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FJ55 with a 6AT installed (Royal Rose)
Bob Brewer's writeup about his 6AT in a FJ55

Compared to other Cummins swaps I think the 6AT is the most doable.  I have now seen one Land Cruiser with it installed.  It's Royal Rose's FJ55 that has a 6AT and SM465 Transmission.  I have also recently learned that there is another FJ55 in California built by Bob Brewer in Santa Clarita.  However, I have no information on it.  Bob mentioned on the LCML that he believed the 6AT would not fit into a 40 but that it would fit into a 55 or 60.

Furthermore, Steve Fox of the Tornado Alley Land Cruisers (Missouri area Land Cruiser club) had been messing around with 6ATs for quite a bit and has a swap planned into his FJ60, "Sherpa." 

The Cummins 6AT is actually a block and engine design from Onan Industries Design from the 80s.  In the late 80s, Cummins bought Onan a did some minor modifications to make it perform better.  They were available is smaller UPS vans and mostly commercial applications only.  I've never seen a 6AT in an automobile other than a Land Cruiser.  Cummins versions of the 6AT are far superior to their Onan predecessors.

From what I've been told and from what I've experienced in Royal Rose's FJ55 with the 6AT installed, generally the motor is quiet and smooth.  The red line is somewhere in the 3500 rpm range and in fact it puts out roughly the same power as a 2F though it is ten time gruntier simply because it's a diesel.  I'm also told at 2900 rpms she's shaking pretty good so I'm not sure you'd want to take her much about 3000 rpms.  Royal runs a General Motors 4 speed SM465 transmission behind his 6AT and while it has great crawling ability he claims he needs a 5th gear to produce better highway driveablity.  However, he can cruise all day at 65 mph.

The dimensions are roughly the same as a 2F except a little taller so in fact it will fit in a lightly modified 55, 60, 80, and maybe a 40?.  Click here to check out the dimensions & specs.

Bolt pattern wise it will bolt directly to an SM420 / 465 so it leaves lots of options available.  Furthermore with adapters from Advance Adapters and others there are a lot of options.