So this came up for me today, time to document this for future Dale (and you guys) .

Stock, the main cat has studs on the 2 flanges. The main cat connects to the precut with these studs and nuts, same deal on the cat-back end - studs and nuts.

More often than not, the studs are rusted/seized up/galled/broken and the easy (and decent) way to fix it is to use a bolt and nut. If you use an aftermarket midpipe or high-flow cat, you will need bolts and nuts since they don't have studs, just straight holes in the flanges.

Now, some of you will head on down to the hardware store, pick up some plain bolts/nuts, put it together with your 5/8" ratchet, and call it done.

I can't do that .

I want metric hardware, with flanged heads, with proper wrench sizes (JIS standard), and stainless steel or properly coated. Also I want the nut to have a serrated head so it grabs on and doesn't back off.

So, head to Clipsandfasteners.com and get

Part number PM33315 - M10x1.25 x 40mm JIS Hex flange head bolt, pack of 15
Part number A16470 - M10x1.25 Metric spin lock nut with serrations, pack of 15

They are OEM-grade, proper M10 size, and use a 14mm wrench to install - not a 15mm, not a 5/8" or some crap.

Just had to order some for the RX-8 and had to dig around to re-learn this.







Dale

 

FYI if you want to grab them in person rather than waiting on shipping - I have found NAPA (in Canada at least) to stock a pretty decent selection of metric and JIS nuts & bolts of varying grades.

I also hate putting incorrect nut/bolt sizes on the car

 

Around here you can get "metric" bolts but they are never with a flange head, always a weird wrench size (15mm, 13mm)....no Thanks.

Those of you in larger US cities or foreign countries may have less of a problem.

Dale

 

I’m the same. Luckily I have local hardware store that has these, not too long though, only up to 2.5” or so.

They used to have a sign that said “largest bolt room on the west coast.” Kinda cool.

 

Nice. We use something similar in house.

 

Hardware stores around me always seem to have similar bolts in appearance to what you posted in stock. Sometimes not in the exact length you'd want though. The flange ones seem to be sold out more often, but they do sell them.

One thing I'd mention though, is I personally prefer the higher rated bolts since these probably get a little warm. Any autoparts store sells them (again not always in the ideal sizes) but none of the hardware stores in my area stock them at all.

 

Cool.
Last time I had to buy bolts for the exhaust, I got them from rx7.com. I also bought the gaskets.
RX7.COM | 1993-1995 Rx7 Exhaust (330)7-ROTARY
Scroll down or ctrl-f for
"Main Cat Bolt and Gasket Kit
Price: $25.00"

 

FYI I've had these same bolts/nuts on my car for YEARS now. Zero problems. Stay tight, fit great, come off nice and easy when the time comes.

I also like that the bolts have a non-threaded shoulder by the head and aren't all thread. If you have thread in the flange hole that can act as a saw over time and also doesn't give good contact to the flange hole.

Dale

 

just for the info, the 1994 and later factory cats have bolts tack welded on, instead of studs, the bolts are 9YA2-01-201
its going to be Inconel, 12 x 1.5 x ??, same thread as the wheels (i have seen lug nuts holding the cat on before)

the nut is a JE11-40-355, again 12 x 1.5 lock nut in inconel with a 17mm head

 

For what it's worth I've tried 12mm exhaust bolts in the past, it's a real hit or miss if they work. Some flanges want the smaller size, some don't have enough room for a 17mm nut. Also sometimes you need that tiny bit of wiggle room to help line things up.

I've owned about 15 different cat-backs and have put each one on my car to test out.

Dale

 

I usually order fasteners from McMaster-Carr. They're located in NJ and have a quick turnaround on orders.

 

I order a lot of MCMaster as well but I haven't had good luck finding the RIGHT fasteners - 14mm wrench head, flange head, serrated nut, etc.

Dale

 

McMaster-Carr is great. They sell small quantities of Bumax Stainless Steel.
I use the Bumax-88 bolt and nut for my high flow cat section and RB dual split section.
I have little to no problem torquing them to OEM specs. I do believe they require a wider head at 15mm.
They do not loosen as I've tracked 20+ laps @ BiR, then checked torque. No exhaust leaks.
I have removed and reinstalled my Bumax stainless steel hardware multiple times with little issue, no damaging threads or damaging engravings from lock nuts.
The best part of Bumax-88 is they turn a most beautiful golden color after being heated by 13b exhaust.

I use Dale's copper lock nuts for my OEM exhaust manifold and aftermarket downpipe, not OEM Mazda.

 

I'm not familiar with the Bumax-88 fasteners, but I'll check them out since I need to replace some my exhaust system bolts. I was looking into the copper lock nuts a while back. I think Pineapple Racing sells them? What's the advantage of using the copper lock nut versus the Mazda nut and lock washer?

 

OEM Mazda Manifold/DP Lock nuts are very expensive and overly aggressive. They seem to lock a bit too well, pulling the $20 Inconel stud out with them, requiring both to be replaced.
If you ever have to do the job again, the copper lock nuts and washers are much easier and cheaper to deal with.

 

Thanks Dale! This is the information I needed. Bought a new exhaust and currently doing a long distance relationship with the car since I'm living in NJ now while the car is in Florida haha.

 

Ti just sayin....

https://www.ti64.com/product-p/5352.htm

 

Titanium is pretty weak when hot, that is why race cars use inconel for stessed connections in the exhaust.

My Magnaflow titanium muffler tip (made by JIC Magic) got dented just from hitting a cone at autox.

Tried hammering it back round and the Ti was just too damn strong. Tried heating it with propane torch out of desperation (reasoning it was hot when cone gave it the flat spot) and to my surprise it shaped back to round super easily..

 

​​​​​​I'll give you Iconel, but compared to steel ti has a higher melting point. It also has a low coefficient of expansion.

Physical Properties of Titanium and Its Alloys

Maybe your muffler was super thin

 

I tried to find a chart that would compare yield strength of Ti versus Inconel, but I just couldnt because to an engineer they are simply not comparable materials for elevated temperatures.

Ti and its alloys strength takes a dump at 500C whereas Inconel starts to drop hard at 800C.

You can find charts comparing Inconel (the old run of the mill industry standard) to more exotic alloys like Haynes alloy and the newer cerametallic alloys.

Regarding material thickness-
the reason race cars use inconel for stressed exhaust components (hardware, headers, exhaust manifolds, endurance race exhaust systems) is they can use a much thinner tube wall of Inconel vs Ti alloy and have more strength still at elevated temperatures and still be as light as the 40% lighter Ti components.

Same reason why turbo exhaust wheels are Inconel or cerametallic (like EFRs) but not Ti alloy. Ti just lacks strength at elevated temperatures.

 

just to add to this, the NON lock nut 12 x 1.5 nut is 8118-40-355, although its getting a little rare
the 10 x 1.5 Non lock nut is 8134-40-355, these come off nicely, but you can't use them on the turbo, because it will fall off...
the 10 x 1.5 lock nut is JE10-40-355, Mazda's is inconel, Kia/Hyundai has an identical one, 0K201-40-355 and it looks the same, but its something softer

 

For the turbo nuts/bolts, I can see needing to use Inconel or something more high-temp. Down by the cat you're in "regular" car exhaust heat territory. Good quality bolts/nuts that are OEM-grade will do fine.

Again, the bolts/nuts I listed above have been on my car for years and I've done a few removal/installs - zero problems, zero seizing up.

Dale