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Our coach (1976) was born with a Cummins 555 and allison mt41 trans.

The 555 is impossible to get parts for so I am planning a heart transplant.

I have a 24 valve 5.9 cummins.  That is slightly tuned up.  I feel this is going to be a good power plant for the bus with a bump in hp/to over the 555.

I have a couple transmissions I am looking into.  The mt41 is not going to handle the torque of the new engine.

I currently have a lead on an mt645cr.  Any experts in here that can tell me if this would be a good trans for this swap?

I am with Carl on using a 5.9 for a vehicle like this, you are hauling the weight all the time around, in a pickup its only when you are hauling a load. What kind of weight is your coach? I would be looking at a mechanical 8.3 Cummins. We have a Wanderlodge member that bought an entire motorhome for pennies and he re-used the entire frame and running gear to transplant to another coach, but you can find some of these older diesel pusher RV's that are water damaged etc and they can be sold for cheaper than you can pickup the engine and transmission combo. Then you will get a transmission that will already be mated to the engine that will be what you need. Just my 2 cents. I am sure a 5.9 would work, its just not going to have the longevity of a larger engine designed for heavier vehicles.

I am assuming you mean the MT654CR, that is what is in my Wanderlodge. From what I have heard the only weakness in the 6 series of transmissions is the forward clutch was kinda small for pulling heavy loads, this is why they ended up with the 7 series transmissions and they put a larger forward clutch in them.