Difference in TH400's

[PlumCrazy]

Ok so this is bugging me, what is the differce between the Nailhead, BOP, and AMC TH400's...is it just an adapter ring or is it a physical bellhousing difference?

I think that Tera has a BOP, and it should be the same for the 65 P*ntiac tranny to go in my convertable, but I cannot tell the difference when I wander the local yard and see racks of trannies. I'm guessing the 78 Waggy is also a TH400 but I have not got around to looking yet.

Other than physical differences are there internal ones that would keep me from getting parts from the wrecking yard/local parts store?

[ January 10, 2005, 08:46 AM: Message edited by: PlumCrazy ]

[joe]

The Buick nailhead bolt pattern(not BOP pattern) was used from late 65(w/327) till 73 along with an adapter regardless if it was behind a 327, 304/360 or even the 350 Buick motor(BOP pattern). From 74 on till 79 the TH400's have an AMC(not including 327) pattern and bolted directly to the motor w/o any adapters.

[tgreese]

The best picture I've found of a Buick nailhead (a 425) is this one from buicks.net:
This is the back of the engine, where the bellhousing bolts to the block. The corner of the valve cover is to the left.

Notice that the block casting sticks out from the back of the engine. This means the TH400 case for the nailhead engines must be shorter in OAL than those used on the 360, for example, where the back of the block does not stick out past the cylinder heads much. The 232, 258, 327, 350 and 360 all have blocks that are flush with the back of the cylinder heads (more or less).

Kaiser and AMC used this fact to adapt the nailhead case to the Jeep family of engines using specific ring adapters. Eventually AMC had GM cast TH400 cases to bolt up directly to the 232/258/360/401. Don't know if this was becasue the nailhead was long out of production, or if production numbers of the AMC Jeeps was large enough to make it worthwile ... maybe both.

[tgreese]

The best picture I've found of a Buick nailhead (a 425) is this one from buicks.net:
This is the back of the engine, where the bellhousing bolts to the block. The corner of the valve cover is to the left.

Notice that the block casting sticks out from the back of the engine. This means the TH400 case for the nailhead engines must be shorter in OAL than those used on the 360, for example, where the back of the block does not stick out past the cylinder heads much. The 232, 258, 327, 350 and 360 all have blocks that are flush with the back of the cylinder heads (more or less).

Kaiser and AMC used this fact to adapt the nailhead case to the Jeep family of engines using specific ring adapters. Eventually AMC had GM cast TH400 cases to bolt up directly to the 232/258/360/401. Don't know if this was becasue the nailhead was long out of production, or if production numbers of the AMC Jeeps was large enough to make it worthwile ... maybe both.

[Green Giant]

The "nailhead" name stems from the fact that the buick engines called "nailheads" because of the valve design (IIRC) had a recess at the back of the block that the torque converter fit in, thus the tranny bellhousing was much smaller than more recent engines. This was dropped when GM went to the "corporate" design engines in the 60s. Kaiser used the short bellhousing GM th400 with an adapter ring (also was a spacer for TC) to fit the odd patterened 50s design AMC 327. Basically a leftover engine, as AMC changed to the 304/360/390/401 block in the late 50s or early 60s. Because of the ready availability of that tranny from GM, Kaiser continued to use it with different spacer/adapter rings with the Buick 350s and the AMC 360. Kaiser used "left over" parts from other manufacturers because they could get them cheap and also save on development/manufacturing costs themselves. Once AMC bought the JEEP line they started using their own engines, but kept the "left over parts" way of saving development expense. (common with smaller automakers from day one)