My TH400 shifts very hard - I'd like to go back to a normal smooth soft shift. In previous owner receipt folder there is a receipt from a trans shop but all is says is "install shift kit"...great.
So is it possible to just move some of the check ***** around and undo the shift kit?

 

It will depend upon the shift kit. I have a TransGo shift kit and it uses different springs for various valves, so reverting to factory would mean swapping the springs.

I suppose the easiest thing would be to replace the valve body with one that has all factory parts.

 

It’s pretty hard to make holes smaller in the spacer plate. Your best bet is to either replace the sparer plate with a stock parts, or get another shift kit and follow the instructions for the milder calibration.

 

^^^THIS. Unfortunately every manufacturer's shift kits are different, and you don't know which one you have or what was done. Short of disassembling the valve body and rebuilding it totally stock, there's no easy answer. The changes could be springs (and there are many in the valve body), governor, valve body spacer plate, or any combination of the above.

 

Also, you need to kind of define "hard" -- at least to yourself. If the car is a 4-4-2, IIRC the OG-coded TH400s were calibrated to shift harder and later than their counterparts found in 98s. If reverting to factory specs, you might end up going to considerable expense and/or effort and still not get the smooth-shifting results you want.

My 80K mile unopened TH400(OG) barks the tires loudly on 1-2 and slams me hard in the back on an 85 MPH 2-3 shift, and every 4-4-2 TH400 I've driven behaves similarly.

 

Very good point. While the 442 and Hurst Olds have much more performance oriented shift calibration, the still would have to be “civilized” for daily use. So before tearing into a perfectly functional transmission, you probably need to think about what you are hoping to accomplish.

If you look at the transmission section of the service manual, there are many changes made to the high performance transmissions. Things like no 2-3 accumulator piston in the valve body, a specific valvetrain setup for the 1-2 accumulator, and I have no doubt specific springs and matching spacer plate. I’m sure the documentation is out there somewhere, I personally would love to see the charts that show what went into every valve body calibration.

Having said all that, if the trans shifts hard enough to knock the dash out of the car, I’d say that pretty much confirms someone hss been into it.

TransGo makes a good valve body kit. It contains the parts to make a “Firm-O-Matic” into an automatic. Just be conservative on drilling the holes in the valve body.

 

Before numbers matched I put a switch pitch Turbo 400 in my 70 442. I put an extra bright light switch to control it. I loved it. Just a thought.

 

I installed a B&M shift kit in my 86 F250 at the "tow" setting because I had a 1-2 shift valve spring failure. It doesn't slam into gear at all.

Of course, I know the TH400 and C6 are completely different animals. Just throwing that out there.

 

Tighter shifts = less friction between the frictions & steels = good... unless the trans has a frequent habit of unintentional rapid disassembly?
I second the request to define hard shift. If the car idles into 2nd gear and rattles the dash and chirps the tires, thats hard....and likely a dual hold modification et al. If 1-2 just chirps the tires at WOT leave it alone.
Get pressure readings in all gears with warn fluid and report back.