A plunder type DTI is then used to find the high point of the mandrel. That gives us one coordinate for the cylinder bore. The other is found by finding the mid-point between the two machined bosses inside the case. A boring head is then used to open out the case for the cylinder spiggot. I'd toyed with doing all this work on the lathe. Machining would have been easier, but it was much easier to align things on the mill, so the mill won. I found that I needed to go out larger than the plans to clean up the "as-cast" bore ID; 0.030" larger! This was perhaps a mistake, though not a bad one as we'll see later when the cylinder is machined.
The holes for the tappet guides and crankcase breather tube are located relative to the cylinder axis, so they can also be drilled at this setup. I regret now not taking the time to make a drilling guide for the cylinder mounting bolts and drilling and tapping them at this setup too. Instead, I decided to "spot" them from the cylinder later. This will be a tricky operation and the time to make a guide thak could be used for case and cylinder would have been an overall time-saver. The shor here shows the case halves, separated, following removal from the mandrel setup.
This over-exposed shot shows the case with crank posed for one of those trial-fits which are really excuses to admire our own craftsmanship (don't worry, all model engineers indulge this practice, frequently carrying the bits around the house with them for hours afterwards). The next operation will be to bore out the cavity for the skew gear and cam assembly (the thing that started this whole obsession!)
This is a critical operation. The axis for the cam assembly must be located precisely in two dimensions to achieve the correct mesh of the gears. Fortunately, that's not too difficuly. First, the mandrel used earlier is cut down, faced and tapped so it can act as a stub to align the rear case against an angle plate. The plate is squared with the mill axis and an edge-finder used to locate the reference plane for one of the crucial dimensions. The edge-finder is then used against the mandrel stub to establish the axis of the mandrel as the other reference.
With the rear case half slid over the mandrel, it is aligned so the cylinder seat is vertical and secured to the jig. The G clamps are a bit crude, but they worked well enough. It's now a simple matter to precisely locate the axis of the cam shaft, vertically from the mandrel axis, and rearwards from the case centerline—adjusting for the case register which needs to be measured before clamping up. I forgot, and had to remove the case to double check this measurement. First, the case is drilled thru undersize for the shaft bushing, then blind bored to form the camshaft cavity. This is simply opened out to final depth with a slot-drill, then finished to size with the adjustable boring head. With this done, the hole for the camshaft bush is reamed to size.
Before pulling down this setup, the case is rotated 180 degrees and the mill y axis adjusted to recenter over the camshaft axis. The casting is them faced and trepanned to provide a seat and clearence for the point assembly. At the same setting, a blind hole is drilled and tapped 4-40 for the screw that will secure a spring steel finger whose function is to hold the point assembly casting in place and apply sufficient friction for advance/retard action. One more use for this setup: rotate the case 90 degrees to bottoms-up position, and the hole for the sump drain screw plug can be drilled and tapped.
Another screw plug is drilled and tapped in the front face of the front case half (shown being tapped in the photo here with a nice tapping accessory that I don't often get a chance to use due to space limitations under the mill quill). This one is arranged so that with the engine horizontal, it can be removed and oil syringed in until is begins to overflow. The oil level will the be such that the crankshaft counter-weight is just brushing the oil surface. When (if!) the engine is running, the spinning shaft and vibration will fling oil all over the place and a mist will develop that will lubricate everything except the rocker arm pivots.
Another Moment Of Truth. After machining and fitting the bronze bush for the camshaft in the case, the driver gear is pressed onto the crankshaft with a spacer whose width provides the axial location and whose diameter—smaller than the shaft—provides clearence for the driven gear. The driven gear was then popped onto a stub of 1/4" drill rod—and—miracle of miracles—it all works. The mesh is correct in two axies and endless hours of mindless, unproductive fun ensue spinning the shaft and watching those gears go round.
This shot shows the way the cam cavity breaks through into the main case cavity, allowing the gears, cams, and tappets to be lubricated by splash and oil mist from the wet sump. The passage also provides the connection that allows the crankcase breather tube positioned between the tappet guides to vent corrosive gasses from the case, while limiting oil loss—unless the engine is operated for any significant period at negative G's (joke). The positions are per the plan, but if I was doing over, I'd move the breather hole aft a thirty-second or so to give more spacing from the tappet guide holes.
Postscript 1: Assembly problem
The way in which the timer is mounted as shown on the Feeney drawings required a narrow recessed track be rebated into the faced off recess on the side of the rear casting. This would require a very tall, thin, lightly curved and very delicate tool to be ground. I cheated and paid the price when the trial fit of the timer was made: my revised mounting worked, but the timer protruded so much that it would be impossible to fit the engine mount and nut to the case screw that pokes out located just in front of the timer. As they say in the classics: bugger.
It's gotta be fixed, but I still want to avoid grinding that special rebating tool (having had them break disasteriously before), so the plan is:
- Mount the case half in the lathe, concentric with the cam shaft axis.
- Counter face down flat to get the required clearance for the nut
- Glue on a disk to provide the rebated recess that centers the timer body
- Kick myself for not forseeing this problem beforehand
To line up the casting on the camshaft bore, a stub of 1/4" drill rod is center drilled on each end in a collet. The case is slid onto the stub and suspended between centers allowing an angle plate to be bolted to the faceplate using the case face for alignment (with a business card under the case to protect the edge). A single clamp holds everything in place adequately for light cuts (after this shot was taken, a counterbalancing weight was bolted to the other side of the faceplate).
The case can now be counter bored (or faced, whatever) down to the required depth using a left hand tool to form the outside half and a right hand tool to finish up to the pressed-in camshaft bushing. The bushing will be used to locate a thin disk of aluminium that will form the bearing that positions the timer body.
The disk is turned to be a close working fit inside the timer body, drilled to be a press fit over the cam shaft bushing, and parted off 1/32" thick. Before parting off, a light rebate 0.001" deep is made from the center to within 1/16" of the edge. This forms a space that can be filled with Lockitie as a bit of added insurance against the disk ever working free after it is pressed onto the protruding camshaft bearing stub.
All fixed! The timer rotates smoothly on the "rebated" track, held in place by the light spring clip that also provides anti-rotation friction. There's now sufficient room to get the engine mount, washer and nut onto the long screw that secures the front case to the rear. It would be a good idea to clamp the rear case down to the mill table and face the rear of all the lug protrusions flat and to the same height to provide a uniform surface for the motor mount. Somehow, I don't think the real Feeney's never bothered with that, so it will be a deviation, but a good, practical one.
Forward to Feeney Log, Page 5
Back to Feeney Log, Page 3
Back to Feeney Journal front page
Back to Ron's Home Page