goofed on the title of this thread. "Hidden area that is not readily visible" is one and the same don't you think?
02-21-17, 10:04 AM
#6
Rotary Enthusiast
Thread Starter
I think I should also mention that my eyesight has not gotten any better over the years.
Apparently, Mazda's primer coat did go into the rear crossmember interior surfaces. If you look at the images I've posted, you can see that it's not shiny metal inside the rear crossmember, that's POR15 that I applied years earlier (which obviously didn't do much to stop the rust). However, in the void where the studs goes, you'll find that the bolt itself is bare metal and will rust, but it's very thick high-quality steel that should have held up pretty well.
So my apologies if I mislead anyone on where the coating is and is not. Bottom line is that in order to stop the rust, you need to get at the metal surfaces on both sides (top and bottom in this case).
Apparently, Mazda's primer coat did go into the rear crossmember interior surfaces. If you look at the images I've posted, you can see that it's not shiny metal inside the rear crossmember, that's POR15 that I applied years earlier (which obviously didn't do much to stop the rust). However, in the void where the studs goes, you'll find that the bolt itself is bare metal and will rust, but it's very thick high-quality steel that should have held up pretty well.
So my apologies if I mislead anyone on where the coating is and is not. Bottom line is that in order to stop the rust, you need to get at the metal surfaces on both sides (top and bottom in this case).