The serial number idea is a giod one. The main differebces between the twi are the bore and cams. The 790s have stronger cams that are desired by 865 owners as a performance mod. The 865 has a bigger bore with milder cams. I had a Speedmaster w/865 before my 06 TBA and I could go 190 miles before hitting reserve which was much better mileage. The other dufferences are in the ECM with a higher rev limit for the 865 ad probably a little different timing map. But a motor that has been passed around may be risky unless you get an unbelievable deal. At worst you get some good spare parts off a not runnung engine. At best you get a good running unhurt engine.
