Part 9  by Ramon Wilson
Part four - by Ramon Wilson

Lapping these, like the other engines described before, has followed exactly the same path. One of the previously made laps was recycled by turning a few thou off.

The liners were lapped before dealing with the pistons. Lapped with successive grit (320, 600, 1000) until the plug gauge would ease in to just below the exhaust ports.

A piston lap was turned until the liner plug gauge was an easy fit then split and drilled and tapped for a closing screw.

The pistons were turned to a fine finish .05mm more than the plug gauge then lapped until they would enter the bore as the plug gauge ie just below the exhaust port.

They were then lapped by hand to their respective liner using minimal amounts of 1000 grit and plenty of lubricant. This is a slow, time consuming operation that cannot be rushed. Patience is the key and mine is usually just about running out just as the piston skirt reaches the lower exhaust face.

it was only after the second had been done that I noticed the top of one piston appeared distorted - I pushed it with my finger at the same time the brain realised what had happened.

Three new ones were made with a slightly more substantial top thickness and lightening them by extra milling inside. These were then lapped as previously leaving them a tight fit at the top of the bore. Further fine lapping will get done on assembly.

Conrods were made from Alumec 85 a tough grade of ali. Cut from a slab the pieces were milled to square section before rough turning and drilling and reaming.

The big ends were formed first - using a previously made form tool. The position is set 'by eye' ensuring the lands either side of the hole are equal, hence the ink.

A fair result for a home made cutter I'd say - always get a buzz out using a home made cutter when it works well. Simple milling and usual treatment - GFS quenched in oil and not tempered.

First stage okay.

The little ends were done in similar fashion - the shank taper turned via the topslide.

This gives a better idea of setting 'by eye'.

To finish the parted off pip a split tapered 'collet', a remnant from another build was used to hold the shank.

Part one here. Part two. Part three. Part four. Part five. Part six. Part seven

Part eight Part nine Part ten



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