C3 Engine Compartment Blackout vs. Undercoating

[Tom R.]

I'm looking at the heater hose clamp and see inconsistency when blackout occurred vs. undercoating. My two 78s, both show undercoating in the fender skirt while in the engine compartment, my Bowtie shows rust while the 56k Pace Car shows a finish peeling off, which suggests blackout. I know that blackout occurred after paint but when did the undercoating application occur in the assembly process.

PS: I'm referring to the bracket that hosts the hose clamp that is riveted to the fender skirt with rivet heads to the outboard side of the fender skirt.

[Edward J.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Tom, I think anything that was riveted to inner fenders and firewall would have been exposed to blackout and exterior paining and undercoating. Both My 63/72 show these riveted or screwed brackets show blackout and overspray(63). Pretty sure this was. Done before body drop, the undercoat was sprayed sloppy so overspray could be near anything that was near by.

[George H.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Tom,
Here is a picture of my LS4 '73 engine compartment, top down view. Not sure if that will help answer your question. The companrent has never been painted. The engine was pulled and overhauled back to factory specs by Jasper Engines in Indiana in 2015. D&M Corvettes, Downers Grove IL did the resteration work. Their crew was taken back that the engine still had the A.I.R.smog air pump system and EGR Valve on it. A push rod had broken through a rocker arm and they found filings in the oil pan. Runs better than ever now.

If you think it will help. what area of the engine compartment are you referring to and I can take a picture, send it to you?

George Hunt
'73 LS4, M21, Coupe
'73-74 Tech Manuel Team Member.

1973 LS4.jpg

[Tom R.]

Re: C3 Engine Compartment Blackout vs. Undercoating

If you think it will help. what area of the engine compartment are you referring to and I can take a picture, send it to you?

George,
I'm referring to the heater hose bracket that hosts the clamp. The rivet heads can be seen from the inner fender side.

I ask because after 78, power antennas were a popular option and that bracket was installed after body drop and all I have seen are rusted...initially natural metal finish. The bracket design is similar to the heater hose bracket.

If you can post a photo, that would be helpful...but then you would have to unhook the heater hose clamp, which may not be necessary. Its looking pretty congested on that right side fender, probably behind the compressor.

[George H.]

Re: C3 Engine Compartment Blackout vs. Undercoating

I just took a look at the heater hose that runs off the manifold and it runs down under the AC evaberator condenser into the firewall. I can't see any suport camp or connectiion into the heater. I looked under the passenger side front fender and all I see is undercoating and several 6-sider small screws (2 per opening) going into the firewall. No revites. I'm probabily looking at the wrong place. You are refering to the hose that comes runs off the maifold, yes? I can take a picture looking down and you can then help me find the clamp you are refering to. Does having AC make a difference?

George

[Tom R.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Look at your 73 AIM, page 127, item #5, clip bracket. It looks like it is mounted behind the compressor.

[George H.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Got it. I'll take another look.
George

[James G.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Tom,
If it is riveted on, as the earlier C3's like my brother's 73 and like my 71, the rivets holding my heater hose retainer bracket to the passenger side inner fender have undercoating shot over them on the opposite side, the front passenger side wheel arch.

Wheel arches were sound deadened after paint, before body drop. - probably after blackout though a careful study of a light colored car should yield some good data.

[Gary B.]

Re: C3 Engine Compartment Blackout vs. Undercoating



Gary

[Tom R.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Tom,
If it is riveted on, as the earlier C3's like my brother's 73 and like my 71, the rivets holding my heater hose retainer bracket to the passenger side inner fender have undercoating shot over them on the opposite side, the front passenger side wheel arch.

Same as in 78...my question relates to the engine compartment side and the bracket finish

[Tom R.]

Re: C3 Engine Compartment Blackout vs. Undercoating

To wrap this up in the event curiosity strikes other late mode C3 owners...got these images from members of the Indiana chapter and they represent early 78s, Pace Cars (April) and a 79. Apparently sometime during 78 production the natural bracket was blacked out and that process carried through 79.

78 Coupe with lightly rusted bracket...now was that rivet black or blacked out?

Bracket_heater hose_1978 SA.jpg

Pace Car with blackout
Bracket_heater hose_1978 PC 2.jpg

79 with blackout
Bracket_heater hose_1979.jpg

Conclusion...can't assume production methods of early C3 were same in later years!

[Gary B.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Good pictures Tom.
I have a few 68-72's around the house right now, I'll get a few pictures tomorrow when the sun is up. I'm scared of the dark !!
In the 1968 Assembly Manual, UPC 11-13, Sht.B2, Item #6 is the plate (3914535) that is riveted to the inner fender panel, I believe, well before blackout. I think this was riveted to the inner fender panel before being bonded to the fenders. It has a square hole that accepts the hose clamp, shown in UPC 1, Sht. M4. Those hose clips, at least for 68-72's, were a black phosphate finish, and was not painted with black-out.
I'm taking a 69 apart, I'll get a picture of it also. It will show the finish difference.
Gary Bosselman

[Gary B.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Picture from a very original 1968.
2024-01-01 10.39.42.jpg

Picture from a late 1970 LT-1.
2024-01-01 10.42.35.jpg

Picture from a 1969 I'm taking apart.

2024-01-01 10.32.30.jpg

I think this carries through 1972.
Gary B

[Tom R.]

Re: C3 Engine Compartment Blackout vs. Undercoating

So what do you conclude that finish was...the black phosphate or blackout? Also, if blackout, were these brackets installed sometime during body build? Or later and after undercoating?

[Gary B.]

Re: C3 Engine Compartment Blackout vs. Undercoating

Tom,
I took apart the heater hose clip on the 69 this afternoon. The heater hose clip is definitely a dark gray phosphate finish. Sparkles.

The steel plate riveted to the inner fender panel is natural steel, covered with blackout.
I think, my own opinion (and we know what opinions are like) the plate was riveted to the inner fender panel before assembly to the top surround and side fenders. It was covered with black-out when the engine compartment was done. And the rivets were covered with undercoating when the inside of the fender wells were undercoated
Picture of the plate with the heater hose clamp removed...

2024-01-02 13.42.57.jpg

There is also a clamp on the heater box, to hold the heater hoses to the box. It is not as wide, but longer. It is definitely painted black before being riveted to the heater box. The heater box was not installed during blackout.

2024-01-02 13.43.30.jpg

The 68, 2-69's, 70, 71 and 72 here at the house are all very similar. I suspect this carried through up thru 82 ?

Gary B