Problems with LT1 Intake Manifold

[thesound]

I've taken up metal polishing as a hobby in the last 8 months. I've recently finished working on an 86 Corvette TPI setup. During the entire process of finishing that manifold, I encountered several areas where tiny air bubbles in the cast had surfaced after I had gone through the initial sanding and gotten all of the rough cast removed, and had started in with the black emery compound. After these air bubbles surfaced, (and after being severely ****ed because I thought I was finally done with sanding after countless hours), I got the sandpaper back out and started to work AGAIN trying to get rid of these air bubbles. Well guess what? It STARTED out with just 3 or 4 air bubbles, but as I began sanding them out 3 turned into 10, which turned into 30, which eventually left the surface of the manifold looking like a damn piece of Swiss cheese. So I threw up my hands and gave up on that endeavor, thinking that I had maybe happened upon a badly or defectively casted manifold. I just decided to cut my losses and try and get as good of a polish on this thing as possible with what I had left. I eventually got the manifold looking halfway decent, (perfectionist), from the "5 feet away" standpoint, but if you get close enough those air bubbles are still there taunting me.
Okay now the good part. I've recently started working on an LT1 intake, with higher hopes this time. Been working on it for about the last 10 days. All was going well in other areas UNTIL I got the the rear part of the manifold down around the area where the distributor hole would be on a conventional intake. Same thing as with the TPI manifold, friggin air bubbles. So now I'm thinking maybe this is just a normal thing, seeing how it's occured on 2 different intakes. So I got ambitious and really went to town on that spot with the sandpaper. I'm now about to chuck this manifold into the street, because these damned air bubbles aren't going away; they only seem to multiply. I'm pretty sure I've gotten at least 1/32's deep into the cast, maybe even as much as 1/16's. I'm also pretty sure that it isn't my sanding technique, because if it was then I'd be seeing air bubbles in every place I'd sanded, instead of in just certain spots. So here's what I wanna know:

(A) Has anybody who's worked on intakes or other similar parts that have thick casts to them ever encountered this problem and what did they do to fix it?

(B) In other threads I've read about the copper plating process, whenever someone referred to "building and filling", was that for the purpose of getting rid of something such as air bubbles in the cast? Or is that for a different purpose?

I'm trying to figure out if I need to quit while I'm ahead with going any further with the sanding in this area and just finish sanding the rest of the manifold and hope for the best when it gets to the chroming stage, or do I need to keep going deeper because these bubbles will eventually clear up? (I'm imagining that the air bubbles would be all the way through the cast, and I'd just end up sanding till there was nothing left.) Please tell me that after all this work I'm not just gonna be stuck with something that looks like a POS. Anyway this problem's got me tied up in knots, and I'm losing sleep trying to figure it out. Any help or suggestions would be greatly appreciated.

[jroger13]

I polished my intake on my 1990 corvette and was very disapointed to see the same result, AIR BUBBLES!!!!!!
i have no idea how to remove them but i believe this problem to be poor quality aluminum!!!!

well let me know if you find a way to remove them.

GOOD LUCK!!

[thesound]

Ravetek: excellent work. I take my hat off to you. I understand totally about what you mean when you said "I learned my lesson." I'm trying very hard not to let this new hobby become an obsession. I'm just the type of person who, if you handed me a pair of tweezers and said, "You see that mountain over there? I bet you can't move it 100 feet in any direction using these," I'd be hell-bent on doing it just to prove them wrong. (I know I'm crazy, the voices in my head won't stop reminding me.) I DO plan on having this intake chromed. But I don't want to just give up halfway through the battle and take it to the chrome shop and ask them to finish the job for me. The air bubbles are just a minor annoyance at this point. I'll eventually get it looking as good as the TPI intake, (I need to take some pics of that thing and put them up on here), but I want the satisfaction of knowing that once I get this motor put together that I can honestly say I had a hand in ALL the stages of building it. If you beat somebody in a drag race, or win a 1st place trophy at a car show, there's usually only two ways you could have done it: you either wrote somebody a big fat check and said, "Build it for me," or you buckled down and wrenched out all the blood, sweat, and tears it takes to have that 1st place car. Even if the guy who's got all the dough to have the best car out there beats me, there's still one thing I've got that he'll never have: respect.

[etyrrany]

What you see as air bubbles are in fact that, air bubbles! They are suspended in the aluminum during the casting process. To the aluminum casting industry this is known as "Porosity".

When I worked in a die casting factory our parts got x-ray'd twice per 12 hour shift just to check for porosity. Porosity can be a very dangerous thing. If it happens on a load bearing member of say a suspension the section of aluminum with the porosity has a much much higher chance of cracking. On a transmission housing casting the section with porosity can even leak. Normally QA standards should prevent parts with porosity from getting on parts. But, as we all know, GM doesn't exactly have the best QA standards or the most reliable cars out there.

What can you do? Well, getting the part x-ray'd would be nice, but who has money or the facilites for that. You could try sanding even more and see if the air bubbles disappear. Just be careful of the intake wall thickness so you don't accidentally sand a hole in it.

etyrrany

PS. We used to do C5 differential housings at this die casting factory. The die was so old that we could only manage a maximum of about 10 parts per hour, and the castings were ugly as hell!

[thesound]

Okay so I've been searching through the different product categories and I think I may have found something that will take care of the air bubble issue. It's the Pot Metal Repair Paste. Says that it is zinc-based, and can be copper plated after it's been applied. It's an extra step that I didn't look forward to doing but if it's what is needed to smooth this thing out then so be it. Caswell, if you foresee any problems with applying this to an intake manifold, sanding it smooth and then plating over it, please chime in.