From http://craig.backfire.ca/pages/autos/boxchevy/manual-transmission
Manual Transmission
I have always preferred manual transmissions to automatics. They offer more precise control, are lighter and more efficient. Despite the fact that no GM B-body ever came with one after 1976, I thought I would go ahead and put one in my Caprice anyways. The transmission I chose to use was a Tremec TKO 500, which had five forward gears, and a reverse lockout.
Installation
To help with fitting the transmission, the old 305 engine was used. It is the same size as the 350, so it is a perfect substitute, and I didn't mind beating it up in the process.
Installing the transmission required a great deal of modification to the car, some of which was outside of my capabilities. To start, a large hole had to be cut into the floor to allow the shift lever to enter the cockpit, and because the new transmission was much taller and wider at the top than the old one.
Crossmember Modification
Once a hole had been cut in the floor, the transmission needed to be supported by the crossmember. Not only does the crossmember need to keep the transmission centered, it needs to hold the transmission driveline at the correct angle.
The car was also being lowered and getting significant suspension modifications, so the exact driveline angle could not be established. Instead, I modified the crossmember such that it would keep the tail end of the transmission too low, which allows the use of shims to bring it up to the right position. The shimming itself was not done until much, much later on in the project.
The transmission comes with a mount for a rear torque arm. This interfered with the crossmember, so I either had to cut off the torque arm or cut the crossmember to clear it. I opted to cut the crossmember because it can be reinforced. On the other hand, once the torque arm mount is chopped off of the transmission, there is no way to reinstall it.
Once the torque arm clearance was established, I removed the front tab on the crossmember because there were more clearance issues with the bushing that the transmission sits on. I then designed a flat platform for the transmission to rest on the crossmember, and brought the crossmember to a local welding shop to be finished up. It cost about $100 for them to do all of the necessary welding to strengthen the crossmember and install the platform.
Covering the Hole
After the driveline angle was established with shims between the transmission and crossmember, the hole in the cockpit could be dealt with.
The first step was to make sure that the transmission had sufficient clearance around the floor of the cockpit. When the engine is putting torque through the drivetrain, the engine and transmission will tend to shift to one side. To avoid contact between the transmission and floor, I dragged a very thick rope over top of the transmission and made sure it did not catch anywhere. This proved that there was around 1 inch of clearance everywhere, which should be sufficient. A small amount of contact during very hard acceleration would not be the end of the world, but it should be avoided.
The second step, once clearance had been established, was to cover the huge hole that was left in the floor. To do this, I put a thick layer of cardboard and newspaper over the top of the transmission and applied about seven layers of fibreglass to it.
A small hole was left around the shifter, which was then covered using a flexible rubber sheet. This allows clearance for inevitable shifting of the transmission, and provides a good seal in the cockpit.
At this point, the car was ready to handle the new transmission, and just needed a shift lever and clutch setup to be complete.
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