A great many motors have detachable heads and their quick removal is a great convenience, when there is carbon to be scraped off, pistons to be looked over, or other internal work to be done. However, replacing them is never quite so easy as removing them, partly on account of the cylinder heads themselves and partly on account of the pistons. The latter are particularly troublesome when the cylinder head is hinged. The cylinder head should be replaced with great care, and after replacement it is fully as important to bolt it on properly. If one bolt or a series of bolts is tightened too quickly and too hard, it is likely to result in cracking the cylinder casting or the head casting or both.
Fig. 38. Rotating Inlet Valve of Roberta Two-Cycle Motor.
Usually on an L-head type of motor, there are three rows of bolts for the cylinder head - one row along the middle, screwing into one side of the cylinders; another row screwing into the other side of the cylinders; and a third along the valve side. These should be tightened in order: first the middle bolts of the middle line, working out to the ends; next, in turn, the middle bolts of the back of the cylinder, the middle bolts of the valve side, the ends of the cylinder, and, finally, the end bolts on the valve side. All these should be tightened but a few turns at a time, and after all are down, a second round should be made in about the same order, to give each bolt a few more turns. In this way the cylinder head casting, which is both large and intricate, is slowly pulled down to the cylinder straight and true so that it is not warped or twisted. Moreover, if the cylinder is pulled down straight in this manner, all the bolts can be tightened more than if the first bolt were tightened very much, for that would result in cocking up the opposite side so that the bolts there could not be properly tightened.
In motors of the detachable head type, like the Willys shown in Fig. 35, the Chalmers, Briscoe, and others, the work of replacing the pistons, particularly if the crank case is cast integral with the cylinder block, is considerable. In fact it is sufficiently difficult to warrant making a special jig for guiding the pistons down into the long cylinder bores; this fastens onto the top of the cylinder where the head belongs.
As shown in the sketch, Fig. 39, the jig consists of a round shell, the interior of which is at the bottom of the same bore as the cylinder, but flares out. considerably at the top. The base consists of the flange needed for turning this in the lathe, and may be of any shape, size, and thickness. The action of the enlarged diameter at the top, gradually reducing to the exact cylinder size at the bottom, is to hold the piston rings in place and slowly contract them as the piston is lowered, so they pass down into the cylinder bore without trouble. One casting must be made for each cylinder bore, but the; time and trouble which they save and the injuries to workmen and parts which they avoid make them well worth while.
Fig. 39. A Simple and Easily Made Jig for Replacing Pistons in Detachable Head Motors.
As has been pointed out previously under Cams, one way to speed up an old engine is to replace the old camshaft and cams with new ones giving more modern timing. Another and a less expensive and troublesome way in which this can be done is by lightening the pistons and reciprocating parts. This the repair man will surely be called upon to do, as the manufacturer probably would refuse. In order to get out any amount of metal worth the trouble, it will be necessary to drill from 12 to 20 or more holes of from ½-inch up to 1-inch diameter, depending upon the size of the piston as to bore and length. In a six-cylinder motor, this amounts to almost 100 holes (even more in some cases), and as these must be drilled with considerable similarity in the pistons, it is well worth while to construct a fixture to aid or speed up this work.
The first requisite is a clamp, Fig. 40, to keep the piston from turning, so that it wilt not break the drill. A good way to begin is to construct a base with a pair of uprights having deep 90-degree V's in them, this being made so that it can be bolted to the drill-press table. The V's should be lined with leather or fabric, and for this purpose discarded clutch or brake linings answer very well. To one of the uprights is pivoted a long handle, having a lined V which matches with that of the upright below it and gives a good grip on the piston.
When drilling to save weight, the holes are put in close together, and in regular form, the idea being to take out as much weight of metal as is safe. In doing this it is well to work out a scheme of drilling in advance and to make a heavy brown paper template, fastening this to each piston in turn* It is not advisable to remove in the first instance all the metal possible, but only enough to show the benefit of the method; after it has proved satisfactory, the first job may be improved upon later. For instance, in lightening pistons it is a good plan to use a ¾-inch drill the first time and not to put in too many holes. If this proves satisfactory and the owner comes back for more, you can go over the same lot of pistons, using a ½-inch or 3/8-inch drill between the existing holes, and thus reducing the weight of the lower end of the piston to its lowest possible point.
Fig. 40. A Home - Made Wooden Stand to Facilitate Drilling Out Pistons.