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FLYING STEAM ENGINES - Lapping

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Lapping

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.

Model Diesels in long term storage

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. These notes are only part complete as at mid Feb 2007 drawings and update to follow on GEW. SUITABLE AIRCRAFT The Parker Comet engine was designed as an integral part of the airframe and it would be difficult to better the combination just as it is. I would substitute the thinnest litho plate for paper or other lightweight model covering under the port wing above the firehole door fire where it is above and adjacent to the worst fire hazard on the whole model. For future work I will certainly use a pusher layout to protect the engine from the worst effects of a seriously misjudged landing. In the case of the Parker Groves hybrid illustrated earlier I think that the Aeromodeller Plans Service F F desiun Ptishv Caf would be an ideal place to start for inspiration. Ifthis desGn were to he used there \\oLLld ha\e to he some weiuht redLtction done as the fuselaue isa sheet slab sided affair and a close eve kept on the CG position and modifications to the fuselaue design and v~inu mount to accommodate the large engine case As I said before Pushy Cat is not a bad place to start hut it lv is only a start If I do get to the point ~ here a real acrobatic performance becomes a reality I will opt for a pusher twin boom layout.


 



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