Exhaust manifold studs: nuts too tight?
 

Just at the point of cylinder head reassembly, hit this dilemma.

I bought new exhaust manifold studs, washers & nuts for my '98 V70 T5, and started putting the new, oiled studs into the head (the old studs came out complete with the nuts during cyl head removal), using a socket drive onto two locked M8 nuts to turn them.

The first one I did went in smoothly, then slightly less easily, before I (fairly quickly) realised it appeared to be turning without going in further... I had been expecting it to stop fairly solid, like any other bolt, once it reached the end of the cyl head thread.

I'd seen a video stating to be careful when inserting the studs "because you don't want to damage them", so I wasn't giving it anything like breaker-bar torque.

I'm assuming it's done something rotten to the cyl head thread, however, it feels well solid, i.e. firm and still stiff to rotate, so I'm hoping to get away with one below-par stud.

Anyway, I put the rest of the studs in no bother, without applying much torque, and they're all sitting the same height.

When I removed the original nuts from their studs I was surprised at how hard they were to turn. In fact one simply wouldn't come off at all.

When I came to try one of the new nuts on a (extra) stud to see how readily they'd fit, the reason for this became evident: they're serious locknuts, not surprising when you consider the variation in temperature they're having to cope with.

What got me was that the serious effort needed to turn the nuts onto the stud (once they reached the locking 'lip' near the end of their thread) far exceeded the torque I applied to that first stud that appeared to cause it to turn without screwing any further into the head.

So now I'm plagued with indecision: do I trust Volvo not to supply nuts which are so tight they'll cause the studs to turn (well before the nut reaches the manifold) and strip the thread in the head, or should I use nuts which I've slackened off somewhat by screwing and unscrewing them onto that old stud?

I thought about using a dab of high-temperature loctite on slackened-off nuts, but that only goes up to 400 degrees or so, and I'm assuming the manifold can get hotter than that, even though it's thermally tight with the cylinder head which doesn't get anywhere near that temperature.

I really don't know what to do next. The cylinder head's in place (but not torqued down), machined and complete with new exhaust valves all round, and now I don't know if weeks of work (and £££) are going to end in quick disaster.

Observations most welcome!

Gutted for you.

My thoughts would be that if you can get it out of the car so access is not a problem I would remove the stud and retap the thread in the head (or get a machine shop to do it for you). Then install a new stud to be on the safe side. If you are lucky just (slowly and gradually) working the tap in and out might clean it up and sort it.

A potential bodge would be to remove the stud and measure the length you've got where it will screw in and either use a shorter stud or a bolt of suitable length - you could even get a bolt with the head drilled so that you could run lockwire through it to stop it from rotating/backing out.
Depends on where the damage to the thread is - if it's just in from the start then a bodge will be too risky. If it's deep into the block then there's less risk.

When I was doing my turbo I removed the manifold to replace it (engine still in the car) and I had one or two studs come out. They got replaced by new volvo studs and they went in easy - I didn't use the volvo nuts to install because as you've said they are SERIOUS locknuts and the studs don't really have anywhere to get a proper grip without risking damage to the threads.

I had to retap both threads on the turbo and one on the exhaust manifold collector but they were spare parts I was preparing in advance of the work to swap them over so not as big a deal for me, still a PITA but that's ebay "good condition" for you...

The studs and nuts from volvo are seriously expensive - I think because they might be made from inconel - the nuts anyway.
Locktite will do sod all as I think the temperature will be too high - tried that on some nuts on my exhaust downpipe on my mx5 (non turbo) and it worked initially until it melted - I ended up using aerospace spec nuts/bolts which sorted it but for £££'s.

Tip - use a V50 T5 exhaust manifold gasket as it's a single piece (much easier to fit) and (for me at the time) was considerably cheaper than buying the 5 single gaskets.

Another option is to have a stud made with a slightly increased OD – quite a common practice for similar problems on classic cars. Depending on the diameters required this will give you a stepped stud so the spigot will still fit through the manifold: ~ a descent engineer should be able to supply the same in the appropriate material.
Completed this on a few cylinder heads without any problems but the taps & dies I have are all imperial (mainly BSF) so not much use to you.

HeliCoils may also be worth looking at for damaged threads.

Good Luck.

As an alternative to helicoil's there is a similar thing called a TIME-SERT which might be more suitable due to the way it installs - it expands the lower thread portions when installing to stop it backing out.

Are brass nuts still used on manifolds ?

If so they will have to be Volvo !!!

"Are brass nuts still used on manifolds ?"

I think it's diy Volvo owners who have brass nuts...

Thanks for the replies, chaps, I'm giving them careful consideration. (160 views before anyone had any suggestions..! At least I'm not asking faqs, eh? :tounge_smile:)

What I think I'm going to do is remove the cylinder head (I only snugged down the c/h bolts), put a dab of tippex (correction fluid) on the base of each stud and onto the c/h, and see whether the studs turn when I try turning a nut onto them.

I was at our local Volvo dealer yesterday (unrelated cause), and asked if they perhaps had some relic mechanic somewhere with experience of cars before they were all modular and digital. They're splendid people there, but the best they had that day was some guy in his thirties (I guess) who didn't have grubby fingers, and all he could say was "Well, they shouldn't be a problem..." :Not_Impressed:

Thanks again for the contributions. Much appreciated.
I'll report back later.

It might be a bind but i would get the head off and get a machine shop to put a M8 helicol in there , that will be better than new ... the studs and nuts are the highest quality exotic materials , and tightening the crimped nuts should not cause any problems .. be sure to use a torque wrench to tighten however , it will be the last one you tighten that snaps off otherwise ! If you use manifoild studs from the D5 they have 5.5 mm hexagons on the end so you can tighten the studs fairly easily without having to lock nuts together .

they died out with the morris 1000 ! With today's turbochargers on just about everything, exotic materials are used for manifold fasteners to cope with the huge forces expansion causes , An exhaust manifold can reach red heat these days ... ( it did on the 760 turbos 37 years ago ... too )