Does anyone have an answer for which is the correct GM RC-29 radiator cap ... a scalloped circular edge, or a smooth circular edge? I have seen pictures of both, claiming to correct. Thanks for any help in this matter. John
The smooth edged caps were what originally came with the cars when new. The scalloped ones were the later replacements. I've used the scalloped one from CARS for over 6 seasons with zero issues. It functions as designed. One thing to note regarding the repros: the rivet is different than an OEM piece. I think both are acceptable in terms of judging but maybe Brad or Duane will chime in to verify.
AFIK, everything said here is true. There is one thing with the scalloped cap, the reason for the scallops. There is a blue line on an original cap that aligns on an original radiator with the overflow hose. At the time, this closed type system was new. By aligning the stripe with the overflow hose, you could be sure it was sealed right and would work. I assume years later some radiators or tanks had been changed. The original cap wouldn't align right. So they made the scalloped type so that if your blue line didn't line up you could turn the disc on top so it would. Then you were supposed to stick a small screwdriver or pick and bend two of the scalloped edges down into the slots that are there. Again this was all due to the new system, and the necks having like a double cam deal on them for the cap. 'Course, you can put the cap on any way you like, just thought you'd like to know the "why".
The radiator cap on the 1970 GSX in the Sept 1970 issue of Road Test magazine doesn't match the original on my GSX. So apparently there were at least two styles used. Both had smooth edges.
Comparing the pictures, the Road Test cap top is a black circle with a white stripe thru it. The other cap top is a white top with a blue strip thru it. They look similar but the colors are different.
Just to add another piece to this puzzle, if you look at the pics of the cap that Brett posted, you will see it has 2 “tabs” on the smooth cover that slip into 2 slots on the body of the cap. That is the way the originals on the cars looked. The factory replacements, with the scalloped edges, have the 2 slots in the cap body, but the cover plate has no tabs on it. On the replacement caps you were supposed to align the stripe with the overflow tube on the radiator and then press the edges down into the slots to lock it in place. That way it allowed the replacement radiator caps to fit radiators with different overflow tube placements. Duane PS, I didn’t see that this was answered above.
Could some one at the factory just decided to put on a different one making it a rare factory defect?
If you’re referring to the photo that Gary posted earlier, I wouldn’t bet the coloring on that black and white photo is any different than the color photo he posted. Just looks different with lighting, flash photography, etc.
Is the rc29 cap used on all 68-72s? I have a 68 350 no a/c and trying to get that factory look going, is the 29 cap correct for it?
