moved from a tube on the front of the intake manifold to a cap on the left- or right-side valve cover, depending upon the application. 1970 - The "400" is introduced in September 1969 for the 1970 model year, with a bore of 4.125" and a stroke of 3.75". This engine introduced the "large-journal" crankshaft with a main journal size of 2.65" and rod bearing journals of 2.10". This engine also introduced "Siamese bores" to the Chevrolet small-block line, requiring "steam holes" in the cylinder heads and head gaskets, which were used to prevent hot spots and subsequent overheating. The connecting rods, due to the long stroke, are also shorter at 5.565", differing from the 5.7" length of all other Generation I small-block connecting rods. 1980 – Weight reduction though thinner cylinder wall block and light weight head castings. Heads are prone to cracking and blocks typically cannot tolerate an overbore more than .040". 1986 – The rear main seal was changed from a 2-piece rubber design to a 1-piece rubber design that used a mounting appliance to hold it in place. This necessitated a change in the flywheel/flexplate bolt pattern as well as requiring an externally balanced flexplate/flywheel. 1987 – The valve cover surfaces were changed so that the mounting lip was raised and the bolt location was moved from 4 bolts on the perimeter to 4 bolts along the centerline of the valve covers (this design debuted on the Corvette in 1986, and the Chevrolet 4.3L 90 degree V6 the year before). Also changed were the mounting angles of the two center bolts on each side of the intake manifold (from 90 to 73 degrees), and the lifter bosses were increased in height to accept roller lifters; the aluminum-alloy heads for use on the Corvette engines retained the non-angled bolts. Also, all carburetors(except some 1987-89 F-body and B-body models, also the 30 and 3500 model 1-ton trucks through 1989 with either the M-code 350 or W-code 454 engines, in which all retained the 4bbl rochester E4ME carbs) were replaced by TBI (throttle-body injection) fuel injection. 1996 – The cylinder heads were redesigned, using improved ports and combustion chambers similar to those in the Generation II LT1, resulting in significant power increases. The intake manifold bolt pattern was also changed to four bolts per cylinder head instead of the "traditional" six bolts. The cylinder block timing cover lip was