OLIVER TIGER MK III - 5cc ENGINE BUILD
Part 6  by Ramon Wilson
Part four - by Ramon Wilson
The main outstanding items left to do are the piston/liners, crankshafts, con-rods and needle valve assemblies. Three pieces of free-cutting EN1a were cut for the liners.
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First off was to establish the bores. These were drilled 5, 10 and 16 mm diameter for a 17.74 finished bore.

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These were then held in soft jaws - for boring to size and facing one end. This end was marked to ensure that when replaced on the mandrel for turning the outer diameters the marked end would be the lower end - any slight taper usually occurring from a boring operation assisting in the lapping process to achieve a slightly tapered bore. A plug gauge was turned to ensure the bores were similar for fitting to the expanding mandrel for later turning. I have long considered it much easier on subsequent machining to establish the bore first then carry out all other work on a mandrel. The main reason for this is the potential for distortion particularly on thin walled liner sections - if held in a three or four jaw chuck even the lightest nip will show high (low?) points when the lapping begins. By leaving the wall thickness as large as possible this virtually eliminates that potential.

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A previously used mandrel was turned to size. As said so often before these are so easy and quick to make. Expansion is by a cap head socket screw which has been turned to a 60 degree taper to match a deep centre drilled hole. The lightest nip on the screw providing a really firm grip for all subsequent operations - accurate removal and replacement too.

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All three liners were roughed to within .2mm first.

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Then finish turned to dimensions against a fixed stop.

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Utilizing an old ground block a simple fixture was quickly made for holding the cylinders for cutting the exhaust ports.

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A silver steel cutter, previously made for another engine which happened to be the right thickness and diameter, was pressed into service and all twelve ports were cut with ease. No coolant was used as it has previously been found that doing so tends to create swarf clogging and subsequent broken cutters. Low speed and feed is just fine and the cutter still has plenty of life left

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With the exhaust ports cut and deburred the liners were set back on the expanding mandrel to cut the transfer ports. These were plunge milled using a 2.5mm diameter cutter then opening with a 3.5 diameter.

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Incidentally, in order to hedge off any possibility of galling, the mandrel had been relieved beforehand to clear the internal burs created by this op.

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Last op was to turn the taper and deburr the ports; the liners are now finished ready for lapping which will be done later after the pistons are made.




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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