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392 placed in position and last of all the low pressure cylinder. All cylinders are bolted together and securely fast- ened to their columns. The assembly is now about half completed. With the cylinders in 'position and correctly aligned, the columns are firmly tied in place in which condition the heavy connecting rods and crossheads can be assembled without throwing an undue strain on the columns. This might happen if they were placed in position before the cylinders were installed. As a matter of fact, it would be much easier to assemble the connecting rods and _ crossheads + before the cylinders were positioned because they could readily be lowered in place through the gap between the tops of the columns. Safety . however, re- quires that the cylinders be put in place first. The connecting rods are first bolted in place on the crankpin boxes. The upper end of the connecting rods are forked to span the crosshead. These units are now assembled. First, the wrist pin is placed in position and the slippers installed approximately the correct distance from the center line. Then the crossheads are placed in their working position between - the crosshead guides and carefully aligned Mm relation to the cylinders. It is obvious that the crossheads must be -centrally located, that is, the hole to receive the lower end of the piston must align correctly with the bore of the cylinder, both fore and aft and sidewise. If this is not done properly, an undue strain is thrown on the finished engine that would ultimately cause heating. The pistons, to which the piston rods have already been fitted, are lowered in place and the piston rods fitted to the crossheads. The engine can now be turned over slowly and the crosshead slippers adjusted to bear properly on their guides. The guides for the valve rods are fitted in place next after which the valves are lowered in position and connected up with the valve gear. Getting Cylinder Clearances Correct clearances at the top and bottom of the cylinders are im- perative and they are marked plainly for the guidance of the engineer in subsequent overhauling operations. If the clearances are not correctly de- termined and plainly marked, this in- attention might result in a cylinder head being knocked out while at sea. This is one of the most serious acci- dents that can happen to a marine engine and for this reason, ample pro- vision is made to guard against it. , The lineal clearances on the en- THE MARINE REVIEW gine being built at Hamilton are ¥B- inch at the top and 9%-inch at the Lotta of all cylinders. Both the clearance and striking points are plainly marked on _ the crosshead guides. In the assembly operation, several liners are interposed between the lower end of the connecting rod and the crankpin bearings. ( these liners can be adjusted to give the necessary. clearance. : The reversing engine is now in- stalled in place. This unit is a single- cylinder engine, equipped with a slide valve and bolted to the medium- pressure, back column. It has float- ing links and levers and is controlled from the handling station at the front of the engine. Now that the valve gear and re- verse engine are in position, the valves can be set This.is oné.of-the most important of the assembling opera- tions, due to the fact that the eccen-. trics are permanently keyed in place. Thus; if the gear is not properly set, it is impossible to change it without resorting to offset keys which are slipshod appliances at best. How the Valve Gear is Set In 'setting the valve gear, the eccentrics are first positioned in the approximate positions to give the de- sired annular advance... This is de- termined by the assembler's eye. Thus the go-ahead eccentric of each separate valve gear is set approxi- mately 90 degrees ahead of its crank, plus lap plus lead. Due to the ac- cepted marine engine practice, the bottom ports are given more lead than those at the top. The leads on this engine are: High pressure, top, 15/32 inch; high pressure, bottom, 19/32 inch; intermediate, top, 9/16 inch; intermediate, bottom, 5% inch; low pressure, top, 11/16 inch; low pressure, bottom, %-inch. The high Pressure cylinder valve is of the in- side admission type while the inter- mediate and low pressure valves are arranged for outside admission. Piston valves are employed on the high Pressure and intermediate cylinders while the low pressure cylinder is equipped with a double-ported semi- balanced slide valve. Due to their design, the high pressure and inter- mediate valve are, of course, balanced. With the valve gear thrown in full go-ahead position, the go-ahead eccen- tric is set to give the approximate lead. Then the gear is thrown in fuli gear astern and the backing eccentric fully approximately set. Now the assembler has a definite point to start from. Next the engine is set exactly on its center. This is accomplished by making a mark on the crosshead Thus, August, 1919 guide when the connecting rod is 4 few degrees away from its highest center point. At this juncture a mark is made on one of the coupling flanges and a pointer set to align with this mark. Then the engine is set off center in the other direction until the crosshead coincides with the mark previously made on the crosshead guide, and another mark made on the coupling flange. By dividing the dis. tance between these two marks on the coupling flange scribing a center mark and setting the shaft in posi- tion so that this center mark is in alignment with the pointer, it is obvious that the engine is exactly on center. It is important that this operation be carried out carefully due to the fact that the angularity of the main rod is at its least poiut of effective- ness while that cf the eccentric rod is at its greatest. Thus a slight error in setting the engine on its center would result in a faultily adjusted valve gear. Now with the engine exactly on center, the valve is given the correct lead by advancing or retarding the eccentric. Then the engine is set on its lower center and the valve lead measured. If these coincide with the leads above given, all well and good. If they do not, however, the diffi- culty is remedied by shimming be- tween the eccentric straps and their eccentrics by means of liners. As a rule, all marine engines carry liners at these points to facilitate valve set- ting. This applies to the high and intermediate pressure valves _ only. Due to the fact that the low pres- sure valve is provided with an ad- justment on its stem, as shown in Fig. 21, it can be adjusted to give the correct travel without shimming the eccentric rods. Setting the Reverse Eccentrics The next step is to set the reverse eccentrics. In this operation, the crank is placed successively on the top and bottom centers and the leads measured. If the leads for the back- ing motion agree with those for the forward motion, the eccentric has the correct annular advance. The astern - eccentric is shifted until these leads agree. Now with the backing eccen- tric in its proper position the engine is carefully placed on the top center and the top lead measured. If. this lead is the same as that given by the go-ahead eccentric, all is well, but if it is found to be less, the astern eccentric rod must be shortened by taking out some liners between it and its eccentric strap to give the correct lead. Conversely if the lead is longer, (Concluded on Page 397)