73-87 Chevy _ GMC Trucks > 73-87 Chevy & GMC Trucks

What am I looking at?

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Hello All,

New to chevy trucks and just bought a rough 75 C20 as the name implies. Question I am having right now is about some ports on the engine.
Not sure what im looking at. most of the thermostat housings ive seen dont have any plugs coming off of them where this truck has two of them. Also any idea what the welded shut port was? Im looking to try to get the gauges working as only oil pressure works right now, so im hoping one of these is the water temp sensor port, but could be wrong. (Dont mind the bullet casing plugging up one of the ports on the carb LOL). Thanks in advance.

I'll tell you but first I need to know what caliber the Winchester casing is... 8)

30-30 LOL


The figure with tubing ports is a thermal vacuum switch. It reacts to heat in the T-stat housing to signal, or allow, vacuum to open or close, to effect some desired result. 1975 did not have as many of these as later years.

If you live in a smog required state, this site has manuals that guide you to the correct vacuum routing scheme. Ask bd for more info.

The welded fitting is really easy. It's a 3/4" nipple that routes hot water to the heater core in the cab. Some did this when their core leaked and they wanted to stop the flow. Smarter guys just routed the hose from the manifold to the radiator, bypassing the problem.

We look forward to seeing this move from the 'rough' column to the 'sweet'.

I used to have a water neck that looked just like that when I first bought my 81 K10 while living in Colorado. There was a great deal of pollution control stuff on my 305 engine such as a smog pump and other junk. One day I swapped the engine with a 350 and cleaned out all the junk.


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