Before starting on the rear cover, you should complete the air guide since you may have to make some cuts in the casting that are not on the plans to get the air guide to fit. If you are making your own carb mount, you’ll want to review that page and finish most of the work on that as well to save yourself a little time later.
The rear seal plate is one of the last parts to be made from my 125mm diameter by 300mm piece of Aluminum 7075-T6. I bought a stick of this long enough to make the front cover, crankcase, rear seal plate, air guide, impeller, and a few fixtures. The diameter was needed to accommodate the crankcase so I did end up wasting a little material on the other parts.
Many of the gears used in this engine need to be re-bored to a larger shaft size. There are drawings in the plans for holding fixtures to do this, but I found it easier just to use standard collets. This should also do a better job of maintaining concentricity between the bore and the gear O.D.
For the oil pump housing, I chose C63000 nickel aluminum bronze (known as QAL9-4 bronze here in China). This material has high strength because of the added nickel and good wear resistance. The downside is this material is very “grabby” and tends to warp if you get it hot. Therefore, I’m going to machine it in two phases with a stress relieving operation in-between.
I rough sawed some 0.5″x0.9″ blanks from some 1/8″ 7075-T6 aluminum plate for the mounts. I decided to go with aluminum instead of brass since the intake pipe does not get soldered to this and I thought aluminum would look better next to the aluminum crankcase.
I clamped up a scrap tooling plate in the mill vise and used a couple of small clamps to hold a 3/32″ sheet of yellow brass on it. I then laid out a 1.375″x0.600″ grid of 15 flanges, drilling a clearance hole in the brass, and tapping the tooling plate below with a #6-32 tap. Once some #6 screws were tightened down, I was ready to roll.
I had some 10mm diameter 416 Stainless heat treated to HRC35 left over from a previous job. Even though most of the time was spent converting it from round to square, overall it was not very hard. However, don’t underestimate the added complexity such small parts can add to fixturing.