Cat - Manifold flange bolts arrrrghhhh

[HarryC]

I agree that stainless steel nuts may seize on stainless bolts, but at least they will not corrode away. Use some copper based anti-seize paste on the threads during assembly to minimise the risk. The nuts I used were flanged with a serrated face to prevent loosening (see picture). If you need to split the joint at a later stage and the nuts have seized solid onto the bolts, the hex heads should still be intact so, with decent ring spanners/sockets, you will be able to twist and shear the bolts.

I've used similar flanged hexagon nuts before, but like you said, they still seize. I imagine that a completely different metal would be better for avoiding galling, there must be something that is gall resistant, strong and corrosion resistant? Do you think something like these phosphor bronze nuts might work : [link=https://www.accu.co.uk/en/hexagon-nuts/268731-HPN-M10-PB]https://www.accu.co.uk/en/hexagon-nuts/268731-HPN-M10-PB[/link]

 

[glenn duval]

Hi I have swapped all my rusted exhaust bolts, On my 981. For brass nuts and bolts with no problem. I am a Electrician and use them all the time. For their anticorrosion properties. Hope this helps regards Glenn

 

[ianthow]

[/quote]

I've used similar flanged hexagon nuts before, but like you said, they still seize. I imagine that a completely different metal would be better for avoiding galling, there must be something that is gall resistant, strong and corrosion resistant? Do you think something like these phosphor bronze nuts might work : [link=https://www.accu.co.uk/en/hexagon-nuts/268731-HPN-M10-PB]https://www.accu.co.uk/en/hexagon-nuts/268731-HPN-M10-PB[/link]

[/quote]

Harry, you are correct that nuts from a different material will help avoid the galling problem with stainless on stainless. Brass/phosphor bronze nuts were common on exhaust manifold bolts many years ago. Typically these were longer than standard nuts to increase the load bearing and shear areas because these materials have a lower strength. The link you gave is for M10 nuts, you need M8 for Cayman/Boxster flanges. M8 nuts that are 10-13 mm long should work (a standard M8 nut is only 6.5mm long).

 

[HarryC]

Thank you. Do you know where I can get longer versions of phosphor bronze nuts please?

 

[ianthow]

Sorry Harry, I don't know where you would get longer M8 nuts.

I did find a supplier for long UNC manifold nuts. [link=https://www.spaldingfasteners.co.uk/brass-unc-unf-bsf-extra-long-manifold-nuts/]https://www.spaldingfasteners.co.uk/brass-unc-unf-bsf-extra-long-manifold-nuts/[/link]. You could use these with a set of 5/16" UNC bolts .

Another option would be to contact Accu.co.uk (the supplier you linked to originally) and ask if they can supply M8 nuts 13mm long. They indicate that the bronze nuts are made to order so I suspect that they will be able to do this for you. You could also ask the max torque that the standard length nuts will handle. Porsche specify 23Nm torque on these nuts. Aim to get something that would allow you to achieve close to that because the Porsche gasket is a thin metal one that needs to be tightly clamped. The torque that a nut will handle increases in direct proportion to the length of the thread so you can easily work out a minimum length for new nuts once you know the max torque of the standard length nut.

Alternatively, as previously mentioned, you could just use stainless nuts with a copper based anti-seize compound, e.g. [link=https://www.amazon.co.uk/Carlube-Purpose-Copper-Assembly-Compound/dp/B07F1W6T3J/]https://www.amazon.co.uk/Carlube-Purpose-Copper-Assembly-Compound/dp/B07F1W6T3J/[/link].

 

[HarryC]

Thank you

 

[GordonT]