Crankcase - Stage 2
The chuck and crankcase are now moved to the rotary table on the mill to complete the cylinder mountings. But first, I decided it was time to re-swing the mill head as I'd noticed that the large diameter end mill I'd used to face the gearcase had been taking just a wisker off on it's trailing edge when cutting right to left, indicating that the mill head was not perfectly perpendicular to the table. The drill chuck holds parts liberated from a magnetic base used to mount a plunger type DTI so it can be swung through an arc about 18" across. The head is rotated so the DTI reads the same at both sides. Well worth doing more often--damn thing was out by 0.04 in 18". That's only 0.12 of a degree, but significant nevertheless.
The rotary table is aligned to the table axis using an engineer's square (I've previously convinced myself this is accurate by clocking across the flat face of the table when aligned this way) and the plunger DTI mounted in the quill to set the #1 cylinder seat flat. On my table, 72 degrees (the angle between cylinders) is exactly 18 turns of the crank, so the micrometer collar on the handle is zeroed at this setting--you can see it on the extreme left of this picture--and we're ready to mill the cylinder seats flat and set the deck height relative to the table axis.
The Morton drawings give the deck height as 0.938" +0.001" -0.000", but how to measure this? At first I thought, easy! This is why Bruce's instructions said clean up the gearcase register for 3/16" when the gearcase only intrudes 1/16". Half the ID, plus the thickness from ID to cylinder seat gives the deck height. But alas, when the crankcase was mounted last, it was adjusted to get the center hole of gearcase running true, so the gearcase register in the crankcase could be anywhere!
This photo shows the first attempt. All seats were milled flat, but left oversize 5 to 10 thou as measured from the case ID as described above. My plan was to bore all the cylinder holes, then average the readings and mill the seats to deck height by rotating the case with the quill locked at this averaged height. The Heath-Robinson support was cobbled up from clamping kit bits and while it provided support against downward drill loads, the rotating radial load produced a noticible movement of the case. Clearly, a better arrangement was needed.
The solution I arrived at was to machine a sholdered plug, the small end being a close slip fit in the gearcase bearing hole and the other end, a concentric 0.6000" diameter. This was done in the 4 jaw chuck as my 3 jaw was quite obviously tied up! The plug was parted off, reversed with the 0.6000" end set to run true, and center drilled. The oiled plug was pressed into the gearcase which was securly bolted to the crankcase. A mill table tailstock center was then brought firmly into the plug and a DTI used to check that the stub turned true as the rotary table was cranked around (it did). Now I had a reference point (the stub OD) which is 0.3000" above the case axis allowing all cylinder seats to be milled to the required deck height. Additionally, the assembled case being effectively supported between centers, was absolutely rigid in all axes.
This photo shows the steps for boring the cylinder liner holes. These are supposed to be positioned 0.571" back from the case rear, but this face is not running true (due to following the instruction to clock on the gearcase bushing ID, not the gearcase register!) I could either average out difference (nearly 0.003"), or set each one individually meaning my cylinders will not all be exactly in the same plane. For better or worse, I chose the former; so the process is:
- Rotate case to locate the quill at the averaged joint location
- Wind across to 0.571" from edge, then center drill, followed by 5/16 and 5/8" twist drills.
- Bore for slip fit of cylinder liner (0.688") with micrometer boring head
- Wind around 72 degrees (18 turns) to the next location and repeat
- Finally, set an end mill 0.638" above the 0.6000" plug and finish machine all seats to height.
As a precaution, the center of each flange was located approximately with odd-leg calipers before step 2 so I'd know if the point I was about to drill was off flange center. Fortunately, none were off to any degree worth talking about, or so I thought (see later). Visible on the mill table is one of the cylinder sleeves obtained from Vernal Engineering. These are chrome molly tube of the correct ID (0.625"), ground on the OD to the correct diameter. They will be lapped before being glued into their cylinders.
The final operation for this stage was to machine the carburator flange flat and drill the inlet and mounting holes. This is located between the #3 and 4 cylinders, directly opposite and under the #1 cylinder, so the position was easy to derive as 9 turns from a cylinder face. It's interesting to note a drawing change here. The Morton drawings I got from Vernal Engineering show a 1/4" inlet bore with mounting holes either side, 1/2" between centers. An earlier drawing (in the set available from ECJ) have the mounting holes 0.563" between centers. I'm guessing the dimension was reduced because the 9/16" spacing placed the holes too close to the carby casting edges. Regretably, I drilled the Morton case to the old drawings before discovering the change. The Satra case is to the new dimension, so my carbys will not be interchangable.
Note To Self:
- If I had it to do over, I think I'd clock the crankcase true on the ID, then bore the gearcase to diameter for the 0.563" bushing, or larger if it did not quite clean up at that diameter. The plug mentioned above would be machined to fit and used as described. The advantage is the case join would track true making the location of the cylinders relative to this face consistent. Hindsight is a wonderfull thing, right? Oh well, next time...