just acquired a 76, 455. My plan is to do a stock rebuild, this engine also came with a pair of 1968 heads. The 68 heads are bare heads. No valves, springs or rocker arm Assy I had the heads checked for cracks, came back OK no cracks. My plan is to transfer the 76 valve and springs to the 68 heads with harden seats. My question is witch rocker arm Assy. is best to use. The 68's alum, or the 76 steel ? It's my understanding the 76 might have slightly larger exhaust valves but that should be no problem for the Mach shop to handle. Thanks Jerry
The 68 rockers are a better rocker, with a 1.59 ratio, if they are in good shape still use them. Will need to plug the 1/4in hole in the driver's side deck in front of no 1cyl
How do you know these heads are of the '68 vintage? casting number? How about some pix of the rockers you are choosing from?
First off you should be having your machine shop tell you weathered 76 heads valves are still in good enough shape to use as anything over 001" of stem ware makes them junk in my book! Also I would not run those used 1976 springs either , just bit the bullet & save up the extra 100 bucks for a new set of springs . Whatever valves you use should get a 30 degree back cut to pick up some nice flow or cheap!
The '68 seats won't be deliberately hardened. They might be work-hardened, but that gets ground away when the seats get cut. Are you planning to have exhaust seat inserts installed? Watch for wear on the rocker arms, at the pushrod end, the center pivot, and the valve tip end. The OEM springs and hardware weren't all that great when they were new.
Why did GM harden the exhaust seats starting in the early '70s? Even GM knew that seat recession was likely to be a problem with unleaded gasoline. The only advantage enthusiasts have now is the number of miles--we're unlikely to put 12,000 miles a year on the car, and we're not likely to pull a trailer unless this is a pickup-truck conversion.
Hence why I said you wouldn't drive it enough. Buick iron is much more durable than the sweepings they used to make Chevy junk, ask any machinist they can cut .030 out of a SBC like butter, but a Cadillac or Buick is rough on bits. My local guy who has done several for me and God knows how many engines used to bitch about it all the time. I've put more miles on 455's than alot of guys as I have daily drove them since I started driving. Put close to 40k on the iron head 425hp combo I drove to high school. They were fine. I would be more leary of having an improperly installed seat dropping and tearing up all kinds of stuff before I would worry about valve recession. But that's jmo
I hear this on Pontiac, Olds, Buick, and Cadillac forums. There's some truth to it, especially in terms of bore wear. It's not so true when it comes to valve seat recession. All of those makes found it in their best interest to induction-harden the exhaust seats years prior to the nationwide reduction in leaded fuel availability. Wear on valve seats is different from wear on cylinder walls. Adding some tin or nickel to the iron alloy doesn't make as much difference on the service life of valve seats. If the seat insert falls out, you need a different machinist.
There is another reason why you want to use the '68 rockers. The '76 steel rockers are not offset on the pushrod end-there are not left and right rockers and the pushrod cups are all centered. It seems the '76 heads had slightly larger pushrod holes, or maybe they just moved the holes over a tad? (and so they had slightly narrower intake ports) so that the pushrods would clear. I ran into this issue when I used 76 rockers on 72 heads and I had 3 intake pushrods rubbing the sides of the pushrod holes creating valve train noise.
