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#41
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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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Skip Fix 1978 Trans Am original owner 10.99 @ 124 pump gas 455 E heads, NO Bird ever! 1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand 1965 GTO 4 barrel 3 speed project 2004 GTO Pulse Red stock motor computer tune 13.43@103.4 1964 Impala SS 409/470ci 600 HP stroker project 1979 Camaro IAII Edelbrock head 500" 695 HP 10.33@132 3595lbs |
#42
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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. Last edited by Kenth; 12-13-2020 at 12:03 PM. |
#43
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Quote:
Tom V.
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"Engineers do stuff for reasons" Tom Vaught Despite small distractions, there are those who will go Forward, Learning, Sharing Knowledge, Doing what they can to help others move forward. |
#44
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By chance would you know if both engines used the same head gaskets or know the GM part numbers for them? |
#45
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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! |
#46
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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
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Nominal thickness .040/.045", crushed .038" |
The Following User Says Thank You to Kenth For This Useful Post: | ||
#48
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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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#49
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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. Last edited by PAUL K; 12-13-2020 at 06:24 PM. |
#50
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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. |
#51
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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. |
#52
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Thanks for taking the time to do that measurement!
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
#53
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#54
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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 Last edited by Jay S; 12-14-2020 at 09:23 AM. Reason: Edit |
#55
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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. |
#56
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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. |
#57
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Awesome. Thanks Kenneth! Wow the 69 RA4 had tiny valve reliefs.
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#58
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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
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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.
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#60
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