- Centre the lap in the chuck, gripping it on the stub end. Move the saddle right out of the way. Cover the lathe bed with plastic wrap and rag to prevent any contamination from the lapping abrasive. This is a household metal polish, called Brasso in Australia and used, funnily enough, for cleaning brassware. Any similar polish should work. It's not as fast as diamond paste, but used with patience will result in an excellent job. Forget valve-grinding paste, by the way.
Aluminium Oxide powder in 500-800 grit may be used and will cut faster than Brasso.
- Transfer some Brasso to the lap and one drop of thin oil. With the top end facing the chuck, slip the cylinder over the lap. Some resistance should be felt, but you should be able to turn the cylinder on the lap fairly easily. Grasp the cylinder between the thumb and index finger wiith a strip of leather, making sure that the surplus material is held well clear of the chuck jaws.
Start the lathe at 250rpm and traverse the cylinder back and forth along the lap, running it over a maximum of 10mm at each end of the lap. Stop as soon as the torque resistance drops to zero. Wash the cylinder thoroughly and wipe the lap clean. Tighten the M5 grubscrew until resistance is again felt between the two parts. Re-coat with Brasso and oil and repeat the process.
- Now you have a good idea of the amount of torque required to restrain the cylinder, discard the leather and hold the cylinder in your fingers only. Wash the cylinder and lap each time you expand it. Be careful not to expand the lap too much, or the cylinder will become too hot to hold and may jam on the lap. Beware of the spinning chuck jaws at all times! Continue to lap the cylinder, stroking it back and forth without rocking it from side to side. The stroking should incorporate a spiral motion, twist in your wrist as you stroke at about one stroke per second. Make sure the cylinder does not run over the cutting length of the lap by more than 10mm at each end, otherwise a bell-mouthed bore will result.
The amount of time it takes to achieve a round bore with a good surface finish is determined in the beginning by the original bored finish. However, the longer the cylinder is lapped to reach a suitable finish, the greater the chance of bellmouthing the bore. When you have lapped the cylinder some seven to ten times, wash it very thoroughly and inspect the surface finish under a strong light. It should be smooth and dull grey. There may be a faint trace of tool marks from the boring operation, but to continue lapping in an attempt to remove these entirely may result in the problem mentioned above.
- When you have an acceptable surface finish, again very thoroughly wash both the cylinder and the lap, having removed the latter from the lathe. The lap should have an even wear pattern towards the front, tapering away to the chucking end. This is quite normal, as this simple type of lap does not expand parallel. Now slip the top end of the cylinder over the lap and expand it with the grubscrew until it is a close fit in the lower end of the bore. Using the lap as a simple gauge, withdraw the cylinder slowly and feel for any noticeable change in the bore size from one end to the other. Ideally you will have introduced a taper at the lower end. If the top end is bellmouthed you're in strife, but we'll rectify that condition in a moment.
- The final lapped bore will be round, with a relatively smooth, matt surface finish and will taper inwards from the bottom towards the top. This is achieved from a parallel bore by selectively lapping the lower end. Slip the top end of the cylinder over the lap, and lap as before, stroking the top end 10mm past the lap, but shortening the stroke so that the lower end does not extend past the lap. The spindle speed can be increased to 500rpm. Continue to lap and test as before, until the lap (as a gauge) will smoothly enter the bottom of the bore, but start to tighten up as it passes the exhaust belt, finally jamming towards the top end of the bore.
If you have bellmouthed the top end, you will have to selectively lap the bore to correct this, until the bore is parallel at least down to the exhaust belt. Then proceed along the lines above. When lapping is completed, wash both cylinder the and the lap thoroughly to remove any traces of abrasive. Clean the lathe too at this stage. Measure the lower cylinder bore and note this dimension on the piston drawing.