#41  
Old 12-13-2020, 10:52 AM
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When I had my original 6X-8s form my 78 and a set of 6X-4s that are on the 81's 400 I did not measure depth, chambers looked pretty similar but the OD of the chamber was smaller on the -4 than the -8. Very noticeable when you laid a head gasket on them. So not sure if they could have done the same on some 48s and be a subtle change.

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  #42  
Old 12-13-2020, 11:55 AM
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"It would make sense the 66cc 48 heads may have been originally intended for the 350 engines and somehow ended up on the 400's. It makes no sense they wanted a d-port 400 to have more compression than the top dog RAIV engine."

Still, they put 66cc #12 heads on the second top dog 1970!
It wasn´t compression alone that made the RAIV faster than the RAIII either year.

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Last edited by Kenth; 12-13-2020 at 12:03 PM.
  #43  
Old 12-13-2020, 12:22 PM
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Quote:
Originally Posted by Dragncar View Post
Dan Whitmore told me otherwise.
He took heads right off Pontiacs in the 60s and cc,ed them. 350 HO #48 actually had 64.5 ccs.
He gave me a list of all the actual head cc when by the book they were 72 ccs.
Not an expert by any means on the #48 heads but both of the two sets I checked many,many,many years ago at a speed shop were in the 64-65 cc range IF they had a small dust off on the head surfaces with a Winona Van Norman Surface Mill. I would agree with Mr Dan Whitmore (RIP Dan).
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  #44  
Old 12-13-2020, 01:14 PM
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Quote:
Originally Posted by Kenth View Post
"It would make sense the 66cc 48 heads may have been originally intended for the 350 engines and somehow ended up on the 400's. It makes no sense they wanted a d-port 400 to have more compression than the top dog RAIV engine."

Still, they put 66cc #12 heads on the second top dog 1970!
It wasn´t compression alone that made the RAIV faster than the RAIII either year.
Agree and that still doesn't make sense. If one ordered the right options on a RA III it would hold its own against a poorly optioned RAIV.

By chance would you know if both engines used the same head gaskets or know the GM part numbers for them?

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  #45  
Old 12-13-2020, 05:18 PM
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I can't comment on why Jay's head shows an "X" but I'm very curious and would love to find something on Pontiac letterhead that explains it.

While writing my Firebird books, I had a chance to look at some original blueprints. According to the notes I took, the #48 blueprint has chamber volume of 66.16 cc while the #12 shows 69.27 cc. The chamber shapes of #48 and #12 were identical. The added volume comes from the fact that the #12 chamber is positioned approximately .030-inch deeper into the casting. Deck surface milling makes that a moot point!

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Old 12-13-2020, 05:40 PM
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So were the OEM RA4 pistons identical to a ra 3 other than being forged. My old books I have the L2398 written down as the replacement ra4 pistons and I can’t remember which piston was for the 3, I was thinking it was the defunct l2279 and it had a big bevel on the top. 6.7 cc vr’s and a 4 cc bevel around the top? Been awhile, I could have that wrong, there was a 8vr piston that looked similar on paper with a heavy pin and he l2262 was for the rest of the 400s.

Was 9790910 the factory head gasket from 67-70? I recall there was one other thicker head gasket used in the lower performance apps, and the 9792995 was the thin gasket, I always thought it was a GM dealer over the counter deal.

The 48 and 12 do like alike to me kenneth. But it would be interesting to compare the head I have to a 12. The head I have could just be a production anomaly. Either way that head I have is clearly a little less cc than the other 72cc small chamber d port’s. As near as I can tell the first castings dates for the 12s occurred 7 days after the casting date of 48 head I have. Hard to believe they cast a 48 after they started the 12s. Unless like Rocky mentioned, it could have been something to do with the delayed Firebirds.


Last edited by Jay S; 12-13-2020 at 06:00 PM. Reason: Edit
  #47  
Old 12-13-2020, 05:52 PM
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Originally Posted by PAUL K View Post
By chance would you know if both engines used the same head gaskets or know the GM part numbers for them?
Yes, the same gasket, #9790910.
Nominal thickness .040/.045", crushed .038"
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Old 12-13-2020, 06:02 PM
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Rocky would there be a way to tell if that combustion chamber on the head I have matched the 12 ? 69.2 cc is darn close to what I measured.

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Old 12-13-2020, 06:14 PM
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I just CC'd a 12 head and it came in right at 70 cc.

Didn't we find out recently Nunzi claimed the X stamped on the 670 head meant something? I wonder if it's the same deal with the 48 heads.

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Last edited by PAUL K; 12-13-2020 at 06:24 PM.
  #50  
Old 12-13-2020, 06:24 PM
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So battery ☠️ in my calipers.

Using a 8515PT FelPro I measure 9/32’s from scribe to chamber with machinist ruler.
Also their doesn’t seem to be any stamping on either head but my heads were steel shot blasted and that’s a typically a heavy corrosion area.
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  #51  
Old 12-14-2020, 12:12 AM
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Luke will measure that head I have and see what it measures.

In regard to the ra 3 having higher compression than a ra 4. I had the pistons wrong for the 69-70 ra3, the piston part # I thought was for the ra 3 was the old domed pistons. The L2440F was the RA 3 piston with the bevel on the top, the total volume was 10.5 cc total. It seems like that was the piston style on all the 4 bbl engine from that era except the ra2 and ra4. Not something we run into anymore since they have not been made for years.
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  #52  
Old 12-14-2020, 06:41 AM
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Thanks for taking the time to do that measurement!

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  #53  
Old 12-14-2020, 08:48 AM
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Quote:
Originally Posted by Jay S View Post
Luke will measure that head I have and see what it measures.

In regard to the ra 3 having higher compression than a ra 4. I had the pistons wrong for the 69-70 ra3, the piston part # I thought was for the ra 3 was the old domed pistons. The L2440F was the RA 3 piston with the bevel on the top, the total volume was 10.5 cc total. It seems like that was the piston style on all the 4 bbl engine from that era except the ra2 and ra4. Not something we run into anymore since they have not been made for years.
I'm not clear on what you are saying?? But the piston in your picture looks like a forged version of the 1975 and later cast 400 piston. That's when they started to using the piston. The 70 RA III used a cast flat top that looked visually the same as the RAIV version except the RAIV is forged. I believe the RAIV piston is identical to the old TRW Power forged pistons.

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  #54  
Old 12-14-2020, 09:15 AM
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Quote:
Originally Posted by PAUL K View Post
I'm not clear on what you are saying?? But the piston in your picture looks like a forged version of the 1975 and later cast 400 piston. That's when they started to using the piston. The 70 RA III used a cast flat top that looked visually the same as the RAIV version except the RAIV is forged. I believe the RAIV piston is identical to the old TRW Power forged pistons.
Yes, agree, that piston does look 75 and later cast pistons. I have thought that to. I have seen that piston listed as the replacement piston for 67-70 400s. But that could be wrong. My old TRW book listed them as 67-70 except ram air. If it was correct it would explain why Pontiac used a smaller cc head.
But it has been years since I have been into a original ra3 or one of that era 400s. I was hoping for clarification. Thanks Paul
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Last edited by Jay S; 12-14-2020 at 09:23 AM. Reason: Edit
  #55  
Old 12-14-2020, 09:33 AM
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Quote:
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Yes, agree, that piston does look 75 and later cast pistons. I have thought that to. I have seen that piston listed as the replacement piston for 67-70 400s. But that could be wrong. My old TRW book listed them as 67-70 except ram air. If it was correct it would explain why Pontiac used a smaller cc head.
But it has been years since I have been into a original ra3 or one of that era 400s. I was hoping for clarification. Thanks Paul
Gotchya Jay.... Just to be clear (for anyone following this thread) the RA III used a flat top cast piston which didn't have a chamfer around the edge.

I'm wondering if the cast pistons actually had smaller valve reliefs that the IV. If I have a few minutes I'll check that out.

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  #56  
Old 12-14-2020, 03:42 PM
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Here are pix from left 1970, middle 1969 and to the right 1968 Pontiac shop manuals.
Obviously Pontiac mixed up the pistons for the 1969 manual.
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  #57  
Old 12-14-2020, 03:52 PM
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Awesome. Thanks Kenneth! Wow the 69 RA4 had tiny valve reliefs.

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Old 12-14-2020, 03:58 PM
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Here are a couple of pics of NOS pistons I have. One is a stock cast unit and the other is a forged RA2/RA4 piston.
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  #59  
Old 12-14-2020, 04:48 PM
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Thanks Rocky. Those look identical. Now that I am looking closer the 69 ram air from the manual appears to be a 350 piston.

  #60  
Old 12-14-2020, 05:03 PM
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Quote:
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Thanks Rocky. Those look identical. Now that I am looking closer the 69 ram air from the manual appears to be a 350 piston.
When flipped the other way, everything is suddenly labeled correctly.

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