The Offy's cylinder sleeves will be sealed inside the coolant-filled engine block using Loctite. Each sleeve has a machined ring around its top end that will fit inside a recess in the head. The bottom ends of these rings will be sealed inside similar recesses that have already been machined around each bore in the top of the block. Unlike the Merlin that used these rings for metal-to-metal seals, the Offy will use a head gasket. When machined and assembled according to the drawings, up to .125" of each ring will wind up inside the head depending upon the thickness of the head gasket. This means that matching the heights of the sleeves above the Offy's block won't be critical as it was in the Merlin, but each sleeve will have three diameters and an inside corner that will require careful machining.
The drawings specify Stressproof for these sleeves which, in comparison with 12L14, offers improved corrosion resistance in a liquid cooled engine. During efforts with the Merlin's Stressproof liners, I struggled to obtain bores with surface finishes that wouldn't require inordinate amounts of honing.
Stressproof turns easily on both my 9x20 Wabeco and 12x36 Enco lathes, and fine surface finishes are easily obtained using tooling with carbide inserts designed for steel finishing operations. When boring, however, the rigidity of the cross slides on both lathes is lacking, and the resulting chatter at the ends of typical boring bars that fit my lathes limits the quality of a finished surface in Stressproof. This isn't an issue with similar operations on 12L14 using the same machines, but Stressproof's 2x hardness and .5x machinability creates problems for boring operations in my shop.
The solution I eventually worked out began with drilling out the cylinders to within .050" of their final diameter using a series of drills. My Enco lathe's back gear was then used to reduce the spindle speed to 100 rpm for use with its slowest possible (.0047"/rev) power feed rate. A much slower feed is available from the Wabeco and, even though I replaced its lightweight cross slide long ago with a large block of metal, its VFD doesn't produce usable torque below 300 rpm.
I used a 3/4" diameter boring bar that had to be notched to fit my lathes' tool holders. The insert was a Korloy TCGT32.51-AK with a .032" radius nose and three polished razor edges designed to turn aluminum. This combination, along with dark thread-cutting oil, produced an acceptable bore inside Stressproof, although the insert's life was extremely short. I was able to complete two sleeves per edge, with the second one having somewhat poorer surface finish.
Depth of cut is important. If it's too shallow, even the Korloy insert will skip across the surface and spoil its finish. If it's too deep, the insert will wear out even faster and produce a poor result. I found .020", or so, (diameter) to be a sweet spot. With .050" excess stock remaining after the drilling operations, I had a couple chances during machining to calibrate the lathe's DRO for consistency between parts. The goal was to bore all the sleeves to within a thousandth of one another in order to minimize the amount of (messy) honing that would be needed to bring them all to the same diameter. Since the piston rings will be machined to fit, the final honed diameters aren't important so long as they're identical.
I had enough 1144 of the correct diameter to make four parts plus a spare. The spare was finished along with the rest of the sleeves for later use as a light-test fixture that will be used to select the rings that will be used in the engine.
Before beginning machining, a Delrin plug gage was turned to exactly match the two diameters already machined into the block. These were mic'd and their measurements recorded as targets for the sleeves' o.d. machining. This same gage will be used later to machine the cylinder recesses in the head.
Using a convention steel finishing insert, the sleeves' o.d.'s were turned with fine but unpolished surface finishes before starting the drilling and boring operations. A parting tool was used to turn the sharp inside corner needed to seat each sleeve inside the block. The top outside edges were slightly beveled to later aid insertion of the head down over the block. A short taper was also machined in the bottom of each cylinder's bore to ease insertion of the ringed pistons during final assembly.
When machining was completed, each sleeve was marked with a unique number so its progress could be tracked during honing. Record keeping isn't all that important with a small batch of cylinders, but when honing a sizable lot, wear on the lap itself can drive the process around in frustrating circles. Initial bore gage measurements showed four of the sleeves were starting out within a few tenths of one another, but one outlier was a full thousandth over.
A barrel lap and 280g Cloverleaf paste was used to bring the four cylinders up to the outlier's i.d. Once this was accomplished, faint machining marks left over from boring could still be seen on three of the five cylinders - probably the second bores from the insert's edges. These machining marks with their familiar period of some 200 spindle revolutions, often show up on my Enco during power-fed finishing passes even when turning. I've inspected the lathe's easy-to-get-to gears for defects but have never been able to pinpoint their exact cause. The marks left during turning polish out quite easily, and so I've just been living with the problem. They're a real PITA inside a cylinder bore, however.
I spent a few hours trying to clean them up with 600g paste, but ultimately had to return to using 280g to make any real progress. Final polishing was done with 600g, however. Some .003" eventually had to be honed away from the cylinders' i.d.'s to obtain five clean bores within a tenth of one another. The actual machining time for each bore was about an hour, and honing wound up adding another. - Terry
and the really awesome and clever engineering used to right the wrong. You always come up with an elegant solution.
