This article deals with application of a pre-chamber type ignition device in a heavy duty engine operated with natural gas. A particular pre-chamber ignition strategy called Avalanche Activated Combustion (originally ‘Lavinia Aktyvatsia Gorenia’ in Russian), commonly referred to as LAG-ignition process, has been studied by performing a parametric study of various pre- and main chamber mixture strength combinations. This strategy was first proposed in 1966 and has been mostly applied in light duty automotive engines. A majority of published data are results from developmental studies but the fundamental mechanism of the LAG-ignition process is unclear to date. To the best of authors' knowledge, the study presented in this article is the first generalized study to gain deeper understanding of the LAG-ignition process in heavy duty engines operating with natural gas as fuel for both chambers.The experiments are performed on a single cylinder 2.1 L engine fitted with a custom made pre-chamber capable of spark ignition, fuel injection and pressure measurement. Measured cylinder pressure is used to perform heat release analysis to understand combustion in the pre-chamber and subsequent ignition and combustion of fuel-lean charge in the main chamber. Results show substantial extension in lean limit of operation and corresponding improvement in indicated efficiency by burning fuel rich in the pre-chamber. Data related to engine-out regulated emission species are also presented.