911hillclimber Posted March 24 Author Share Posted March 24 Interesting! The piston top is by me and a pillar file... I watched a video of an engine (glow) being built the other day, and the builder pushed the piston and rod to the top of the cylinder and covered the top with his finger, sealing the head. A sharp pull down of the rod and the piston gave a loud 'pop' as it came out of the cylinder. The assembly was dry. Mind does that but not as loud and my piston was oiled. If I make a new piston I may as well machine a new counter piston and get it 'tight' in the head but after the fine lapping past has been used. I do not have a ball bearing anywhere near that diameter. When this is all done i will clean up cosmetically all the external surfaces to try to get it smart.... Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted March 25 Share Posted March 25 (edited) Hi Hill climber. The ball bearing does not need to be the same diameter as the CP but dont use a tiny one as its likely to crack the CP. Cast iron is slightly maleable so a good tap often expands it and as its effectively being squashed in the middle it expands equally outwards a few microns but often enough to get a good friction fit. When i make pistons i get them to a sliding fit up to the exhaust port. Then clean and test assemble with plenty of oil and if possible a prop fitted to assist turning the engine . This determines if it still to tight turn over or if it needs a bit more lapping . As soon as the piston passes the exhaust port it should start to compress the air and a bouncy resistance felt . The bounce might not be pronounced if its still tight but be careful if you do any more lapping at this stage as it easy to go tooo far and ruin the piston. Everyone will have their own method . Keep going and you will get there 👍 Edited March 25 by Engine Doctor Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 25 Author Share Posted March 25 Thank you for this valuable advice Dr. Trying to get in touch with Hemmingway today to get a bar of cast iron while I await the 1200 grade lapping grease to arrive. I have had a rod through the prop 'nut' as in the pics to turn the engine over repeatedly. Without the head on there is a light resistance as the piston is driven by hand up the bore, this could be a touch of binding due to the piston not being dead true to the various parts. This is with the cylinder tight to the case via the 4 nuts but no gasket between the case/cylinder. I think the piston is too slack in the cylinder, hence making a new one to match the honed bore. I made the piston really close to the un honed bore, my mistake. Same for the contra piston. Will try to make something to spread the C/P as an experiment and if it works will save me making one again! 1 1 Quote Link to comment Share on other sites More sharing options...
KEITH BEAUMONT 1 Posted March 25 Share Posted March 25 While you are wating for the new Cast Iron rod, Try " cherry Bombing" both pistons. Heat to bright red for a minute, then drop into water. This often adds a thou to the diameter and after pickling you can re- lapp with the 1200 grit. Keith, Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 25 Author Share Posted March 25 (edited) Thanks Keith, worth a try! While I wait for the fine lapping grease and some fresh cast iron for the piston thought I would do some boring bits. The needle itself has been made as my friend suggested, in the chuck and with a grind stone in the dremmel (and it at maximum rpm) shaped the needle point quite well. Turned a needle collar and soft soldered the needle and collar together with little fuss. All screws up and down with some nice resistance which I hope will resist the fine vibration of this turbine like engine... Next came the top of the cooling head and decided to add some more fins as the rest of the sleeve, more heart-stopping parting off tool work, what a din! Soaked the steel cylinder in citric acid solution, 20% crystals and had a good coat of bubbles on the cylinder after 10 mins. all rinsed and it looks much better, but still needs some swiss file work. Oddly, now ground to a halt, little else to do until the iron and grease turn up. Can this adventure (to me) actually be drawing to a close? Plan was to have it ready to run (really?) by my birthday, end of this month. Will i make it? Doubt it! 73T 911 Coup Edited March 25 by 911hillclimber 4 Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted March 26 Share Posted March 26 Fingers crossed for your birthday 🤞 Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 27 Author Share Posted March 27 Thanks, but no chance. Hemmingways are on holiday for Easter, so no cast iron from them soon, but have ordered a lump from ebay, but doubt that will arrive soon either. Will try the red heat and quench as I've nothing to loose, everything to gain. Pipped-at-the-post! Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 30 Author Share Posted March 30 Some advice needed please. The selection of lapping greases arrived today from China in 6 days (!), all 20 of them... They are numbered from 40 to 1. All different colours. My first lap was with '20' and I've just tried a good dose of 10 but not sure if there is much difference in surface finish in the cylinder. The 40 feels like the 1 to my fingers, so which is the coarse one please? Less subtle is the bar of cast iron which also landed, so I'm keen to lap and then machine a new piston, contra piston, but need to lapp it all first. As to the surface finish, i would say it is good as it stands if i were looking at a good car engine bore. Quote Link to comment Share on other sites More sharing options...
Jonathan W Posted March 30 Share Posted March 30 Just my opinion, but I don't think you should get too hung up on the finish. If you look at a brand new PAW, the bore is as ground while the piston has very coarse honing marks. The honing marks promote oil retention and allow some bedding in to occur. You won't get that if everything is to a mirror finish. I think dimensional accuracy with a slight "pinch" at TDC is far more important. Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 30 Author Share Posted March 30 That was my stance too. Oddly I have a new PAW from the 80s so will have a peek. Maybe a new tight piston tomorrow on the agenda. Quote Link to comment Share on other sites More sharing options...
KEITH BEAUMONT 1 Posted March 30 Share Posted March 30 At this point I can only suggest you re-read my post of the 15 March. It is the recognised method to achieve a working engine. I have used that system with satisfaction,with 10 model Diesels that all work. The finish that is acceptable on a full size engine is not good enough for a model compressionn ignition engine. Take your time and you will succeed. Keith. Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 30 Author Share Posted March 30 Ok, I'll persevere! Is '1' coarse and 40 fine or the other way round for the lapping grease? Quote Link to comment Share on other sites More sharing options...
Jonathan W Posted March 30 Share Posted March 30 It might be that the numbers are the micron size of the diamond grit??? In which case 1 to 40 = fine to coarse. Just to add that I never used this diamond stuff myself and your Chinesium product might be using a different system, who knows? I do have some silicon carbide lapping compound at 600 grit which I use to good effect on model 4 stroke valve seats, but the manufacturer of this product has unfortunately disappeared. Equivalents of FEPA F grain numbers to microns are 400 = 17, 600 = 9, 1000 = 5, 1200 = 3 etc. See HERE It would surely be helpful if your man Keith would point you towards the specific products he has successfully used. 😊 Quote Link to comment Share on other sites More sharing options...
Jonathan W Posted March 30 Share Posted March 30 Also see HERE At least this is a UK company you can phone up and talk to 😏 Listing diamond paste in the same range 1 -40 which you seem to have. They are saying W1 = 15000 to W40 = 400, so the "W" numbers are apparently not the same as microns then. If your paste conforms to this system, I would have thought the difference between 1 (super fine) to 40 to be obvious. Quote Link to comment Share on other sites More sharing options...
KEITH BEAUMONT 1 Posted March 31 Share Posted March 31 (edited) First of all I hope you are enjoying your Birthday! I suggest you test your lapping paste on a piece of perpsex or shiny aluinium, using a cotton bud and samples from the highest and lowest numbers, to rub a small area to see what marks the easiest. If you have purchased Diamond paste youwill need to be ultra careful with the cleaning, as diamonds penetrate the Ali surface and will quickly wear the piston/ cylinder fit. If you do not have an ultra sonic cleaner,you will need to boil the parts and plenty of scrubbing with a tooth brush,several times. I did use that diamond paste for a few engines, using the No 12 to start, followed by No5 to polish, but have changed to using the Fine grit with plenty of oil from the double ended valve grinding paste, left over from years ago, when we did such things to our motors, to start the process and get the overall grey finish. This is followed up with 1200 grit silicon carbide powder, mixed with oil,to achieve the polish, I puchased 500 gramms from the Internet from a company trading as Etsy. The idea that the surfaces should be left a trifle rough,is, I believe ,a carry over from the steam engine makers and even then it is a debated subject, Even a shiny surface to the naked eye is pretty rough when looked at by magnification, so plenty of oil retention is possible. Hope all this helps. Keith Edited March 31 by KEITH BEAUMONT 1 Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 31 Author Share Posted March 31 Thank you both. Birthday today is 'my day' so have to fix the brakes on my car trailer, the season is soon to start, then to focus on the lapping. Being old I have the double ended paste from various engine valve jobs done over the many years, so may go back and start again there. Between the fingers 40 feels like 1! I do have some fresh alum plate which is mirror smooth so can see how 40 scrubs the surface compared to 1. I can borrow a small US cleaner I've used for motorcycle carbs in the past to clean the cylinder after polishing, but like the idea of the 1200 powder and engine oil as the final pass. All part of this project! Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 31 Author Share Posted March 31 Lapped the cylinder using fine cutting compound I've used on car/bike valve seats, and what a difference that made! Changed the look of the surface, but removed a lot of the imperfections. Have ordered the 1200 powder for £6 off ebay. Thought I would try 'expanding' the piston by heating to cherry red and quench. Did that twice. The previously slack piston would now only enter the cylinder 1/2 way, so the growth was far greater than I expected. Started to mess about with the lower part of the piston as it was the tightest only to find 2 hair line cracks in the 'skirt', so now scrap. Further wait for the 1200 powder to arrive and then can progress I hope. Quote Link to comment Share on other sites More sharing options...
Jonathan W Posted March 31 Share Posted March 31 When I have expanded pistons in my impecunious youth, it was just to heat up and allow to cool naturally. So probably the quenching has caused the cracks. Sounds like you are on the right track with the lapping now though. What kind of finish did you have before lapping? Was it from lathe boring tool marks or did you use a reamer? Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted March 31 Author Share Posted March 31 Dark grey, but imperfections in the bore. The bore was fine turned, don't have the right reamer, so tried to work the surface with a new boring tip etc. I have to wait for the 1200 powder to come a few miles from me, it is in Birmingham. Quote Link to comment Share on other sites More sharing options...
Indian Posted April 4 Share Posted April 4 Small quantities of silicon carbide grit is available from lapidary suppliers and inj a wide range of grits. Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted April 4 Author Share Posted April 4 The fine 1200 grit powder arrived 2 days early, so mixed some with a drop of engine oil and set to polishing the bore some more. I really doubt i can improve it further, so have called that a day, and started turning the cast iron piston. I'm experimenting of getting a good chatter-free cut as I reduce the bar, and may have found the happy spot of cut, feed, tool overhang and angle. Quite a few variables but will try some refinement tomorrow starting with a fresh tip. Plan is to make a new 'tight' fit piston by lunch tomorrow. Quote Link to comment Share on other sites More sharing options...
KEITH BEAUMONT 1 Posted April 4 Share Posted April 4 You ae on the way! Dont forget that all important taper in the cylinder,so that you can achieve that"pinch" at TDC, when you are working the piston back and forward from its tight spot just above the Exhaust. A trick I find works when doing that is to double up on the gasket thickness on the cylinder face when doing that operation. Then, when you are just able to go over TDC and stop for the final clean up,only replace the original gasket for the final assembly for the attempt at starting. The piston then has a few thou more of tightness at TDC, which seems to make a dfference. Good Luck1 Keith Quote Link to comment Share on other sites More sharing options...
911hillclimber Posted April 4 Author Share Posted April 4 Thank you Keith. The prop is in the post to give me a bit of incentive! Any thoughts on the fit of the contra piston? Quote Link to comment Share on other sites More sharing options...
KEITH BEAUMONT 1 Posted April 5 Share Posted April 5 (edited) The fit of the contra must be gas tight for the engine to run successfully. The best method of achieving this first time, is the thin walled, taper fit, method attributed to the well known model engine builder, Ken Croft. You have mentioned your lathe does not allow you to turn taper accurately, so I will outline a method for you make the contra using this method. 1, Turn your bar to 0.005" larger than the bore at the top of cylinder. 2, Bore the bar to drawing dimension depth, but to a diameter that leaves the wall thickness at 0.020". 3,Part off leaving about 0.020"for final clean up. 4 Make short stub mandrel about 1.5 inch long, any metal, to make a good fit in contra. This needs a hole through it, at least 1/8" diameter, preferably threaded to allow a screw or plain pin to be used to push conta off mandrel when finished. I usually put a drop of super glue on the mandrel when fitting contra. Make sure this is pushed on mandrel square. 5, Turn the contra to finished length. 6, Obtain a piece of accurate flat steel as wide as the contra. Using this with 450 grade wet and dry and plenty of oil and a lathe speed you are happy with, concentrate on the solid end to achieve a taper to allow test fit in cylinder for about 2 thirds of its length. Make sure this is a flat taper. Do not over do the small end. When you are happy with this fit, eject contra off mandrel. If you used glue ,heat it up before pushing/screwing off. 7, After cleaning, contra can now be pushed in cylinder to be flush with top. I always apply some copper grease to assist this.. It should require a fair bit of pressure to push in, I usually use a vice with soft jaws to do this, but it should not be a force fit. Once you are happy with it, knock it out with a piece of wood and refit after you have finished the piston fit with the prop on. Hope this helps. Keith. Edited April 5 by KEITH BEAUMONT 1 Quote Link to comment Share on other sites More sharing options...
Jonathan W Posted April 5 Share Posted April 5 The other option is a redesign to use a brass contra-piston with o-ring seal. I guess they did not have o-rings in 1946, or whenever it was. If the brass contra is made to a close transition fit in the cylinder, at running temperature it should expand to be a tight fit. Quote Link to comment Share on other sites More sharing options...
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