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Engine Restoration - Part 3

Updated: Jul 19, 2022

Two of the engines six connecting rod and piston assemblies


This part will cover the restoration of the connecting rods and pistons. All six connecting rod and piston assemblies were removed after the crankshaft was lifted away from the crankcase. Next the two cylinder assemblies were detached from the crankcase.

Each cylinder bore was measured for wear, and roundness. These measurements indicated that the wear was sufficient to require all cylinders to be bored over-sized. The standard bore was 4.375".

Once again lady luck smiled on us, we found a set of six Ohio L-13150 NOS 4.5" semi-finished pistons with fitted wrist pins on Ebay! The original application of these pistons was for a 1930 Autocar truck. Incredibly the compression height of the piston pins matched that of the Simplex!


The box label for the pistons

The diameter of the piston pins was just slightly smaller than the original. We fabricated new bushings to fit and pressed them into the rods.

Fully machined forged connecting rod and new wrist pins and bushings


At this point we had 4.5" diameter semi-finished pistons and six 4.375" cylinders with a wide range of wear. We decided to have the cylinders bored 1/16 of an inch (0.0625") over-sized and then we machined the pistons to fit our new 4 7/16" (4.4375") bore.

The Simplex, Crane model 5 engine has a 6 1/4" stroke. The cylinder assemblies do not have a detachable head. To re-bore these cylinders it would require a special machine to "blind-hole" bore the cylinders to a 12.5" depth. We were fortunate to engage the services of a well known bay area antique automobile machinist. For this project he used a specially modified jig borer. The machine was eight feet tall!


For proper fitting the pistons were turned 0.010" smaller than the new cylinder bore and cam ground for heat expansion.


The piston ring grooves needed to be widened to accept the new 3/16" compression and oil control rings. The depth of the ring grooves needed to be increased for proper inside ring clearance, this was no problem as the semi-finished pistons were extra thick.

Each ring was placed inside a cylinder and the end gap was filed as necessary to achieve 0.018" clearance for heat expansion.


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