I completed my C4->T5 swap a month or so ago and wow, what a difference. I definitely agree with the folks that say a T5 can be one of the best mods done to a vintage mustang. However, I do have one slight problem.

I've got a nagging leak at the seal between the case and the extension housing. I've verified that the leak is not coming from the speedo gear. It's a big enough leak that warrants some attention. Not to mention the grief from the missus after finding our 18 mo old playing in the "neat red pool".

Originally, I noticed that it appeared to be leaking from the lower three bolt holes that attach the extension to the case. I thought maybe I didn't circle the bolt holes with RTV properly. So, I dug in last weekend, dropped the tranny, separated the case from the housing, cleaned the surfaces again, and put them back together making sure that the RTV formed a complete seal around the mating surfaces. I payed close attention to all of the bolt holes to ensure they had sealant around each one of them.

However, after taking it out for a spin, I noticed a leak again. However, this time it only appears to be coming from one of the holes.

One thing I did differently the second time was after I applied the bead of RTV, I supported the tranny in a vertical position with the output shaft up. By the way, a Craftsmen portable worktable works well for tranny repair because it allows the input shaft to hang free while the weight is supported by the case. I then placed the extension housing on the case and let the RTV set up for a day. I then came back and torqued the bolts to spec.

My thought process here was that the first time, I torqued the bolts immediately and possibly let the RTV squeeze out, leaving some gaps in the seal. The second time around I thought I'd let the RTV cure before torqueing the bolts in order to ensure no gaps. Obviously, this was not the case.

My question for you guys is what is the trick to getting a good seal between the case and the housing? Is it possible to use too much RTV? I was using the Permatex Ultra Black RTV. As a stop gap, is it possible to use a crush washer, like on an oil pan, to totally seal up the bolt hole(s) that are leaking?

I'd like to put this issue to bed. I'm going to have some free time this weekend, so I'd like to address it then if I can.