The importance of measuring bell housing runout.

Kern Dog said
The topic of bell housing runout has gained a lot of attention the last few years with the rise in popularity of the aftermarket overdrive manual transmission kits.
What is it about these new transmissions that the vendors require such precise alignment?
.005 is the limit for the SST kit. If you’re at .006 they won’t honor the warranty??

Here is a screen shot of a YouTube video showing the tapered roller bearing used on the input shaft of the Tremec transmissions.

01 r o t.png

That explains why the runout has to be so small...The bearing can't tolerate much misalignment.