Try the wrist-pin. It should be an excellent smooth fit in the marked boss, but not enter the other. Then continue to ream this boss until the wrist-pin will just enter it.
Always turn the reamer smoothly and in one direction only while both cutting and withdrawing. You are using the tapered lead of the reamer to ream an undersize hole in effect, into which the wrist-pin will be a press fit, so it can't float and score the cylinder walls. Scribe an arrow on the smaller boss inside the piston indicating the direction in which the wristpin will have to be fitted during assembly.
NOTE:
A later, alternate method of contra-piston preparation was devised by the Author and can be viewed under the title DCO Thin-Wall Tapered Contras.
If this approach is used, the contra-piston should be bored Ø11.5mm to a depth of 4.2mm. All external dimns remain as per the drawing.
Total stroke should be no more than 10mm. Do not allow the lap to extend past the ends of the piston more than a few mm. Clean and test the piston for fit in the lower bore regularly. As soon as the piston actually enters the cylinder bore excercise extreme care. It won't take a great deal more lapping to final size, which is reached when the piston top (crown) starts to get tight slightly above the exhaust belt and jams at about 12mm from the top of the bore. Use lard as a lubricant when testing this final fit. Remember that you can rectify a too tight fit, but you've had it if it's too loose! So err on the tight side. All fits are judged under hand pressure only, of course.
Place another piece of wood on top of the contra piston and tap this wood with a light hammer, driving the contra piston into the bore until it is flush with the top face. If it is far too tight, tap it out with a drift from beneath and mount it on a mandrel. Cyano adhesive will hold it for further lapping to size, but don't forget the through hole or you won't get it off again. Oil the piston assembly and place it in the cylinder bore, before wrapping it up again. The hardest work is done now and, following the completion of several straightforward parts, your "MATE" will be ready for final assembly and test running.
So, back to the grindstone!
This page is Copyright © David Owen 1989-2006. All rights are reserved world-wide.PREPARING THE PISTON LAP (5-3)
LAPPING THE PISTON AND CONTRA PISTON
ASSEMBLY OF THE PISTON AND CYLINDER UNIT
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