TH350-NP203 to TH700R4-NP208

[1985K10]

My 77 GMC has a TH350-NP203 setup, but I was considering swapping to a TH700R4-NP208 setup. I have a 700R4-208 truck(1984), I know id have to use the 208 crossmember and drill the frame, but what about the trans tunnel, could I bolt in the "cover" from the NP208 truck? And are the u-joints(driveshaft to diff) the same??

I would like to use the 77 axles, and is the shifter the same? I know the marker thing would need to be changed(P R N L1 L2 vs. P R OD D 2 1)

Im sure someone has done this!!!

Thanks!

[72 CustomCamper]

I'm not sure about the 84, but my 86 K/10 doesn't have a removeable tunnel...


Chuck

[u2slow]

The older truck should have ample tunnel clearance for the TH700/208... I have a buddy that went the other way (TH400/203 w/86 cab) and the clearance was tighter because it was longer overall...

Hard to say on u-joints... don't expect them to be the same. Rear shaft will need to be different style for the 208 slip-yoke. Lengths on both shafts will likely need to be changed.

[1985K10]

I have the front and rear driveshaft from the 84, for length nothing should change(assuming the axles and motor are in the exact same spot from 73-87)
im just concerned about what u-joints to use for the 84 driveshafts, and 77 axles....

And the tunnel...I know in 77 the shifter comes up through a hole in the floor, and in 84 the shifter is mounted to the floor...can I just "cut and paste"?

72 CustomCamper i remember now that my 85 tunnel did not unbolt, so im sure its the same 81-87.

If this swap is gonna be too much work im just gonna put a part-time kit in the 203, and have the TH350 "freshened up".

Thanks, keep any info coming!

[u2slow]

http://www.neapco.com/pdf/2005universaljointcatalog.pdf

Looks like the u-joints are the same. (Spicer 1310)

The bigger question is if both trucks use the GM front axle (in which case, the driveshaft should swap.) However, the '77 may have a Dana axle, and that uses a different length driveshaft.

[1985K10]

I think both trucks have the GM front axle, theres a 4-bolt flange that bolts the driveshaft to the t-case, vs. the spicer driveshaft with the u-bolt setup.

[us111]

hello,,i'm no genius on t-cases but just recently had to pull my 203 and tranny,along the way done alot of reading up on the 203,,it's fairly cheap and easy to swap to part time,lot easier than the swap your talking about..here's a link if it works you can check out if you want..


http://www.4wheeloffroad.com/howto/47538/

[Jonboy]

but the swap will net the 700R4 also, resulting in a LOT better mileage that a part time 203 ever thought about. You should be able to use the front driveshaft for sure. If they have the same wheelbase, you should be able to use the rear driveshaft as well (LWB to LWB, etc.). If both trans tunnels unbolt, I would go that way. If not, just use the 84 floorpan as a template, and cut away. I would guess everything will bolt up. Make sure you get the TV cable adjusted correctly.

[1985K10]

us111-Thanks for the link, I checked that site out before, good info. I still am considering the part-time conversion kit and rebuilding the TH350(although it works fine), but as jonboy said, ill get better mileage with the 700R4, just hoping someone else did the conversion and could warn me of any big problems.... Both trucks are LWB, I have all the parts for the 700R4(tv cable, lockup wiring, etc.) and the tranny hump, but the tranny itself is toast....

Thanks for the info!