This is Part VI of a series we are sharing with you showing some of the more interesting details involved in rebuilding a 100-year-old 1914 Simplex 50 h.p. Speedcar for the Collier Collection. This episode covers connecting rod rebuilding operations, and machining new connecting rod and camshaft bearings.
- The lead image shows one of the 17-inch long connecting rods in a shop built connecting rod boring fixture. Just above it is an adjustable boring head, boring bar and cutter in the milling machine spindle. The boring head spins and is fed downward while machining the big end bore.
Just as the main bearings that were covered here earlier, the rod caps and bearing inserts had been filed numerous times to take up on wear to the babbitt-lined bronze-backed rod bearings. This was an effort to keep the bearings tight in the connecting rod bore and caps; in addition shims were inserted behind the bearings. Patch jobs like this generally only last a short time, and all of the bearings were quite lose in each rod when taken apart.
New connecting rod bearing inserts were needed, and the first step before making them is to rebore the lower ends of the rods round again for the precision fit that is needed; the job can be done on the shop’s special LeBlond connecting boring machine. Instead, it was accomplished using an adjustable connecting fixture fabricated in-house some time ago, set up in a vertical milling machine. This method will work better in this instance because the mill’s digital readout is very useful to deal with the varying center-to-center distances found in this set of rods.
- After boring, the connecting rod lower ends are given a smooth honed finish with a Sunnen honing machine. This provides for more complete contact between the bearing insert and the rod that helps in transferring heat out of the bearing.
While being powerful, durable, and state of the art in the 1908-1912 period, Simplex engine castings, and components were machined the old-fashioned way and many times the parts are not interchangeable. Bolt holes and stud locations vary in a single engine, and no two are exactly alike.
The center-to-center distance of this set of rods varied as much as .030 of an inch (30-thousands), and the four rod cap bolt holes are also are not in any standardized location. The caps had been filed as much as .008 in the past, and the lower end bores were oval.
One other consideration that needed to be dealt with was the connecting rod cap bolts. The location of the bolt holes vary considerably, and some of them and the bolt are open right at the edge of the bore. The use of sacrificial bolts instead of the originals while boring and honing allowed the originals to remain full size; later the bolts that protrude into the new bore are accommodated by relieving the o.d. on the new bearing inserts.
- The four new replacement connecting rod bearing inserts are seen in this photo on the far-left and right; in between are partially machined cam bearings. All are made out of cored bearing bronze of the type in the middle top of the image.
The next part of the job is to machine the new connecting rod bearing inserts out of cored bronze stock in a lathe, and then cut them at the parting line in the milling machine. We have already shared with you in the past the replacement main bearings for this engine, and as the rod bearings are quite similar, refer to the link to learn how they are made. The only difference in the two types is how the babbitt is bored, and you can learn here how the connecting rod bearings are finished off.
- Cutting a cam bearing in half with a slitting saw in the milling machine. This operation is followed by milling the parting faces to a smoother finish than a saw cut provides.
After the new camshaft bearings have been partially machined round and the holes for the fasteners that hold the halves together have been drilled and tapped, the bearing is cut in two on the milling machine with a slitting saw. This is followed by further operations that include boring the inside diameter of the bearing close to final size in the lathe, and then honing that bore to the finished inside diameter on a Sunnen honing machine.
- Setting up to finish machine the outside of a cam bearing in the lathe on an expanding mandrel.
Next each bearing is set up on a precision expanding mandrel in the lathe on the previously finished bearing bore. A dial indicator is then used to check that the o.d. of the bearing is running true to the i.d. Next the outside diameter is finish machined to a size this is only .0002 of an inch (two ten thousands) larger than the bearing housing in the crankcase. This fit will keep the replacement bearing from ever becoming loose again.
- The finished cam bearings and a shop made tool used to adjust the position of the bearings if needed after being installed in the crankcase. The pockets on either side of the top of the bearings are for collecting oil to lubricate the bearing and camshaft.
Just like every other early engine with an aluminum crankcase and high mileage that has been rebuilt here in the past, the original Simplex cam bearings were worn and also loose in the housing that holds them in the crankcase. In use, the aluminum crankcase has a higher coefficient of expansion when at operating temperatures than the bronze cam bearings, and in time looseness sets in and becomes an issue.
The fix is to line bore the bearing housings in the crankcase and machine new oversized diameter cam bearings with a slightly tighter fit in the bore than originally used. Many times the fit that should be a force or press fit (needs some force to drive it into position) was not tight enough to help ease the assembly process.
Quite often with a bit of planning and the construction of a simple tool, the correct fit can be used. The device if needed, is used after insertion of the camshaft and the bearings to orientate the bearing in the correct position so the anchor bolts that prevent bearing rotation can be inserted.
You can look back on all of the previous parts of this series here.