Talking transmissions with Fredward

[bevans6]

Fred Thomson was kind enough to call me last night and talk about his experience doing the same transmission conversion as I am in the middle of.  He confirmed a lot of what I have been understanding and learning about this deal, and gave me some great tips.  One thing that may be different is that my late model 5C has a big, solid cross-member right below the tail-housing of the transmission, while the stories say that earlier buses do not.  I looked closely for chassis cracks and found none, and I credit the addition of that cross-member with strengthening the bay of the bus significantly.  One thing we discussed was the need to check for driveshaft clearance when the axle is up against the bump stops.  He changed his bump stops and added spacers to keep the axle a bit lower, and I plan to do much the same thing.  This is also why there is a need to shim the rear of the engine up by 1/2" to 3/4" at the rear engine mounts.  I expect to have more clearance there than Fred had, since I have a close-coupled U-joint output from the transmission while he used a flange.  That makes my driveshaft longer, and reduces the angle adding some clearance over the axle.  How much more only time (and the delivery of the driveshaft) will tell.

That brings up a point I wanted to mention.  The ultra short driveshafts we use on these things are called "close coupled assemblies", and the Dana-Spicer catalog lists many of them to choose from, in a wide variety of useful lengths.  When your guy calls the warehouse to order one, all of the part numbers turn out to be obsolete.  D-S no longer makes such assemblies.  When your guy searches on the individual part numbers, the yoke sleeve (the part with the internal splines) is readily available and less than $200.  When the search is for the yoke shaft the problem arises.  D-S doesn't make them anymore, they are all obsolete part numbers and you need to call around to about a dozen distributors (not the main warehouse) until you find a part number that will work left over on someone's shelf.   It turns out I really dodged a bit of a bullet on the project, if we hadn't found a workable yoke shaft this bus would have been an automatic noise maker, not a self transporting mobile home.  And yoke shafts are apparently not only scarce, they are pricey as heck.  The one I am getting is "special" in that the yoke is designed for up to 45 degrees of operating angle.  That means it costs no change from $1,000.  Plus shipping over about all of North America.

This makes four parts that are critical to this, and for posterity and subsequent google searches, an Allison MT-647 install into an MCI MC5 needs four unique and hard to find parts - a crank-shaft hub adaptor part number 6884867, a flex-plate with ring gear part number 6884866 (both available new from Allison, again no change from $1,000 each, but call first before you buy your new transmission) and a close coupled 1710 type driveshaft roughly 14" collapsed length (varies depending on if you swap the drop-box to a yoke input or keep the flange.  I kept the flange on that end.)  The final critical part is the correct SAE 1 to SAE 2 spacer assembly, part number 6881378.  This is an adaptor ring that is also a 0.890" spacer.  We found a used one, odd as that may seem, but spacers and adaptors that are 1" thick are readily available and can be machined to fit.  Again no change from $1,000 if you buy new, I paid $200 for mine.  The Allison drawing that specifies all the part numbers and dimensions to mount a MT-644/647/654 to a DD 71 series SAE-1 engine is AS 04-200.

Cheers, and thanks Fred!

[Fredward]

One more thing to record for posterity's sake. As discussed last night on the phone, the OEM drive shaft is clocked 45 degrees out of phase. I had driveline vibration at 90 degrees and of course 180 with my new close coupled assembly. It wasn't until I re-re installed the driveshaft clocked at 45 degrees that the vibration disappeared.

And I think we may have covered this in another thread, but we will still want to discuss the oil cooler.

Fred

[Lin]

As mentioned in an earlier thread, Fred's 5a did not have that cross member while mine did.  Clifford or someone said that they think that some 5a's did not have it.  Also it was possibly removed from Fred's bus.  Although my 647 was installed leaving the cross piece intact, it made it impossible to remove the transmission pan.  Therefore, I had it cut on both sides of the transmission and modified to be reassembled as a bolt on.  I did this for access to the transmission pan, but later it turned out that it also allowed that transmission to be removed without pulling the engine.

[Fredward]

Yes that's correct. That horizontal cross member has been removed on my bus. It came that way when we bought it.