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I have done a fair bit of this, reboring clapped out model Diesel Engines, I have always managed to get them to run but I lacked the 'edge' in my technique to get it just right every time, however a final point or two dropped into place some while ago and I think it may be of interest to some of you.
The lap for the bore is turned and then at the business end I drill and tap a hole of suitable size (about one third of the bore of the engine). I use a taper (Number one) Tap to produce a tapered thread entry where it will expand the lap as the adjusting screw is screwed into the end,see sketch. This produces a smooth gradual adjustment as the lap does its work and wears. The lap is then cross sawn to produce four sectors, I do this in the mill out of choice but years ago before I bought the mill I had managed it ok with a junior hacksaw. I use just three cuts if the lap is smaller than say 0.250" (6mm) diameter and the smallest bore I ever lapped was 0.125" (3.2 mm) and for that I split it in two with a single cut and a 12BA expanding screw.
This all works fine and the length seems to be of little relevance to the cylinder but the lap for the piston is a bit more fussy, I now believe the length of the piston lap to be of great significance.
In my sketch you will find the piston lap is very substantial at about 3 times the bore or more and made of solid brass. I also use the lap as a jig for machining the piston internal form to reduce weight. I am now as certain as I can be that it is always better to make the piston lap shorter than the piston's finished length. I use a simple pipe clip to squeeze the lap onto the piston. Also during lapping if I want to stop to have a check on progress I always STOP THE MACHINE FIRST, it is of course a bit safer AND it is obvious that as the lap leaves the component whilst it is moving it can generate a taper where you don't want it.
The sketches and photograph shows the two laps I made to fit a new piston in a 0.5cc Allbon Dart diesel of mine which I brought to life again quite recently. I scrapped two pistons as they ended up 0.0001" or so barrel shaped. I reduced the thickness of the lap and got it perfect. I have a very nice Imperial/Metric digital micrometer, with a resolution of 0.00005"and so I can say with some certainty what taper there was at each end of the piston. Whilst writing this I realise that the Allbon (later DC) engines all had quite short pistons as did the later Frog engines and they have usually presented more problems than earlier designs like the Mills and ED 2 cc Comp Special series which used pistons nearly two diameters long. This is due I now believe to the simple fact that the lap WAS often shorter than the piston out of convenience with no thought beyond that simple fact.
I won't go any further into the process of lapping as it is well covered in many 'Model Engineer' articles over the years, several small diesels are available in UK as plans and castings, the lapping of cylinders and pistons being well covered. Never do I recall has the advantage of having a short piston lap ever been mentioned but I may be wrong in that. I do not pretend to have read every ME magazine published during its 100 odd years of printing.
As a further point to free flight Aeromodellers a diesel piston does not tolerate much wear before it is clapped out I reckon it is less than 0.00005-0.0002" obviously the smaller the bore the less the tolerable loss of diameter will be; 0.00005 (0.00254 mm)for those miniatures of about 0.250" bore and perhaps up to 0.0002" or 3(0.00762 mm) for pistons of 3-5cc engines whose bores are in excess of 0.5" (12.7 mm) diameter. I do not suggest I can accuratly measure these sizes but it is certainly of that order.
Have you ever noticed that a model diesel 'wears out' if you store it for years and years, something you may not have heard before, it obviously doesn't wear, it rusts. If you want to store a diesel for a long time remove the cylinder, do not move the contra piston, and wash the piston and cylinder bore first in petrol, methelated spirits or methanol to get rid of the old oil, dry the spirit away then rinse it in clean water with a small dash of washing up fluid, rinse very thoroughly and dry it all as quickly as you can, ( I use a hot air blower and get the cylinder and piston to boiling point) you are doing all this to remove the acid deposits of combustion. Then lubricate the whole engine with new oil and replace the cylinder. I never leave the piston at top dead centre as if it rusts at TDC it does the maximum of damage, store it at BDC or just closing the ports but not at TDC. I stuff soft loo roll wads into and around all the inlet and exhaust ports and oil the wads in situ with the motor oil, then I seal it into a new polythene bag and keep it in the dark. It takes maybe 30 minutes to do but it works. The oil does not degenerate over many years in these conditions. I do the same if they are in a fuselage, in my dark loft they look like they have been bandaged up, I use cling film if it is in a model, they keep their compression that's all that matters. I recently acquired a new boxed DC Spitfire from an old Friends widow, unrun since an initial 'try out' after purchase and well lubricated with engine oil as I suggest above and suggested to my deceased collegue 30 years ago. That engine was run for an hour or so on the bench all those years ago, it started third flick and restarts hot no problem.A New Engine for Skylark
Ever Since I got the first run at 100 psi pressure (7 Atm.-bar) the performance of Skylark had been consistent and reliable until, as a result of an overheat caused by a feedwater pump failure a considerable quantity of black copper oxide formed in the tube and ended up causing a blockage. This blockage resulted in the final failure of that monotube after I had repaired the feed water pump by progressivly stopping the flow of water into the tube. This was of course after I had left the pontoon and was progressing ever slower than usual down the cut. There was an ear splitting explosion easily as loud as opening a can of beer and a small cloud of steam and progress ceased. This event caused me to look more carefully at the cost of Vehical brake pipe made from a 90% copper nickel alloy compared with the cost of Stainless steel (st/stl in future). It turned out to be 30% more expensive so the new monotube is of this superior material. From here I decided to look at a new engine for the boat, in the book Experimental Flash Steam there is a section showing a boat made years ago by a Mr. Bert Squires, the engine is a small 3 cylinder unit of 32mm bore and 50mm stroke and it is stated as being able to produce 10 HP. From the Hydroplane section on this site, if you read it you will understand why I believe that figure. A mere 30 years ago I rescued/liberated a small twin cylinder compressor pump simply because just looking at it in my head it became the base for the construction of a smaller engine than Mr. Squires quite capable of powering a smaller boat like Skylark. Not surprisingly I still have that compressor and having dismantled most of it, clearly it will indeed make an excellent base for such an engine. In this section I will go through the process of design and build of the components needed to effect that transition from a chunk of scrap iron into an efficient little high performance steam plant. PIC The first three pictures show the compressor more or less complete, the fly wheel was to drive the machine by two vee belts and is not needed, the cylinder head will be replaced by a new one with two small poppet style inlet valves made from small motorcycle or lawn mower exhaust valves. The exhaust will be uniflow slots cut in the cylinder walls just before Bottom Dead Centre of the engine's stroke. PIC The poppet valves will be actuated by cams, four of them, two per cylinder, these will be placed at 180 Degrees apart per cylinder set and each pair will be in clock wise and anti clockwise positions so the engine can be reversed as most marine steam plants can be. These cams will be mounted on a square shaft and arranged so the whole shaft can be shifted maybe 15-20mm axially to reverse the engines rotation. PIC None of this is my idea, Serpollet did it in his steam cars a century or so ago. There will be no need for Stephensons links or eccentrics to adjust cut off because the dynamics of a monotube are such that heat control is enough to ensure all the control needed from dead slow to full power. Possible Problems The biggest difficulty may be keeping the crankcase cool enough because the cylinders and crankcase are one casting where Bert Squires engine had a gap between the cylinders and what appears to be a fabricated crankcase. In my favour there is a separate sump plate which is bolted to the bottom of the crankcase, this could become a very effective heat sink as there is ample supply of relativly cool water not far away. The whole crankcase area could thus be water cooled perhaps by using a simple water scoop as the sump will be below the water line this should present no serious difficulties to achieve. I would like to avoid a separate lubrication pump and I hope the existing lubrication system designed for what is a commercial continuously rated compressor will be adequate for the job. Only time will tell, I am confident enough to say I expect it to be fine. Can it develop the power? Yes I think it can given that I can supply enough steam at 350 psi. (24 Bar) a simple comparison can be made using a multiple of the cylinder area x stroke x the number of cylinders will give a fraction that will fairly represent what may be expected as a fraction of 10 hp. The cylinder areas are so similar that they may be counted as unity so if we make the smaller engine the numerator we get:- 1x28x2 over 1x50x3=56 over 150=0.373 or 0.4 will do. If I get 4 hp in Skylark she will be grossly overpowerd so I feel fairly secure in the ability of this small well engineered compressor to provide the power I need with relative ease.
These notes are only part complete as at mid Feb 2007 drawings and update to follow on GEW.