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The making of a Neracar piston.
General
Considerations
The aluminum piston. Casting was done at:
M.P.H. Aluminiumgieterij Boundary conditions, considerations and other info Piston rings were bought at:
Technische Handelsonderneming Fa Nederland b.v.
The process of making this piston mimics that of a home-made tapered Cleveland
Piston. See drawing below:
Process steps 1. Clamp the piston casting with the lower end in the self centering chuck of the lathe. 2. Mill a little flat round area in the deflector and drill a hole for the tailstock centre. 3. Turn the outer diameter cylindrical over the full length of the piston skirt to a slight oversize, taking the targeted lower skirt (larger) diameter as a reference (64.88 mm). Do not yet process the piston ring grooves. Processing aluminiun 231 is not easy. It contains 10.5 – 13.5 % Si. During cooling down after casting, Si crystals of 0.3 mm are formed. These crystals are very hard and tend to be dragged through the surface during turning, leading to forming of irregular grooves. Different cutting tools, including diamond, and different angles were used, methylated spirit and paraffine was applied and different rpm’s were tried. Finally a straight cutting tool and butter (no margarine) as a cooling / lubricating aid was successful. Although this was just a cylindrical pre-cut, this finish would be good enough for the final cut, provided wet (oil) sanding with grain 400 was done afterwards. Later in the process, CCGT 06 02 04 indexable insterts were used, again with butter as a coolant/lubricant. 4. Make an aluminum auxiliary clamp to hold the crown of the piston. This clamp serves to be able to hold the deflector side in the chuck, not to prevent damage to the pre-turned piston. I was lucky to be able to borrow one from Wim Schulze. He has used it for his Cleveland piston. For photographs see last five pictures at http://www.geutskens.eu/neracar/cleveland_piston.htm 5. Clamp the piston in the aluminium clamp and saw off excess length. 6. Fit the clamp-piston assy in the 3 jaw chuck of the lathe and align. I used a 3 jaw chuck, because alignment needed very little adjustment. 7. Cut the piston to exact length. The total length is 95,3 mm, skirt length 87 mm. 8. While the piston remains in the clamp in the 3 jaw chuck, turn a rim inside the piston, with a diameter of 50 mm and a depth of 5 mm, allowing to correctly align the piston after successive removals from the lathe in later process steps, by using a billet. The inner diameter of the rim must be larger than the anticipated final inner diameter of the piston. Because we aligned the piston in #6, the pistons outer diameter and the rim inner diameter are pretty much in line. Deviations of a few hundredth of mm will disappear when we taper turn the piston. 9. Drill a hole in the piston bottom and tap M10 thread to accept the bolt for securing the piston on the milling machine. 10. Fit an angle plate on the milling support plate in order to drill the gudgeon pin boss under a right angle with the piston skirt (which in this step is still cylindrical. Orient the defector at a right angle; the gudgeon pin bosses run in parallel with the deflector. 11. Secure the piston with an M10 bolt and check if the bottom of the piston skirt is fully supported by the angle plate and check if the piston skirt is supported by the milling support plate over its full length. 12. Determine the position if the gudgeon pin boss centre, longitudinally with an edge feeler, the ‘highest point ‘ with a dial caliper. 13. Drill a small dimple for the final check of the position to drill. 14. Drill and mill one side of the gudgeon pin boss to allow for later reaming to accept the 15.84 mm wrist pin. 15. Turn a mandrel that tightly fits into the boss. Do not use an aluminium mandrel in an aluminium piston as this can casue severe ‘biting’. 16. Fit the piston with the boss over the mandrel. Use Molycote to allow for relatively easy removal of the piston from the mandrel. In my case the balance between a tight fit and possibility of removal gave problems, so for later processing I secured the mandrel in the piston with an M6 bolt. 17. Drill the other side of the gudgeon pin boss in line with the mandrel/first boss. 18. Mill the second boss to the required diameter. 19. Mill the recess for 406, Wrist Pin. Diameter 20.2 mm. The depth of 3.2 mm on the high side and 1.7 mm at the low side has to be compensated for the reduction of the piston diameter after fine taper turning. Hence the depth has to be increased with 0.3 mm. For milling the wrist pin bosses and the recess, CCGT 06 02 04 F-AL KX indexable insterts were used, first dry but with the last cut butter was used again. 20. Remove the mandrel. 21. Weigh the piston (693.0 g) and calculate how much weight will be lost by fine turning, making the piston ring grooves and gudgeon pin hole (23.5 g). There is a separate section on engine balancing; the calculations done so far teach us that the ‘bare’ original piston, which weighs 389.0 g should be 504.0 g, so 115 g heavier. I will make the piston 504.0 g to start real life testing and mill material off until the balance is optimal. The section about balancing can be found on http://www.geutskens.eu/neracar/balancing.htm . So, I shoot for 504.0 + 23.5 = 527.5 g. I have to mill off 693.0 – 527.5 = 165.5 g corresponding with 60.1 cm^3 or approximately 40x40x40 mm. Later processing will reduce the weight with 23.5 g ending up with a weight of 527.5 – 23.5 = 504.0 g. 22. Clamp the piston in the milling machine and align the work piece with the milling head. Mark the orientation of the piston in the clamp to allow the piston to be placed back in the original position after subsequent weighing. Mill the inside of the piston away until the anticipated final weight is achieved, taking into account the weight calculated in # 21. Fine tuning with respect to the weight was done on the lathe. The final weight was 529,7 g which seems close enough at this stage. 23. Anneal the piston in an oven at 220 - 250º C during a couple of hours to remove material stress. The actual process was 3 hours @ 230C. Measures taken before and after annealing were within 0.02mm. 24. Set the lathe’s top slide at the correct angle for taper turning by using a dummy. Make sure this is done accurately. The calculation should take into account any differences in cylinder diameter due to the boring process. In my case the cylinder diameter at the bottom was 0.05 mm larger than at the top. The top slide was placed under an angle of 5.5’ (minutes) 25. Make a round billet that tightly fits in the piston skirt bore. 26. Fit the billet in the chuck of the lathe, place the piston skirt bore over it and secure the piston with the tailstock centre. In case of a one sided billet, DO NOT REMOVE THE BILLET FROM THE LATHE UNTIL LATER TEST RIDING HAS BEEN FULLY COMPLETED! By making a billet with a cylindrical section to fit into the chuck, the billet may be removed and fitted again for further processing. The billet and the centre hole in the deflector will allow the piston to be removed and processed repeatedly. Make light cuts only, with not too low revs to prevent the piston from slipping. 27. Execute the final tapered turning of the piston by using the above mentioned clearances. 28. Polish the piston skirt with P600 abrasive, using paraffine. Do not use too much force to prevent Si crystals to be drawn out out of the surface and causing grooves. 29. Cut the piston ring grooves. Make sure the grooves are deep enough to avoid piston rings sticking out. The rings must be able to move freely without noticeable vertical play. Clearance 0.05 mm. The edges must be chamfered with P600 abrasive. Cutting was done with Kenmetal insert KC730 NG3M 300RK. Other tools failed. This type of inserts is not subject to aluminium build-up (false cutting edge). 30. Check the piston ring slot. This was 0.3 mm for all three rings. 31. Intermezzo. I encountered numerous problems with the fact that the gudgeon pin bosses were not true. I tried a number of times to drill a slightly larger true hole over an out–of-centreline hole, still remaining within the final target diameter. It appeared that the milling machine was not true. Finally fine processing of the bosses was done using mandrels on the lathe, similar to step 19. At the end the gudgeon pin could be pushed with a finger into the bosses without noticeable play. Do not use aluminium mandrels for an aluminium piston, at the materials ‘bite’. Rather use brass or steel. End of intermezzo and end of my headache. 32. Check the gudgeon pin in the bosses. 33. Fit the piston rings in their grooves and check a smooth movement. This has to be done before the securing pins are in place just in case the grooves might need some further processing on the lathe. Insert the piston with rings in the cylinder to check for (relatively) smooth movement. Make sure the slots are kept away from the ports. 34. Determine and mark the place where the piston ring securing pins have to be placed, away from the ports off course. 35. Mill a half-cylindrical recess with a diameter large enough to accept the head of the tapered securing pin. Drill a dimple with a centre drill. 36. Drill a hole with a spiral drill with a diameter just 0.1 mm below the thinnest end of the pin. 37. Ream the hole with a 1:50 taper reamer, similar to the pin. Start at the lowest groove, as this is the only one out of three that does not have a blind hole. Ream carefully and clean thoroughly after each cut before carefully checking the depth with the tapered pin. The head of the pin must be sticking out approximately 2 – 3 mm over the cylinder skirt surface. The reason for using this through hole is that the pin can be tapped back. A tapered pin that gets stuck in a blind hole will be impossible to pull out. Tap the pins with a hammer using moderate force. 38. File the half round slots in the piston rings where applicable and put the piston rings in place. 39. Fit the piston in the engine and do a test run with an 1:15 oil-fuel mixture. 40. Open the engine after a test run to check any high spots that need reprocessing.
Remark: The weight of the piston is 533.9 g, 4.2 g less than the targeted 538.1 g. Thanks to Wim Schulze for his advice
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