Sudden increase in noise!

[Roni]

What is wrong in this picture?[frame][/frame]
First time I've seen that happen.

[MarkyB]

Did it crack before the engine was run?
Maybe someone jacked the car up on the exhaust box?

[philthehill]

I have seen similar before but just below the manifold joint.
I suspect that a Harmonic vibration had set up in the exhaust pipe/system and it manifested itself in the fracture of the manifold.
I note that you used the heavy duty exhaust clamp which if done up tight would easily transmit any harmonic vibration to the manifold. I am certain that if it was as I believe caused by harmonic vibration and you had used the ordinary pressed type the fracture to the manifold would not have happened as the joint would have had some give in it and slackened before the manifold fractured.

[PaulTubby]

Have had this happen to me on a Austin 1300 years ago, went with a bang on start up for no reason really and sounded like a stock car! I used the car daily for 3 years before this happened and I used the cheaper pressed clamp so don't think it had anything to do with the clamp you have used. Just think its due to age and maybe weak engine steady bushes and or mountings....

[JOWETTJAVELIN]

It's cast iron so we have to be extremely careful when handling it. My bet is on the clamp being tightened-up too much. As the parts expand with the incredible heat, if the clamp is too tight there is no 'give' with the above catastrophic results.

A caution to all, especially owners of cars for which parts are almost impossible to find. The exhaust clamp need only be sufficiently tight to hold the pipe steady for a gas-tight seal.

[Roni]

The manifold's previous history is unknown.It is a big bore type from a Marina 1275, had only just been fitted and a new front pipe made at an exhaust shop. I only drove a handful of miles, not quite home, before the bang happened. The fracture originally was clean and shiny so it looks like it all happened at once. When I cleaned the manifold for paint there was no sign of any cracking.
Now, on with the hunt for another one. Next time I must also not think "That should be ok now."

[katy]

Depending on the actual size, you could run a pipe thread tap in there, screw in a pipe nipple and go from there. Alternately you could get a good welder to weld that up w/cast iron or braze it.
That would keep you going untill you found a replacement manifold

[bmcecosse]

The paint has caused the manifold to overheat........and the exhaust system has been too rigidly mounted........

[MikeNash]

And I think you should include the "Declan Exhaust Bracket Mod" - which if I could find the appropriate thread I'd show a picture of! Basically, our Declan supported his exhaust pipe with a bracket extending from the sump(?) bolts out to the exhaust pipe as it falls from the manifold. (Anyone else know where it is?)
Something I've got to do myself, MikeN.

[bmcecosse]

Dec's bracket is from the gearbox mounting bolts...not sump. I've never found the need for it - but it can't do any harm to fit it.

[Roni]

An update on this. The manifold, a big bore type from Marina/Allegro is now replaced and seems fine now. Time will tell.
It took a while to find a replacement. There were a few available but all had the flange area broken, one having been repaired and then re-broken. One was still attached but on cleaning it up it had the start of a crack.
I found a used one at a BMC parts shop, where he kept a supply of them as "its not that uncommon" for them to break in that area. All of the fractures were right at the edge of the machining and "as cast" finish around the flange area. In some cases quite a step results. It would seem to me to be a stress point created, with the expansion + contraction over time creating the crack.

[bmcecosse]

Thanks for coming back with that info....I'll avoid the big-bore manifold then!