bob k. mando
Moderator
Both the Chevy and BOPC ( Buick, Olds, Pontiac, Caddy ) transmissions were 6 bolt housings for most of their production life. The original Chevy housing was 7 bolt and thats why their trans housings have a peak. In fact, most Chevy blocks were manufactured with the top center bolt hole drilled and tapped long after Chevy had changed to a 6 bolt housing.
So how do you tell the difference between the common as dirt Chevy pattern and the far more desirable ( to us ) BOPC housing?
1. Well, the first and most commonly stated visual reference is the Chevy peak versus the BOPC saddle at the top of the bell housing.
Here's a picture of the Chevy ( L ) and BOPC ( R ) pattern bells:
Notice the engines in the background? We'll be taking a closer look at them in a second.
2. Also, only the two bolts below the alignment dowels share the same position between the two patterns. This is why a dual-pattern housing has 10 bolts, all the holes above the dowels must be duplicated. A Chevy bell has two bolts on each side aligned vertically with the respective alignment dowel. A BOPC bell only has the bottom bolts aligned vertically with the dowel and the higher bolts are moved inboard.
Here is the closeup of the blocks. A Chevy 350 and 1970 Buick 350:
Then we have the differences between the most common GM automatics, the TH-400 BOPC short-tail ( L ), BOPC long-tail ( C ) and Chevy TH-350 ( R ):
Here's a Super Turbine 400 ( early name for what is basically a Turbo-Hydromatic 400 ) for a 1965 401 Nailhead. Notice that even though it's still a 6 bolt pattern ( the second hole up from the bottom on both sides is for a guide pin, not a bolt ) it's wildly different from the later 'standardized' patterns. I have no idea if this pattern is found on anything besides Nailheads.
back of 215 / Land Rover engine
So how do you tell the difference between the common as dirt Chevy pattern and the far more desirable ( to us ) BOPC housing?
1. Well, the first and most commonly stated visual reference is the Chevy peak versus the BOPC saddle at the top of the bell housing.
Here's a picture of the Chevy ( L ) and BOPC ( R ) pattern bells:
Notice the engines in the background? We'll be taking a closer look at them in a second.
2. Also, only the two bolts below the alignment dowels share the same position between the two patterns. This is why a dual-pattern housing has 10 bolts, all the holes above the dowels must be duplicated. A Chevy bell has two bolts on each side aligned vertically with the respective alignment dowel. A BOPC bell only has the bottom bolts aligned vertically with the dowel and the higher bolts are moved inboard.
Here is the closeup of the blocks. A Chevy 350 and 1970 Buick 350:
Then we have the differences between the most common GM automatics, the TH-400 BOPC short-tail ( L ), BOPC long-tail ( C ) and Chevy TH-350 ( R ):
Here's a Super Turbine 400 ( early name for what is basically a Turbo-Hydromatic 400 ) for a 1965 401 Nailhead. Notice that even though it's still a 6 bolt pattern ( the second hole up from the bottom on both sides is for a guide pin, not a bolt ) it's wildly different from the later 'standardized' patterns. I have no idea if this pattern is found on anything besides Nailheads.
back of 215 / Land Rover engine
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