Miata Exhaust Gaskets
Published: September 20, 2014; updated: November 21, 2017
For some reason ordering Miata exhaust gaskets seems like an exercise in receiving wrong parts. This page lists the gaskets I've tried and how well they worked.
NA8 Header To Front Pipe
Fel-Pro 60866
This should be the correct gasket. It has a 2" inside diameter.
This part is listed for NA6 Miatas but not for NA8 Miatas by RockAuto for some reason.
Walker 31561
This is not the correct gasket for the application but it can be made to work with a modification.
RockAuto shows this picture of the 31561 gasket:
However the gasket I received looks like this which is virtually identical to the Victor Reinz F7485 gasket:
The real 31561 gasket is not symmetrical and does not go on the header studs as it is. The stud holes can however be enlarged with a dremel tool to make offset hole gasket go on the car. The inside diameter of the real 31561 gasket is 2".
While the gasket does physically attach with enlarged stud holes, the center hole ends up not centered on the airstream which is bad for horsepower and could be making the gasket wear out quicker.
Victor Reinz F7485
If this gasket is accurately pictured by RockAuto, it is equivalent to the Walker 31561 gasket, in which case it is not the correct shape for the application but it can be made to fit the car by enlarging stud holes with a dremel tool.
NA6 Front Pipe To Tail Section
The ID of front pipe is 2" and stud spacing is 3 5/16". This measurement was taken on a 1990 Miata.
"Spec Miata" and other exhausts often have slotted holes at the front pipe junction to accommodate different front pipes/headers - if the car has a stock front pipe, the gasket generally needs to match the front pipe rather than the tail section.
Fel-Pro 60620
Dimensions: 2" ID, 3 5/16" stud spacing.
This gasket is an exact match for NA6 front pipe to tail section junction.
Walker 31388
Dimensions: 2" ID, 3 5/16" stud spacing - dimensionally almost identical to Fel-Pro 60620.
This gasket is an exact match for NA6 front pipe to tail section junction.
Fel-Pro 60771
Dimensions: 2 3/16" ID, 3 1/2" stud spacing.
This gasket just fits between the OEM NA6 front pipe and the tail section. Stud spacing is on the high end of the acceptable range - a slightly narrower spacing would be ideal but 3 1/2" still works.
The ID of the gasket is a bit larger than the ID of the NA6 front pipe, the gasket is not impeding the flow of gases but a slightly narrower ID gasket will also work.
NA8 Front Pipe To Catalytic Converter, Catalytic Converter To Tailpipe
For some reason RockAuto specifies different gaskets for the front and rear of the catalytic converter. From what I can tell the OEM catalytic converter has exactly the same ID and stud spacing front and rear, hence the same gasket would be used in both positions.
I measured my OEM 1997 catalytic converter to have 2 5/16" ID and 3 5/8" stud spacing for the entry and 2 1/4" ID and 3 5/8" stud spacing for the exit.
Note: NA8 front pipe has a larger ID and bigger stud spacing than NA6 front pipe. As such an ideal NA6 gasket will not fit NA8 and vice versa. However, I believe Fel-Pro 60771, while not being ideally sized for either NA6 nor NA8, will fit both cars.
Bosal 256-837
Dimensions: 2 7/16" ID, 3 5/8" stud spacing.
This is the correct gasket for OEM front pipe and OEM catalytic converter. It also fits perfectly on the rear of the OEM catalytic converter with OEM tailpipe. The ID of the gasket is slightly larger than ID of the holes on the OEM catalytic converter, meaning this gasket does not get in the way of exhaust gas flow.
Walker 31537
Dimensions: 2 7/16" ID, 3 5/8" stud spacing - dimensionally almost identical to Bosal 256-837.
As Bosal 256-837, this is the correct gasket for OEM front pipe and OEM catalytic converter. It also fits perfectly on the rear of the OEM catalytic converter with OEM tailpipe. The ID of the gasket is slightly larger than ID of the holes on the OEM catalytic converter, meaning this gasket does not get in the way of exhaust gas flow.
Walker 31537 does not fit between NA6 front pipe and tail section - stud spacing is too large. The ID of the gasket is also larger than the ID of the NA6 front pipe, although I expect the gasket to seal if it weren't for the stud spacing being too large.
Fel-Pro 60771
Dimensions: 2 3/16" ID, 3 1/2" stud spacing.
This gasket has a slightly narrower ID than OEM and a bit smaller stud spacing than OEM. I have not tried it on an NA8 car but my feeling is it is likely to work. One of the above ideally sized gaskets would probably be preferable due to the ID of Fel-Pro 60771 being slightly smaller than the piping ID.
Bosal 256-390
Dimensions: 2 1/4" ID, 4 3/8"-5" stud spacing.
ID matches OEM but stud spacing is too large.
Fel-Pro 60831
Dimensions: 2 5/8" ID, 3 5/8" stud spacing.
OEM stud spacing but ID is bigger than OEM. This gasket may not seal correctly.
NB - Racing Beat
The flanges on Racing Beat NB exhaust parts are not flat, but instead have grooves for exhaust gaskets. It seems that the correct gaskets for Racing Beat exhausts are O-ring type gaskets with no bolt holes.
NB Header To Front (Connecting) Pipe
A while ago I installed a Racing Beat connecting pipe to replace the OEM one which rusted out. For whatever reason I was lacking a gasket in front of the connecting pipe. Initially everything was fine, but with time an exhaust leak developed. Once I installed a wideband oxygen sensor immediately past the leak this leak had to have been addressed as the oxygen sensor readings were completely bogus.
Finding the right gasket turned out to be a long and frustrating undertaking. None of the gaskets shown on RockAuto have dimensional specifications, and as the Racing Beat connecting pipe has a larger diameter than OEM exhaust getting the dimensions right is important. I went to the local AutoZone and their system offered a Fel-Pro 60831 as the appropriate gasket.
This is not a usable gasket though. I cannot find specifications on it online anywhere and I no longer have one in my hands, but its inside diameter is if I remember correctly 2.5 inches, and the OEM exhaust has 2 inch outside diameter. The reason why the gasket is unsuitable is it is too thin - a thicker gasket may have worked, loosely, but a gasket that is this thin can move entirely outside of the flange at which point it was not doing its job.
As I ran out of time to find something to close the exhaust leak, I settled on the following two pieces:
Fel-Pro 60771 gasket. This gasket's applications are 1985-1991 GM 4.3L V6 and 1985-1990 Oldsmobile 5.0L V8. The inside diameter on this gasket is 2 inches which fits perfectly over the OEM front pipe, but the holes are not matching the holes in Miata flanges. I ended up trimming the gasket around the holes to basically make it into a donut. This gasket is also too thin to cover the space between the front pipe and the mid pipe as both pipes are concave to allow for a thick gasket. I would have been happy with 3-4 of these gaskets stacked up but AutoZone only had a single one.
Mr. Gasket 5980 which is a set of 2 gaskets for a V6 or a V8 application. These gaskets have a 2.5 inch inside diameter which is larger than the pipe outside diameter, but the gaskets are wide and thick. Again, the holes are not matching the Miata flange pattern, therefore I trimmed the gaskets into donuts and used both on the connecting pipe side.
With this impressive contraption of gaskets the exhaust leak was almost but not quite gone, but more importantly, my wideband sensor was reading sensibly. Success!
Researching parts for this post I came across this Miata Turbo topic which linked to this Vibrant Performance catalog. The 2" ID gasket should be the appropriate one, although the bolt holes look to be too close. Fel-Pro 60771 are about half the price of these gaskets at AutoZone, I am still considering just buying 3 more of them.
Researching further I came across this Racing Beat header identification chart that also includes header to connecting pipe gasket part numbers. In case of NB Miatas, the gasket part number is 56301. 949racing sells them. This is a donut gasket and according to its description it "has a larger ID than the factory gasket that would normally be used at this connection". In other words, still no clear inside diameter spec. Sigh.
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