I think I had a good day in the shed yesterday. I may descend into bleeding obvious mode again but remember that there may be someone out there needing to know all the basics which some of us can take for granted.
The task is to make the crank bearings and main connecting rods to the pistons. The first obvious fact is that I can't measure the length of the rods till I have made the bearings and can measure accurately so all I did on the rods was think about thickness and connection to the bearing. You need to have this in mind as you make the bearing otherwise you could finish up with not enough room in the foot of the rod for the connecting bolt heads, don't laugh, I have done this in my time! I started by looking at Mr Bahrett's design which is based on having CI castings for the body of the assembly with a small split bronze bush for the actual bearing surface. I don't have the castings so I shall make the entire bearing out of bronze thus doing away with the need for the bush. I decided that like his, and unlike the eccentric rods, the two bolts that hold the split bearing together will also be the fastenings for the rod. They will be nutted together dead tight so no problem there. Any subsequent adjustment to tighten the bearing will be done by relieving the faces and the bolts will still be tight. So, dead simple, two bronze blocks bored for the crank journal and held together by two 2BA bolts through both the bearing halves and the foot of the rod. One decision was that the bolt hole in the bottom half will be threaded and special bolts made, the effect of this will be that the bolt effectively becomes a stud, this will mean easier fitting and adjustment, no need to worry about the bolts being loose when you fit or tighten them.
You may remember that before I started on the eccentrics I cut some scrap brass for the bearings. When I came to look at them I found they weren't thick enough so I ditched them and started again with a new piece of brass bar. Incidentally, this is the bar I found at the back of a cupboard in the house when I moved in.
It took about half an hour to get to this stage. The fag-packet drawing with all the measurements checked at least twice and a bar of brass.
First job was to cut four blanks on the HM leaving some spare meat for machining.
Four blanks ready for shaving. First job is to get a flat face on one side and then reduce to a shade over 7/16 thickness. The journals will take 1'2" but always leave a bit of room for the rods to float as they run. If you look at any full size engine you'll find the old fitters allowed this, it gives expansion room and the bearing faces last longer if there is some side play.
A little tip. If you have a good machine vice it will have a dead flat face in the bottom of the jaw. Using an accurately squared packing piece means to get a true face. When you have tightened the workpiece, hit it with a dead blow until you can't move the packing piece underneath it. You will tell from the feel when it is dead flat. Then give it a final nip before cutting.
Once you have a clean face, blue the edge and mark to 7/16" from the clean face. Cut the other face almost down to this mark (leave a little for finishing) and once you have done one you can cut the other faces without altering the cutting height. If you have done everything right they will be identical.
Close of play. Four bearing blocks accurately milled for thickness with plenty of meat at the sides, top and bottom for finishing to size. The next job will be marking and drilling the bolt holes to 2BA tapping size straight through.
