Exhaust Manifold Bolt Holes: Why So Few?
I have an original '78 WS6/W72 T/A that has been stored for a number of years. While getting it roadworthy, I found an exhaust leak between the driver-side exhaust manifold and the cylinder head. This was due to a broken bolt on the last threaded hole of the manifold/cylinder head near the firewall. I removed the head and manifold (unfortunately breaking a few more bolts), which brings me to my issue.
The two end bolts are different from the center four. Each of them is hollow at the tip (threaded end) to a depth of approximately 8-10 mm. This appears to be where the first bolt broke. The bolt at the other end was removed intact.
What is the reason for this design? Is it a thermal issue? It threads into an open passage. Do I need to replace it with a similar bolt, as it seems to affect bolt strength and durability?
I'm the original owner of this Bird, so I know there haven't been any previous repairs.
Ted Cochran
Amherst, NY
Rocky Rotella Responds:
It's not uncommon to find Pontiac engines with exhaust manifolds retained by less than six bolts. It seems that as early as the late-'60s and early-'70s the division assembled some of its engines with four or five manifold bolts per side. By 1972 some of the cylinder heads were cast without provisions for outer exhaust-manifold bolts entirely. It's unclear if this was an attempt to save material costs, but it seems that engineering found satisfactory sealing quality with fewer than six bolts for a short time.
In 1973, the division began casting its cylinder heads with provisions for six holes once again, all of which were drilled and tapped. I can't recall seeing any D-port engines from that point forward assembled with less than six exhaust manifold bolts per side. Pontiac literature contradicts this, however.
According to Service Information Bulletin No. 74-I-34, dated April 5, 1974, the end exhaust-manifold retaining bolts and locks were eliminated from all engines (except the SD-455) with Engine Unit Number 191552, claiming it improved manifold durability. It's unclear whether this practice carried over into the '75 model year, but the '75 Pontiac Engine Assembly Manual shows 12 identical exhaust-manifold bolts (PN 943537) used on all engines that year. The same information appears through 1977, and this seems to correspond with what I've seen over the years.
Hearing that your original '78 Trans Am has different outer bolts is interesting but not shocking. I've never noticed unique fasteners in those positions when disassembling engines, but that doesn't suggest my observation is correct or that your Firebird's engine was assembled incorrectly. I can't imagine a hollow bolt having as much integrity as a solid bolt, however, and suggest replacing the outer bolts with conventional 3/8-16-inch units in the appropriate length. It's a simple fix that will get your Pontiac back on the road in no time.
Cylinder-Head Clarification
I read Jim Taylor's Tech Q&A response to Jeff Knautz concerning his No. 48 heads. A few years back, I was given a R/A-III H.O. 350 out of a '69 Firebird, with an XC block and No. 48 heads. Due to my inability to glean info on this combo, I contacted HPP. I was told the 48 heads have larger cc combustion chambers and a 10.5:1 compression ratio. Mr. Taylor states differently in your Nov. '08 issue. Could you please clarify this for me?Lee CourtneyWhite Salmon, WA
Jim Taylor Responds:
Pontiac didn't offer a R/A-III 350 H.O. engine in a '69 or any other year. The XC-code 350 engine with No. 48 heads was-and is-known as a 350 H.O., having 325 advertised horsepower. This is a very rare engine. The exact same engine with an XJ-code in an A-body was rated at 330 hp. Both had No. 48 heads, and the advertised compression ratio was 10.5:1.