engine swap?

looking for an easy answer-
will the engine of a '81 yamaha xs400 go into a '78 xs400 with no
unforeseen issues?

 

will the engine of a '81 yamaha xs400 go into a '78 xs400 with no

Probably. There were a number of changes made wrt early and late
models, but the Haynes book I have seems to say that '77 and up (in
the US, anyway) are considered "late" so it looks like you're swapping
a late engine into a late frame. Two things to check, though, are:
engine mountings -- early engines were bolted directly to the frame,
later engines had rubber buffers; and ignition -- early had points,
later had solid state CDI. If your '78 has these "late" model
improvements then I suspect it will be plug 'n' play.

 

thanks for the good news.
this is encouraging and much appreciated.

 

Yes. Same lump. I *think* the later XS400s had electronic ignition,
which might complicate things.

Unless you're talking about the monoshock XS400, which is a totally
different bike, but I think that first appeared in 1982.