L-78 Exhaust Manifold Machining

[Daniel K.]

Need to have the interface between the exhaust manifold and head machined. I want to be very specific with the machine shop as to the "DO NOT EXCEED LIMIT" when surfacing the manifold. What should they be told?

Thanks.

[Mike T.]

Re: L-78 Exhaust Manifold Machining

Daniel, tell them just what you said in your post. Are you getting the mating surfaces surfaced because of corrosion/pitting or that they are uneven? No doubt the bolts will draw up the manifold enough if there's a 'little unevenness' but you don't want to have too much stress on these cast items. Any idea if they've been surfaced before? Are you going to use gaskets upon reassembly or go without gaskets? I'm sure if you explain your desire to keep the cutting to a minimum, a decent shop will understand.

[Daniel K.]

Re: L-78 Exhaust Manifold Machining

Mike,

Thanks for responding.

No clue if the manifolds have been previously surfaced. They are installed with gaskets now and the intention is to use no gasket. Car has been sitting for most of this year, but last year, the bolt under the alternator backed out and I put a little mileage on the car in this condition. Other posters at that time suggested I have the manifolds surfaced and reinstall w/o gaskets. On reinstallation, I'll use new bolts as recommended by others on this forum along with new french locks. Anything else I should be considering?

Thanks again and Happy Holidays.

[Mike M.]

Re: L-78 Exhaust Manifold Machining

i've never had to have the head's surfaced as i've never seen then warp. exhaust manifold however, commonly warp from years of high temp. i've never had to machine more than about .010" to correct the warpage. with the manifold removed, simply take a straight edge across all the manifolds mating surfaces and you'll be able to determine the amount its warped. use a feeler gauge to determine the air gap between the straight edge and the manifold. if you're going to have the car judged, i wouldn't use gaskets as they weren't used originally. any machine shop should be able to surface your manifold. good luck,mike

[Daniel K.]

Re: L-78 Exhaust Manifold Machining

OK, Thank you Mike. Was only interested in the machine surface on the manifolds and not the heads. Not even sure I need to do this yet and I will use your technique for determining if machine work is required.

Have a good holiday season.

Dan K.

[Timothy B.]

Re: L-78 Exhaust Manifold Machining

Daniel,

If you search in the archives I believe this is discussed with someone using a sealer at this interface and no gaskets. The product used escapes me but it's something to consider.

I think in all GM complete engine gasket kits there are gaskets for the head/manifold area. They probably would not do this if this was not a problem.

[Pete B.]

Re: L-78 Exhaust Manifold Machining

Papa smurf Whittington suggested in a previous post Permatex Ultra Copper. I machined mine at work, and put a "schmear" of it on for good measure. You cannot see it, hope it helps.

[Daniel K.]

Re: L-78 Exhaust Manifold Machining

Tim & Pete,

Thanks for sending your information on the manifolds. It will be interesting to see if they are in fact warped any at all. None the less, it is time to "pretty them up" as sometime this next season, I want to have the car judged again. Also, if Dick has a suggestion on a fix, I'd venture to say this is what should be done.......

Thanks again & Happy Holidays.

[Tony S.]

Re: L-78 Exhaust Manifold Machining

Make doubly sure that when you machine the inner face of the intake manifold mating flange that the planing remains parallel with the corresponding plane of the outside of the flange. The inside surface and the outside surface needs to remain parallel. If they don't remain parallel, when you tighten down your exh manifold bolts, it can snap an exhaust manifold flange. Trust me, I learned the hard way.

Good luck.