Stuart No 1 - Connecting Rod

It is important that the 2 holes in the connecting rod are parallel in both planes for the engine to run smoothly.

The process described here machines both the critical holes using a boring tool on the face plate, to keep things as parallel as possible.









First the con-rod was mounted in the 4-jaw chuck as shown here and set to run true. Then the locating spigot at the far end was faced off and centre drilled so the tailstock could be added for support.

With the tailstock in place, the con-rod stem was turned, along with the top and bottom faces of the foot. The outer diameter of the foot was tidied but left oversize.

The top slide was used on a 1 setting to get the taper on the stem.

Before the con-rod was removed, the 2 bolt holes were marked the base of the foot.









Next - the piece of bronze to make the big-end bearing was cut in half and the 2 halves tided up using the 4-jaw chuck.

The 2 parts were stuck together with double sided sticky tape and mounted together in the 4-jaw chuck. An indent to match the spigot on the base of the con-rod was drilled in the middle of the exposed face and then the face was machined to take one of the bronze parts to the final thickness. A 2.5mm hole was then drilled right through the 2 parts.

Then the 2 parts were reversed in the chuck and the other half was machined to its final thickness.









The 2 bearing halves and the con-rod were now assembled together. The shank of a 2.5mm drill was used to hold the 2 halves of the big-end together. The 2 bolt holes were drilled right through the set of parts so that they could be bolted together.









The 2 critical holes on the con-rod (little end and big end) would be machined on the faceplate but the top of the con-rod wouldn't fit through the gap in the lathe bed. So first it was placed in the vertical slide and machined on both sides, but left oversized.









To bore 2 parallel holes in the con-rod the assembled parts were mounted on a piece of gauge plate, which was then bolted to the faceplate of the lathe. The gauge plate could be moved around on the faceplate whilst still maintaining a common reference, so that both holes could be bored parallel, even when re-centring the work piece.

The assembly was held with a single bolt drilled and tapped into the top end of the con-rod and the two big end bolts at the bottom. This mounting process allowed the con-rod to be flipped over so that both sides could be machined. The top bolt hole would be machined away once the top slot was created.









The images below show how the holes were drilled and bored. The cross-head bolt hole was made first and in the same setting one side of the little end was faced to thickness. A reamer was used for the cross-head bolt hole as this was too small for the boring tool. The big end bearing was made using the boring tool and again the bearing was faced to thickness and also had the shoulder machined.

In addition to the drawing a slight chamfer was machined on the inside of the big-end bearing to allow for the small fillet of silver-solder that was at each end of the crank-pin.

















To complete the con-rod assembly, the part was reversed on the gauge plate and matching profiles were then cut on the second side.

Next the con-rod was returned to the 4-jaw chuck and the bevel side faces were turned. The 2.5mm through hole drilled in the big-end bearing meant that set up was easy and it ran true from the outset.









When the con-rod was clamped on the crank-pin for the first time it was a little tight although could be rotated with some force.

Then metal polish was used with progressive tightening of the big-end bolts to help it bed-in. Once a smooth turning fit was achieved the polish was cleaned out and replaced with light oil.







The final operations on the con-rod were to shape the cross-head end to its final size. First some filing buttons of the correct diameter were made and put in place. Then in the vertical slide, the 2 sides of the cross-head end were milled right up to the filling buttons. This ensured that the 2 sides were equidistant from the centre hole.






Then in the vice the end was filed to shape again using the filing buttons as a guide.






The assembly was placed back on the vertical slide to machine out the forked end profile.

This was done by first drilling a hole of the correct diameter and then opening out the hole using a milling cutter. In practice, it was actually found that a milling cutter was better for drilling the hole because the normal drill tended to wander off line. This problem was created because this hole intersected with the cross-head bolt hole drilled earlier.





Stuart 1 Index