One of the more common questions I get from gents finishing an XL250/350  top-end repair is "how do I mount the &@#!)$ cam sprocket".

With a new (or good) cam chain tensioner, with a distinct "bow" to it, and even with loosening the cam chain tensioner adjustment bolt, getting the cam sprocket to slip onto the cam can be a chore.

Some time back, can't remember just when, I tired of fighting this procedure and built a jig (this is an actual picture, but not actual size.)

The jig was cut from sheet metal with the overall dimensions of:

a = 3-1/2" and b = 3-7/8".

It mounts on the left side of the engine, where the point's plate sits, using mounting holes "c" and "d" and the same Phillips head machine screws used to mount the point's plate.

To perform this procedure, before mounting the jig to the engine, pull up on the cam chain, position the end of the cam (lobes down) thru the hole in the cam sprocket (index lines horizontal), then mount the cam chain around the cam sprocket. The sprocket will now be sitting loose on the shaft end of the cam. 

Position the cam in the journals, then mount the jig, with the end of the cam through hole "f".

Now, insert a flat-blade screw driver thru hole "g" and catch just the inside lip of the cam sprocket hole with the very end of the blade. 

GENTLY, pry the cam sprocket onto the cam/sprocket surface.

Finally, making sure that the engine is at top-dead-center ("T" on the flywheel), position the cam lobes "down",  making sure the horizontal marks (180 degrees apart, scribed on the sprocket) are in-line with the horizontal mating surface of the cam cover. 

Secure the sprocket to the cam with the two bolts. Rotate the flywheel and check for proper alignment.

If the flywheel mark is at the "T", and the cam lobes are pointing up, your assembly has just resulted in the most common engine builder's mistake, the cam timing is 180 degrees out of phase. Been there - done that, got the "dunce cap" T-shirt.

The other dimensions are 

c to d = 2-3/4"
c to e = 1-3/8"
f to horizontal line drawn between c and d = 3/8"
f to g = 15/16"

home dimensions are 

c and d = 1/4"
f and g = 1/2".

As I mostly build XL350 engines, I don't know if the jig will serve for all other XLs, but I do know it also works on a XL250 side port unit.

If you make a similar jig for a different engine then please e-mail me with the dimensions; and, let me know your experiences.