Tall, magnesium intake manifold cracking issue

[1QuickS]

I am currently working a project with the 911R tall intake manifolds and discovered cracks in a couple of the mounting flanges. I have a weld repair service provider that is recommending some reinforcing weld material to help prevent the cracked flanges from cracking in the future.

It is my understanding that these manifolds are susceptible to this type of problem. I think that a reinforcement of all the flanges would be in order to prevent the issue from re-occurring elsewhere. If these manifolds suffer other cracking issues I would also like to address these as future problems to be resolved at this time.

Comments or experiences would be helpful.

[Alan.uk]

Hi Paul, we have mag welded plenty of mag manifolds. In many occasions they are caused by people prying them off or them no longer being flat. We also re-face them once welded.


HTH

Alan

[Frank Beck]

And...

herein lies the problem with using 40+ year old race parts (see: Throw 'em away after they're timed out.)
This is one reason the Carl Thompson/Tilton version back in the day were so cool. They looked just like mag but being aluminum one didn't have to worry about breakage like this.

Happen to have a set if you want to borrow...

[1QuickS]

Hi Alan, I've sourced a good mag welder over here with lots of historic mag welding experience, Thank you!

I figure to have them welded to have thicker flanges with the weld tying into the main runners and fly cut the bases flat...again. I found the defects after my machine work and chromating was finished. I suspect over-torquing, compliant gaskets plus vibration causes this issue.

Thanks for the offer Frank but will stay with these and have them repaired as best as possible.

[Frank Beck]

Oooooooh.

I misunderstood.
I thought you were casting reproductions.

Sorry.

[chris_seven]

Paul,

Not really an issue of tightening or gaskets just really about fatigue and loss of properties in old sand cast magnesium.

There is quite a bit of weight bouncing up and down on the flanges and the cracks will develop in the most highly stressed areas.

Old fashioned sand cast mag alloys are also unlikely to be able to maintain their mechanical properties for more than 40 years so their ability to tolerate these stresses will fall with time. Hence Frank's comments about parts having done their time.

Welding some of the older Mag Alloys can be problematic due to the formation of intermetallic compounds in the grain boundaries of the Heat Affected Zones and a solution treatment, prior to welding, is often recommended.

The intermetallic that causes these problems is a Beta-Mg17Al12 compound and is known as a 'massive phase' and it can occur in alloys that have over-aged. If you weld mag alloys with this phase present it is quite likely to develop hot cracks in the heat affected zones which will then fail again after a relatively short time.

Heating to 400 degC for 24 hours can cause these massive phases to re-dissolve and help to eliminate this potential problem.

It may also be an idea to stress relieve the manifolds after welding and I would suggest that 200 deg C for 16 hours may be a suitable treatment.

[1QuickS]

Hi Chris,

Excellent input regarding the heat-stress relief. I am quite aware of this issue in large structural weldments so the suggested time/temp input is VERY useful.

I do believe that the gaskets compress locally under the bolt tension while the larger diameter doesn't compress the gasket as much resulting in the typical plastic deformation and bending of these ears; I typically fly-cut these flanges on Webers, MFI manifolds and intake manifolds to return them to a flat condition. Any vibration, especially the vibration about the engine centerline results in lateral loading of the Webers which results in dynamic rocking loading of these fragile ears...not to mention the change of section and resulting notch sensitivity at the fillet between the flange and the runner. That the cracks are on the top side and not the bottom implies tensile stress in the top and compression in the flange; compressive stress is fatigue proof in comparison to tension stress; again suggesting the flange is bent down from gasket compression and bolt tension.

[Frank Beck]

Hi Chris,

Excellent input regarding the heat-stress relief. I am quite aware of this issue in large structural weldments so the suggested time/temp input is VERY useful.

I do believe that the gaskets compress locally under the bolt tension while the larger diameter doesn't compress the gasket as much resulting in the typical plastic deformation and bending of these ears; I typically fly-cut these flanges on Webers, MFI manifolds and intake manifolds to return them to a flat condition. Any vibration, especially the vibration about the engine centerline results in lateral loading of the Webers which results in dynamic rocking loading of these fragile ears...not to mention the change of section and resulting notch sensitivity at the fillet between the flange and the runner. That the cracks are on the top side and not the bottom implies tensile stress in the top and compression in the flange; compressive stress is fatigue proof in comparison to tension stress; again suggesting the flange is bent down from gasket compression and bolt tension.

or it could just be crappy worn out parts.

[Brad Davis]

Hi Paul,

My tall mag intakes had cracks on nearly every ear. I had them all welded and they turned out very well. Below are a couple pictures of the cracks. Tonight I will get a few pictures of the intakes after the welding. I was pleasantly pleased with the result of the weld and finish. There was no porosity or anything really ugly.

Best regards,

Brad

[Mr9146]

And Porsche built ENGINE CASES with this stuff. No wonder they leak, eh?