What thread size are the exhaust manifold studs

Adam Vinnicombe · May 19, 2020

Hi all I have a 2001 Ford Mustang GT. I’m installing a new exhaust manifold/header and I thought the manifold studs where M8x1.25 but they are not. They do fit but they are loose does anyone know what size nuts I need?
Cheers

OLD H2S · May 19, 2020

??? You are right at 8x 1.25 I use a product called Tight made by FelPro to lock them in because it is a thick gel and doesn't drip and run off.

Adam Vinnicombe · May 20, 2020

Cheers mate

Adfalenski · May 20, 2020

I worked in a Chevrolet parts department for a while and have had a lot of experience with metric and standard fasteners. I recommend you try a 3/8”-16 bolt to check the fit. If it is too tight, try a 5/16” bolt. As an example, the GM metric engine blocks used 3/8”-16 exhaust manifold bolts but metric for the rest of the engine. 8mm = 5/16” and you said that your bolt fitted loosely.

Adam Vinnicombe · May 20, 2020

The m8 x1.25 nut do wind up and torque up on the studs but they just don’t feel right, they can wobble a little on the threads before you torque them up.

Adam Vinnicombe · May 20, 2020

Adfalenski said:
I worked in a Chevrolet parts department for a while and have had a lot of experience with metric and standard fasteners. I recommend you try a 3/8”-16 bolt to check the fit. If it is too tight, try a 5/16” bolt. As an example, the GM metric engine blocks used 3/8”-16 exhaust manifold bolts but metric for the rest of the engine. 8mm = 5/16” and you said that your bolt fitted loosely.

Would they be UNC or UNF