Fig. 2410.
This is often the case in repairs, and sometimes on new rods, in which slight inaccuracies of workmanship are apt to occur. In this case it is best to mark a line, as g, in [Fig. 2410], representing at each end of the rod the centre of the space f in that figure. Then set a pair of trammels to the correct length of the rod, and with one point of the trammel resting on the point at the intersection of line c with line d (the latter being the line g transferred to the centre of the bore) at the small end of the rod, we mark a line at the other end. If the lines d are too far apart, making the rod too long, the trammels will mark a line r, and the distance between lines r and d at the large end will be the amount the rod is too long, while half this distance will be the thickness of liner to go behind each bottom brass if the error of length is to be equally divided between the two ends of the rod, in which case a line t, midway between d and r, must be marked, the trammel then being rested on t, and the line s marked. These two lines, s and t, are then the centre lines for the bores of the brasses.
If it is determined that one pair of brasses shall be central in its space f, all the error being thrown on the other pair, this may be done by lining one pair up so that its bore is true to line d, and putting behind the back brass at the other end a liner whose thickness is equal to the distance between d and r at the large end of the rod. It is obvious that the measurement for rod length must be taken on the line c.
Having thus determined what thickness of liner is necessary to bring the rod to its proper length, it remains to find the thickness of liner necessary for the other half brass, to bring the key up to its proper position, the process for which has already been explained. After, however, the various liner thicknesses have been found, and the sheet metal selected to cut them from, it is well to try if the thickness is correct by cutting off a small piece of the metal, putting it in place behind the brass, and then, after keying up the brasses, the rod length may be trammelled.
As the liners placed behind connecting-rod brasses require to be very finely bedded, the facility with which their forms permit them to be fitted is an important consideration.
Fig. 2411.