I have been coal firing my UPS BIG BOY 4002 (first series manufactured in 1981).  However, to anyone who is desirous of following in my footsteps I offer the following advice.   

 (1) Modify the blower jets. For gas firing the arrangement is fine, but the jets are too large for coal firing. Furthermore, the twin blower jet fitting in the smoke box is attached to the steam line exiting from the boiler front end by a rather flimsy arrangement using an M2 screw (I believe). All that has been changed on mine.  

(2) Still in the smoke box. There will be need to seal the space between the copper boiler itself and the boiler casing. When I did my conversion, I could not for the love of Mike understand why I was not getting adequate vacuum (drafting), only to realize that that space was the problem. 

(3) The two ash pans have damper openings front and back. I blanked off the front openings on both. The ash pan to the front of the firebox has the front damper opening located very close to the eccentric sheave and cam for the axle driven water pump. No need to tell you why that opening should be blanked off. 

(4) The fire grate provided in the Aster Big Boy kit (4002 series) was a ‘folding’ type. After one steam up session, the grate became badly distorted and I had to condemn it and have one made (in one piece) and placed permanently in the firebox. A couple of years ago Hans Huwyler (Aster Hobby USA) had some made (a 2 piece arrangement) which has so far held up beautifully. That would be the way to go. 

(5) Don’t let the apparent ‘shallow’ grate put anyone off coal firing Big Boy.