Diagnosing and resolving a tappet failure in a modified engine can be a complex endeavor, especially when the problem leads to a complete redesign of the valvetrain. This project began with an abnormal exhaust gas temperature (EGT) reading in cylinder No. 2, which consistently displayed temperatures around 300°F, far below the 700–900°F range of the other cylinders. Adjustments to fuel and timing provided minimal improvement, prompting further investigation. A leak-down test revealed the intake valve was not sealing properly, leading to the discovery of tappet interference and, ultimately, cracked tappets.
OEM Continental tappets were unsuitable due to their smaller face diameter, which was incompatible with the modified camshaft profile. Hudson tappets, with their larger diameter head, were installed, but shortened to work with Isky springs and a set of off-the-shelf valves. The shortened Hudson tappets removed critical features, such as flats required for adjustment, and suffered from mechanical stress that led to cracks under operational load.
To address the failure, alternative solutions were explored. Various configurations of tappet designs were evaluated using 3D modeling. Efforts to re-machine the Hudson tappets included grinding flats and incorporating a jam-nut system, but challenges with thread engagement and locking mechanisms persisted.
Valve spring selection emerged as a critical factor. Ensuring reliable operation of the tappets and camshaft required precise spring pressures to close the valve, maintain contact with the cam lobe, and resist engine vibrations. The Iskendarian 185G springs previously in use were replaced with custom springs sourced from R/D Valve Springs, which offered the required open and closed pressures. Adjustments were made to the installed height, and PAC Racing retainers were used to accommodate the new design.
Additional challenges arose when inspecting the camshaft, which showed damage to multiple lobes, including chips and pitting. This was likely caused by improper break-in procedures and the use of worn lifters on a new camshaft. The camshaft was sent to a specialist for repair, ensuring long-term reliability and compatibility with the redesigned system.
The project also required modifications to the valve stems. To match the new tappet dimensions, valve stems were shortened by 0.360 inches, and new keeper grooves were machined to establish proper alignment. These precise adjustments ensured compatibility between the tappets, springs, and valves, resulting in a cohesive valvetrain system.
This project highlights the complexities of modifying engine components and the precision required for success. From diagnosing initial performance issues to engineering solutions for tappet failures and valve spring compatibility, each step demanded meticulous analysis and innovation.
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