Yeller wrote: ↑Fri Nov 10, 2023 4:50 am
The turbo 400 in the Jeeps behind the Buick is even more odd…. it used the old Cadillac/Buick Nail Head bolt pattern and has an adapter from that to the Buick. These were genuinely put together from whatever they could do on a low budget.
Here’s the adapter
This is an interesting story, but something of a tangent from what the OP was asking.
Jeep adopted the Buick nailhead (and Cadillac?) aka "universal" case TH400 for all their automatic-equipped offerings starting in 1965. The nailhead buick blocks stick out toward the transmission bellhousing a couple of inches, making the nailhead case shorter overall than the case for other BOP engines or Chevrolets. This leaves room for an adapter between the block and transmission in other applications, as you see above.
The switch to TH400 coincided with the end of the 230 OHC engine, which was offered - in at least Wagoneers - with a Borg-Warner automatic and the single-speed Dana 21 transfer case. I kinda wonder if they used the nailhead transmissions because that's all that GM offered as a commodity component in that era. As I understand it, other manufacturers in the era such as Jaguar offered the TH400, and it was also the nailhead case transmission. Maybe there are other examples. Did Checker use the TH400 and was it a nailhead transmission?
Could be that Jeep stuck with the nailhead transmission to unify their automatic transmissions across all models, but they had both the 225 V6 and the 350 in '66 and '69 resp. Kinda seems like they could have used the Buick case TH400s if it were available. Indeed, it was 1974 before they used a bespoke TH400 for the AMC engines, relying on the nailhead transmission for automatic Jeep/AMC sixes and V8s until then.
Indeed, for passenger cars, AMC went from the Borg-Warner transmissions to Chrysler transmissions in '72 (?) and I suspect they would have converted Jeeps to Chrysler transmissions sooner if they would work with passenger drop. The bespoke AMC TH400s would never have existed. Or perhaps Chrysler was unwilling to build A727s with the long output shaft the nascent Quadratrac needed. Perhaps AMC did not have the capital to convert Jeeps to the Chrysler transmissions and upgrade to the Quadratrac at the same time. Just speculation.
BTW there are lots of BOP engines that you could swap in to replace the 350. Most popular have been the Buick big block engines, and the big Cadillacs (472, 500). The Cadillacs work well because they are a big step up from the 350, and are a compact engine compared to other GM offerings of similar displacement. I would hesitate to go ahead with a BB Pontiac or Oldsmobile if the starter was on the driver's side - might not work with the Jeep adapter even though they are both BOP engines.
I would mention - the parts specific to the adapted Jeep TH400s are unobtainium today, except for what you can find from wrecks or part-outs. The following Jeep engines used the adapted TH400: AMC 327, AMC 232 (the pre-71 "Rambler" engine in Wagoneers and J-trucks), Buick 225 and 350, AMC 232 (the post-70 version, which may exist with the TH400 in the Commando), AMC 258, AMC 304 (FSJs to '72, '72-73 Commando), AMC 360. If unique parts are broken or unavailable, it may be better to go to the Buick transmission for the 225 or 350. For the later AMC engines, the solution is the later bespoke TH400, for which parts are available. If you have a 327 or the Rambler 232, you''ll have to rely on junkyards. Note that convertors can be rebuilt - check the classic car specialists, maybe online or Hemmings).