Could I use M6 bolts instead of M8 for exhaust to manifold?

jsmoke · 25 Oct 2022 at 19:57

I replaced the middle exhaust section and all 3 studs snapped with the nut on at the exhaust / manifold connection.

I removed the manifold and have drilled out the 3 studs to M6 size so far.

I'm now considering options. Firstly do these holes need to be tapped and secondly how bad would it be to just use M6 bolts and nuts instead of the original M8 bolts that were on it?

Ideally I would drill out to say 7.5mm and tap to 8mm (M8) then fit M8 bolts but at the moment I'm wondering if I can get away with just M6 bolts and no tapping?

Slam62 · 25 Oct 2022 at 20:04

Err no drill out oversize tap and put inserts in

tom_e · 25 Oct 2022 at 20:18

You've drilled an M8 stud out to M6 and you want to put an M6 bolt into the blank hole you've now got in the middle of a seized stud. Is that what you're asking?

jsmoke · 25 Oct 2022 at 20:19

tom_e said:
You've drilled an M8 stud out to M6 and you want to put an M6 bolt into the blank hole you've now got in the middle of a seized stud. Is that what you're asking?

Yes. Not much of the seized stud left though.

tom_e · 25 Oct 2022 at 20:23

You can give it a go and let us know how you get on, it would make a funny story.

But in all seriousness no that will not work as slam says get some M8 timeserts or similar and drill out to the size they instruct you to so you can fit them and then use the proper size studs again.

azazl187 · 25 Oct 2022 at 20:25

It will be fine. Just remember to use the correct gasket

andy_mk3 · 25 Oct 2022 at 20:25

Definitely not, carry on and do it properly. If you've managed to drill very close to the centre of the existing bolt then you may get lucky and be able to use an easy out to remove the stuck bolt. If not then you want to drill to 6.8mm for an M8 tap ideally. 7mm will do but you'll remove more material than you should and weaken the threads.

Slam62 · 25 Oct 2022 at 20:26

Alternatively get someone who knows what they're doing to sort it out.

jsmoke · 25 Oct 2022 at 20:37

Slam62 said:
Alternatively get someone who knows what they're doing to sort it out.

We all have to learn.

Slam62 · 25 Oct 2022 at 20:40

jsmoke said:
We all have to learn.

Best to learn before you start a job like that.

kaiowas · 25 Oct 2022 at 21:42

I’m looking forward to the Amazon series that follows this two month saga.

Hawk Tuah · 25 Oct 2022 at 21:56

I'd try to get the original bolts out now you've managed to drill into them.

jsmoke · 25 Oct 2022 at 22:08

Bug One said:
I'd try to get the original bolts out now you've managed to drill into them.

And how do you suggest I do that, with the heat in the exhaust I assume they've essentially welded into the manifold. Bolt extractors or left handed drill bits don't work?

Hawk Tuah · 25 Oct 2022 at 22:12

Concrete · 26 Oct 2022 at 01:14

jsmoke said:
And how do you suggest I do that, with the heat in the exhaust I assume they've essentially welded into the manifold. Bolt extractors or left handed drill bits don't work?

If you have already drilled into the bolt cleanly then before you go for the extractors, which can easily snap I've had success hammering a torx bit into the hole and getting leverage on it that way.

I assume you've applied a ton of penetration oil and are heating up the manifold prior to working on it? (be careful when combining the two

)

jsmoke · 26 Oct 2022 at 02:00

Concrete said:
If you have already drilled into the bolt cleanly then before you go for the extractors, which can easily snap I've had success hammering a torx bit into the hole and getting leverage on it that way.

I assume you've applied a ton of penetration oil and are heating up the manifold prior to working on it? (be careful when combining the two )

Well I've drilled right through into an M8 using an M6 drill bit so only 2mm left of stud so I doubt I'll be extracting much.

Haze · 26 Oct 2022 at 07:36

Slam62 said:
Best to learn before you start a job like that.

Have you read any of his other threads.....

Cadder · 26 Oct 2022 at 07:52

M8 thread minor diameter = 6.466mm
You don't really have 2mm left. It's not worth doing.

If you've already drilled it, how can you guarantee you did it to a 0.233mm tolerance on the stud axis, perfectly parallel? Chances are it's not just the stud that has had material removed.

visibleman · 26 Oct 2022 at 15:47

Slam62 said:
Err no drill out oversize tap and put inserts in

I'm assuming it's the flange on the manifold (pic below), so out of curiosity, why inserts rather than drilling out the stud remains and opting for correct sized bolt/nut?

jsmoke · 26 Oct 2022 at 18:09

visibleman said:
I'm assuming it's the flange on the manifold (pic below), so out of curiosity, why inserts rather than drilling out the stud remains and opting for correct sized bolt/nut?

Correct. I drilled this flange out to M8 as advised above (good advise also). I just fitted M8 studs, washers and a nut. Didn't bother tapping as apparently tapping has no structural benefit just more convenient to position the studs. All turned out very nicely, nice and tight and secure.

Took a while to complete though.