Propwash April 2019 web.pub

because this is the hottest part of the piston and it therefore will expand much more than areas on the piston sides. (Note: Jim Wilson felt that the piston taper above the wear band should be lapped to match the cylinder taper at top dead center. This gives a fit like that of the glass plug in a wine decanter at TDC.) The listed numbers above are for The Nelson 45 only! For example, the piston top taper and the cylinder taper for the .90 engine which has a .902" (23 mm) stroke and a 1.125" (28.6 mm) bore are; piston top taper .002" (.05 mm)/side, down .100" (2.5 mm) and a cylinder taper of .002" (.05 mm) on the ID, measured from the top of the exhaust to the top of the cylinder. Notice in the above photo the piston goes to the very top of the liner. Its OD is the same as the ID of the liner at the top. A motor fitted like this will run, but will not make any serious HP. One of the two pistons in the middle will be good to use in this liner. To determine the correct size of a piston for any tapered liner; first, measure the liner top and bottom carefully; second, check for roundness at these points; third, check at different points above the exhaust for roundness and taper to the top of the stroke. Out of roundness at the bottom has no effect. Liners with insufficient taper CANNOT be fitted properly with the correct crunch point. (Continued on next page) Honing is also used. It takes proper technique to hold roundness to within .0005” (.01 mm) with the right taper. The idea of all this is to produce a cylinder and piston with a taper that fits snugly at the top of the stroke for a good seal during combustion. Greater clearance farther down reduces friction, a very important factor in small engines. There is no typical taper amount that is used for a brass or aluminum cylinder engine. The best cylinder taper amount in any particular engine with brass or aluminum cylinder can only be determined by being able to accurately test the engine. In general, a larger bore, shorter stroke engine, can stand more cylinder taper than a smaller bore, longer stroke engine. The accurate measurement of the taper amount being used becomes critical in this process. We have found that different alloy materials, DO NOT change the correct taper amount, but they will change the cold fit of the engine. In the fitting of pistons to a cylinder, Jim uses pistons that are machined in .0001" (.002 mm) steps. Cylinders are honed to the correct taper amount, checked for roundness and are then fitted with a piston. Another very important factor is keeping the cylinder, regardless of material, as round as possible at the engines operating temperature. The piston’s wear band and the cylinder’s wear pattern will tell you if the cylinder and piston are remaining round at the engine's operating temperature. Notice how the cylinder’s lip is designed on the Nelson pylon racing engines. That lip is .100" (2.5 mm) thick on the .45 size engines and it is .200" (5 mm) thick on the .90 size engines. It is clamped between the engine's head and the crankcase. This lip is as large as the OD of the crankcase. This keeps the bore round and gives great heat transfer in this most critical area of any high performance engine. The photo below shows what a new piston looks like after it has been fitted to its cylinder. This piston example is from the Nelson .45 long stroke racing engines used in competition. Notice the position of the wear band on a brand new piston is .090"' (2.3 mm) down from the piston crown. The taper amount for this piston in an engine that has .800" (20 mm) bore and a .800" (20 mm) stroke is .001" (.02 mm) /side, .090" (2.3 mm) down. This means, if it was possible to measure the piston's OD at its top edge, it would be .002" (.05 mm) smaller than its major OD. The taper amount in the cylinder is .0015" (.04 mm) on the ID, measured from the top of the exhaust, to the top of the cylinder. Engines built this way maintain their cylinder to piston fit indefinitely. Any straight sided piston running in a tapered bore will experience wear at its top edge PROPWASH 28 April 2019 Sleeve in Honing Fixture Nelson Piston Wear Band Too Loose Piston Position