I have a problem and I've got a 1972 Ironhead xlch Sportster 1000 recently bored the motor 20 put new Pistons rings wrist pins everything new on the heads and jugs and I started smoking out of the rear cylinder a couple days ago so I know that's not a good sign.. I thought I'd better check it out you know and when I take the heads and jugs off because I checked everything I could check besides compression I knew something was wrong anyway I've never seen anything like this the snap ring that holds the wrist pin had come out of one side and let the wrist pin move but on the opposite side I'll have to post a picture it looks like it's burnt where the wrist pin comes out do it has anybody ever seen this?? and but also like to know what the difference is in the front and rear cylinder just a jug.?? I mean I've built this bike myself from top to bottom and I didn't know that only one set of heads will fit this thing if I'm correct could somebody let me know thank you

 

1972-early 1973 cylinders and heads have to be used together. One of the ways to identify these is they use a six point head bolt. Late 1973 on up have a wider head bolt pattern and have twelve point head bolts, and can be bored oversized up to .070
The 1972 and early 1973 can only be bored out to .040

 

the ironhead is one of the most forgiving machines around but that does not mean due diligence is not necessary.
head bolts might not be original
the xlform and the sportsterpedia are dedicated to the xl and not the bt
really not following since it is somewhat encrypted
there are several mistakes that can cause issues in the pin area. one is that many reuse the pin keeper and two they use the wrong one!
i will throw this slinky out and see if it walks
new pistons could have the keeper land not matched to the keeper. hd used a spring clip that sits deep into the land with a cut out to dig the clip out, aftermarket will use a land that is cut for a square keeper while the original land had a round or half moon shape. if you put a square end keeper into that half moon land, it will probably not stay. if the piston walks, it will put pressure on the keeper and when it comes out, the cyl wall is next in line and that is where metal to metal causes enough heat to melt the aluminum piston. also of note, a poor quality piston only adds to the mix.
did you check the rod assy???? did you change the pin bushing? did you do an alignment check??
not uncommon for a assy to develop a cant or a twist and sustain service since it happens and wears together, but, new parts will have friction since everything is fresh.