TH400 swap wrap-up
Thought I would just wrap this swap up. Hopefully if there's anyone like me out there doing a search about this swap, this thread will pop up. Keep in mind that I'm not a mechanic and most stuff that I attempt like this ends up being a lot more trouble than it would be for someone who is mechanically inclined. Maybe this will answer some questions.
This is what I started with:
1985 GMC K1500 (half ton) short bed with 4" suspension lift, no body lift
stock carbeurated 305
TH700R4
NP208
The 700 had seen better days. I decided to swap to a TH400. Believe it or not it was pretty close to a "bolt in swap".
This is what I had to come up with for the swap:
1. TH400 from a diesel Blazer (bolted right up to my 305's stock flywheel - most any Chevy TH400 will work.)
2. 32 spline NP208 (from same diesel Blazer, although I could have just changed the input gear on my 27 spline NP208)
3. adapter for TH400 to NP208 (got mine together so no problem)
4. crossmember for the adapter you end up with (I've seen two versions - a short one and a long one)
5. kickdown switch that goes on the throttle pedal and the two-wire connector that hooks to it (One wire goes to 12 volts and the other goes to the kickdown solenoid on the driver's side of the trans.)
6. vacuum line that goes from the vacuum modulator on the passenger's side of the trans to the intake manifold vacuum port behind the carb (I made mine from hard brake line)
That's about it. Here's some things worth noting:
1. I was able to use the same driveshafts as before. It seems like the TH400/NP208 combo is about 3/8" or so longer than the TH700R4/NP208 combo was. Seems like there's more shaft showing on the front driveshaft than before and less on the rear shaft. Not enough to matter though.
2. I used the same trans cooler lines that were already there too. They were a pain to hook up because they went to a slightly different location. I never could get them bent up enough to use the stock location on the torque converter cover with the stock length bolt, so I had to use a longer bolt and a stack of washers to attach the lines to the cover.
3. I didn't get a torque converter cover or a dipstick with my TH400, so I had Imiceman44 send them to me, only to find out when they arrived that they were the exact same as the ones that came off the original TH700R4. Who knew? The bolts to attach the cover to the TH400 were standard thread, but the TH700R4 used metric. I reused the metric bolts from the 700 to attach the supports and cooler lines to the cover, so those holes were metric thread.
4. The dipstick tube from the TH700R4 wouldn't work with the TH400. The holes for the tubes were in different spots on the two trans. I was able to use the tube that came out of the diesel truck; I just had to bend the tab that attaches to the bolt that holds the trans to the engine.
5. I got a stock rebuilt torque converter. I never could find out what the difference (if there is any) was between the diesel converter and the gas one. The converter from the diesel truck had six bolt holes to attach it to the flywheel, but only three of them would line up with the holes in the flywheel on my stock 305. The converter that came out of it with the TH700R4 had only 3 holes. The other 3 looked like they would line up, but they wouldn't quite do it.
6. I had to drill 4 holes in the frame to attach the crossmember. The diesel military Blazer that I got the trans and transfer case from had 1" spacers to drop the transfer case down, so I reused them. I still had to grind some metal off the crossmember under the front driveshaft because it was rubbing.
7. The stock linkage for the column trans shifter bolted right up.
The rest is in this thread. Hopefully some of this info will help someone.
So what are the final results of the swap? When I was first thinking about doing this swap, I heard stuff like "stay with the 700 for the lower first gear and the overdrive." "You're mileage will go down." "Rebuild the 700 or buy a custom 700 from TCI or Bow Tie or whoever." "Your stock 305 won't be able to use the 400." "The first gear on the 400 is so high you won't be able to take off like you were before."
Well the conclusion that I've come to is that the TH400 is way better for me all around than the TH700R4 ever was. It doesn't take off any slower than before. In fact, I think it launches better and gets up to speed quicker now. The mileage around town is better now. I haven't done enough driving out on the road to see if the mileage will go down at highway speeds, but I don't think it will. (I don't think my torque converter on the old trans was ever going into lockup though.) The 400 shifts waaay smoother than the 700 and I don't having problems with engine stalling out like I did before the swap. I don't know how the trans could affect this but it did somehow.
Overall I am quite pleased with the TH400 swap. I know I have a good strong transmisson that I don't have to worry about, the mileage and the
acceleration didn't suffer at all that I can tell. At 55 mph, it sounds like the engine is turning about 300-500 rpm faster than it was when I had an overdrive. No big deal to me.
The only thing I've had a problem with is the connector for the kickdown solenoid falling off the trans, but a new connector will cure that really soon.
New, empty torque converter too.
