73-87chevytrucks.com

73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Leevon on March 21, 2008, 08:56:58 pm

Title: TH400 to 700R4 crossmember
Post by: Leevon on March 21, 2008, 08:56:58 pm

I've been swapping a 700r4 in place of my th400 in this week and it was going well until it came time to fit the crossmember up.  The truck is a 1/2 ton, 2wd, 305 TBI, swb.  I have ready many times that the holes exist for both transmission in the frame but I can't get mine to line up with anything that looks like it was meant to be there.  I elongated the mounting hole a little and still no go.  Even if I drill new holes, the driver side just doesn't fit up well against the frame...

By the way, next time a shop is doing this.  What a messy pain in the rear. 

Any thoughts? 

Title: Re: TH400 to 700R4 crossmember
Post by: HAULIN IT on March 22, 2008, 12:13:38 am

Leevon, Is this in the '87 truck? I don't think a 400 came in there originally, but I could be wrong, I'm not up on all of the details, someone else will help I'm sure. There are several tail shaft housings with different mounts on the 700, What is it out of? I'm using the foreward most mount set-up (two bolts up through) & a mid '70's 1-ton 4 speed crossmember in mine, it bolts to the top of the frame & has a big dip in the middle (works well to clear the exhaust). That should be a routine swap, as many trucks had the 700. Lorne 

Title: Re: TH400 to 700R4 crossmember
Post by: Leevon on March 22, 2008, 12:35:56 am

Yeah, the TH400 was an option in these trucks even with the wimpy 305 and 4.3.  It's an odd combo, but factory for sure.  The 700R4 has an overall length on 30 3/4 making it just over 3" shorter than the TH400.  I guess I just need to grow a pair and start drilling holes, it's just that I thought they would be there.  There definitely is another set of holes about 1" forward from what I need, I also thought 700R4's had the same tailshafts...?

Title: Re: TH400 to 700R4 crossmember
Post by: Blazin on March 22, 2008, 12:39:02 am

If you use the police car / taxi tail shaft housing you can mount a 700R4 in the same crossmember position as the TH400. This tail shaft housing has the holes in it for the trans mount.

Title: Re: TH400 to 700R4 crossmember
Post by: VileZambonie on March 22, 2008, 08:47:46 am

yep they kept using the 400 all along just renamed it the 3L80(TH400) or 3L80HD (TH475)

Title: Re: TH400 to 700R4 crossmember
Post by: Blazin on March 22, 2008, 10:07:41 am

I had an 86 GMC 1500 2x with a 4.3 and a TH 400. Never understood why you would need such a stout trans with 4.3?