From http://craig.backfire.ca/pages/autos/boxchevy/bell-housing
Bell Housing
I had to replace the bell housing that the transmission came with because it was cracked. I bought another factory bell housing for a T5 transmission, but it was also cracked. I instead bought a Lakewood scattershield, which should have no chance of cracking because it is made from hydroformed steel. It is also SFI rated, meaning it can be used for racing. On the down side, it cost over $500 instead of $5, and weighs 40lb instead of 5lb.
Indexing the Bell Housing
For this project, I decided to take the time to index the bell housing. Due to manufacturing tolerances of both the engine and the bell housing, it is possible to have the transmission off-center with respect to the crankshaft, which puts a great deal of stress on the front bearings and other internals of the transmission when under load.
To index the bell housing, a dial indicator is fitted to the crankshaft with a magnetic holder, and the crankshaft is then turned. If there is any runout, the dial indicator needle will move.
The maximum runout specified by Tremec for the TKO 500 is 0.005". This means that as the dial indicator is turned, it must not show a difference of more than 0.010" in either the horizontal or vertical direction. If it does, special offset bell housing dowels must be fitted to the back of the engine.
On my engine, the runout was almost exactly 0.005", so I decided to leave the dowel pins in the engine alone. It was not worth pounding them out and risking cracking the block or causing other damage.
Had the runout been greater than 0.005", I would have used offset dowel pins, which come in 0.007", 0.014", and 0.021" offsets. These pins are installed into the back of the engine, and clocked in such a way that they remove the excess runout. For example, an engine which has 0.011" of runout in a downward direction would use the 0.014" dowels in an upward direction to give a new runout of 0.003", which is acceptable. Had I done this with my engine, I would have used the 0.007" dowels to get a runout of 0.002" instead of 0.005", which is the maximum.
It is also possible for the bell housing to cause the transmission to be angled relative to the engine when viewed from the top or side. This is almost impossible to remedy without machining one or both surfaces on the bell housing, which could be costly. Luckily, mine was perfect.
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