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January 21, 2020


Intake Manifold

The intake manifold on the GT6 is an aluminum casting that provides passages from each carburetor to three adjacent intake ports on the cylinder head.  In this sense, the engine is a little like two coupled three-cylinder engines, each with its own carb.  It appears that the designer attempted to provide approximately equal length runners for each port, though this meant that the center runner of each bank (cylinders two and five) has a pretty significant jog in its path.  The manifold also includes an integral balance tube between the carb mounting throats, and a passage for coolant, presumably to improve warmup, and to keep the manifold at a relatively constant temperature during operation.

My manifold was grimy, but appeared to be intact.  I think the non-standard fittings at the far end were my early attempt to mitigate the effects of the stuck-open heater valve.




Removing the various attachments to the manifold casting produced some heartache.  First, the heater valve left most of its threads behind when removed.  Then the banjo bolt was being troublesome.  When I had to resort to an impact driver, the bolt decided to sacrifice itself rather than come out peacefully.




Removing the remnants of the banjo bolt was a couple of hours detour.  The threads in the manifold were a little damaged, but running a tap into them improved them a lot.




There wasn't really a lot to do inside the manifold.  I had already matched the head-side openings to the head gasket decades ago.  Likewise with the carb side openings.  Other than that, there was just a little shaping of some ridges on the carb side where the machined throats meet the cast cores.  It wasn't much--maybe only 1/16" or so is all it took to make a smooth transition.

After cleaning up the casting with soap and water, I remembered that I had polished up the topside back in the 80s.  It still looked sort of OK, but there was a lot of stains and mottling on the shiny surface.




To try to even out the surface finish, I popped the manifold into the blast cabinet and gave it a light once-over.  The result was a more even, but matte, finish.




A little wire brushing and some Scotchbrite brought back the shine. 




Before I went any further with the surface finish, I wanted to check out the mating surfaces.  This is blue layout compound on one of the head side mounting flanges.  Giving the flange a few rubs on a flat plate with fine sandpaper glued to it easily shows the high spots.  Not surprisingly, it was high where it was clamped.  Once I knew the pattern, I could check the other flanges with a straightedge and feeler gauges.  The un-flatness was a little under 0.005" on all of the flanges.  That's not huge, and may not even represent a problem, but on the other hand, it's pretty easy to fix.




Skimming 0.005 cleaned up the flanges nicely.  They are now all flat and co-planar.




I did the the smaller sealing surfaces, too.  They took quite a bit more to clean up--almost 0.020".




Then I had to decide what to do about the surface finish.  The manifold looked OK from a few feet away, but up close, it was easy to see some imperfections--mostly porosity.  Also, I knew that even though the aluminum will take a pretty good polish, it won't last long in the hostile environment of an engine bay.  I decided to try a clear powder coat to preserve the shine.  Clear powder coat goes on white.  It was a tight fit in my small oven.




Well, the clear powder coat was a big fail in several ways.  First, I decided I didn't really care for the high gloss finish.  Second, there was a lot of bubbling on some areas.  I blame this on a combination of the porosity and my cleaning regimen.  Before applying the powder, I washed the part in detergent and water.  Even though I dried it well, I think there was still water in some of the pits.  The water boiled off in the oven, causing the bubbles in the melted powder.




An additional reason for abandoning the idea of powder coating the manifold was that I began to doubt that the coating could withstand the temperatures near the head and exhaust manifold.

In the end, I decided to just paint the manifold with a good high temp engine paint.  This is all of the parts associated with the manifold, including the replated original studs and hose barb fitting.  The heater valve is a popular replacement that uses a much more reliable internal design.




On the shelf with this dude.




I really wish I could have achieved the look I wanted on the manifold without resorting to paint.  But, in the larger scheme, it's a pretty small issue.

Cost was around $30 for the heater valve.

Comments to Ed at mailto:elhollin1@yahoo.com

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