Now that’s what you call an engine !
If Crocodile Dundee built an engine he’d build a Bonza which is exactly what my friend Aussie Jim has done. This is Jim’s first large engine and it’s an absolute beaut !
It is based on a design of the same name by John Williams as published in Australian Model Engineering Sep 2009 – Aug 2010. Changes, in addition to a few cosmetic ones, include the use of an RCEXL CD ignition and substitution of Jan Ridders’ Vapour Carburettor for the original.
As Jim explains – It is called a “Hit & Miss” engine because of the way its speed is controlled. When the speed rises too high, the governor holds the exhaust valve open so it does not fire (a “miss”). When the speed drops again, the valve is released so the engine fires (a “hit”).
Jim has kindly put together some notes and photographs taken during his build.
Remember – if you want to see a larger pic just click on the image.
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- Top bloke Aussie Jim, builder of this magnificent ‘Bonza’ hit and miss engine.
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- Carbide tipped circular saw is fine for cutting aluminium
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- Another carbide tipped blade used in a slitting saw arbor to cut out the frames
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- Cutting out the main bearing mounting slots
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- Frames on box base (later discarded)
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- Cam bearing and cam follower
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- Crankshaft ready for silver soldering
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- Blocking out the conrod with a profiling tool
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- Nearly completed cylinder
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- Drilling the mounting holes in the cylinder
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- Drilling the piston for the gudgeon pin
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- Turning the ring grooves in the piston
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- The govenor arm being milled from bar stock
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- The flywheels were cut from the solid
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- There is still a lot of machining from here to the finished flywheels
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- Power-hacksaw makes short work of cutting a blank for the cylinder head
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- Cylinder head set up for machining the spark plug hole
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- A stainless button silver soldered to a silver steel stem makes a good valve
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- The engine starts to take shape
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- Cutting the hole for the hopper in the water jacket
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- Baking the high temperature engine enamel in the barbecue
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- The assembled water jacket fits neatly onto the cylinder
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- Brass oil cups for the main bearings
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- Rounding the edges of the new base plate with a carbide router bit
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- Make it first draw it later. The changes mounting arrangements
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- Underside showing the CDI and battery box.
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- Brass tube contains the wiring to the Hall Effect device for the CDI
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- Govenor side of the engine. Note the window in the Vapour Carburretor.
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- Timing side of the completed engine
