How To Make and Use Cylinder Hones

Created: November, 2003
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Beyond doubt, the simplest, quickest, most effective way of finishing a cylinder bore for the amateur engine builder is the diamond paste charged, aluminium "lap" (hone, whatever--let's call it a lap for now). In use, the lap needs to be adjustable so its cutting edges (all the embedded diamond grit) can be kept in contact with the cylinder walls as imperfections in said walls are cut away by said particles. Over the past 10 years, I've tried a number of designs and my thinking has now settled on one that best meets my needs (recognize that yours may be different).

At first, knowing nothing, reading old how-to articles convinced me to use the "parallel" variety described by Sparey and others. Two are shown here (from the EZE 3 construction article). The one at top comprises an aluminium mandrel with a tapered shank, onto which is pushed a brass sleeve with a matching, internal taper. The sleeve, being spirally split will expand as it is pushed up the taper, maintaining a parallel shape. The one below it is similar, but this time, it is split longitudinally, with additional longitudinal grooves allowing it to expand in a reasonably circular fashion and helping maintain oil flow to both lubricate and carry away metal particles--as does the spiral cut in the brass sleeve type. Both require a fine taper to provide fine adjustment. The last thing in the photo is the shop-made "D-bit" reamer that matches the mandrel above it. The problems with this type are legion. Making the mandrel and reamer is a pain. Heat treating the reamer without it warping is problematical. Preventing the lap from rotating on the taper is also problematical (there's a small pin in the slot of the longitudinally split one; the spiral type depends on friction). Pushing the sleeve "up" the taper is easy enough. Pushing it back down is far from it. And the big nail in the coffin of this type is we don't want a parallel bore, we want a tapered one!

Enter the expandable, split lap. I first saw this one in an article by Roger Schroeder on building the Deezil compression ignition engine. Roger's lap is on the left of this photo (of tools and snafu's). After turning parallel for a length of about half the bore length to be finished, the lap is necked down, drilled, tapped and taper bored a short distance for a cap-head screw whose head carries a matching taper. The lap is then split quadrilaterally up into the 'necked-down" area so that as the screw is inserted, the lap petals are forced into a light cone whose apex will actually be somewhere out in space, far beyond the end of the chucking piece. This design gives us the adjustable capability required and imparts a slight taper to the bore. If we lap with the top of the cylinder towards the chucking piece, we'll have liners with that oh so necessary "pinch" about TDC to give lovely compression, but that become progressively loose towards BDC. This lowers friction where a tight compression seal is less of a requirement (we still want to develop "primary" compression in the crankcase, but the pressure is comparatively low). Roger's lap--which he's been using for years before the AHC article in SIC--accomplished this very effectively. The only downside is that while one screw may expand multiple laps, each lap needs a tapered cavity in the end that has to be cut with reasonable accuracy.

In his instructions for making the Mate, David Owen came up with a similar design that does not require any taper cutting. As seen in this shot from the AHC project, the split-petal design remains unchanged, as does a tapped hole (done before splitting, obviously). The difference however is that the thread is cut with a #2 tap, resulting in a short length of tapered thread. When the tip of a screw encounters this, the petals will open almost as before (note the "almost"). I liked this idea so much that it quickly became my standard process for cylinder lap manufacture.

This exaggerated drawing shows what is going on and why it is important that the petal slits extend past the working surface; why the point where the taper cut by the #2 tap should not be too far down the body; and why the lap should be a close initial fit in the unlapped bore. The first point should be obvious: the lap expands by bending. Bending implies a radius. We want the working area to be flat, so the bending needs to take place before the working lap surface. The angle rate at which the lap expands per turn of the screw depends on where the force is applied in relation to where the bending occurs. For fine adjustment, we want this to be as far away as possible while still retaining an adequate "start" for the thread. And finally, try to imagine what an undersize lap like the one drawn here would do as it is stroked back and forth in the bore. We are aiming to remove no more than 0.002" from the bore; perhaps less than 0.001". Given springback in the boring tool, the hole will most likely be lightly tapered already (which is why you should always bore from the bottom of the cylinder, never the top). Our lap is intended to regularize the circularity of the bore, finely finish the surface, and impart a taper in the vicinity of 0.0005" to 0.001" in diameter per inch, without creating "bell mouthed" ends.

As the lap is expanded, the screw becomes loose in its thread at the entry point. There is the possibility that pressure from the bore will try to close this gap, inducing a second bend in the tool, this time within the working area. I'd also noticed that this lack of support at the end made re-truing a lap (for a smaller bore) impossible on this style design, while it was practical on Roger's tapered screw type. So what is needed is something which combines the ease of manufacture of the second with the better support for the petals of the first. Duck soup, as they say, provided you have a duck, or in this case, a taper turning attachment.

What you are looking at here is a tapered 10-32 screw. It starts out with a diameter of 0.190" for the first quarter inch, than increases over the next inch to about 0.200", give or take. This means that one turn in of the screw will increase the diameter by approximately 0.0003". The end has been tapped for an 8-32 cap head screw and one has been glued in as a means of turning the tapered screw. The short parallel thread was formed by running a die up the tapered thread after screw cutting to the required depth. This was intended to provide a positive start into the tapped hole, but was probably unnecessary. Laps can now be made to the #2 design, except the hole can be finished off with a bottoming #3 tap, and it does not matter where the bottom is, provided it's deep enough to give adequate adjustment. The expanding force will be applied at the end of the lap, and there's a good chance that the lap will be supported internally by the tapered screw reducing secondary distortion. The screw will be re-usable over all laps, provided they are large enough to take the internal 10-32 hole. Finally, using this screw, I was able to re-size a worn out lap successfully. Only fly is you gotta have a taper turning capability. Mine came from a Hemingway kit and I heartily recommend it (see Suppliers Page).



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