The temperature and pressure of the charge, reached at the end of the compression stroke when an engine is cranked for starting, decide whether it will start and attain selfsustained running. These, in turn, are affected by the crevice volume in, and the blowby from, the engine, particularly at cold ambient temperatures and low cranking speeds. This paper presents a model to estimate these effects. Tentative values are proposed for the parameters that appear in the model based on experiments performed on small engines motored in a cold chamber. The model can be incorporated in engine cycle simulation programs to allow for crevice and blowby effects.It is impossible to prevent gas leakage entirely from an operating reciprocating engine. Gas may leak at the valves, the cylinder head gasket, the spark plug gasket, the injector gasket and the piston rings. The gas that leaks from the cylinder past the piston rings into the crankcase is termed “blowby”. Blowby constitutes the major part of the total gas loss from the cylinder  and, unlike the other leakages, cannot be eliminated completely. Gas leakage from an operating engine leads to a decrease in its performance and an increase in hydrocarbon emissions. In an engine that is cranked for starting, gas leakage reduces the temperature and pressure attained at the end of the compression stroke. Gas leakage is particularly severe at low cranking speeds and, in conjunction with the generally poor fuel distribution and preparation prevailing under these conditions, can inhibit cold starting at low ambient temperatures.There is another feature inherent in the design of the reciprocating engine which has a similar effect on cold starting in that it also reduces the temperature and pressure attained at the end of the compression stroke. This is the unavoidable presence of small crevices in communication with the working space of the cylinder. The working clearances of the engine such as the piston top land clearance, the clearance between the rings and their grooves and ring gaps constitute these crevices. Such crevices could also exist around the inner periphery of the head gasket, spark plug thread clearances, clearance between the fuel injector and its hole, etc. When the cylinder pressure rises, gas is compressed into these crevices where, because of the very large surface/volume ratio, it is cooled rapidly to the metal surface temperature. Thus, in effect, the crevices augment the heat losses from the charge and hamper cold starting. Though much of the crevice gas is returned to the cylinder when the pressure falls, some of it in the ring pack leaks into the crankcase as blowby.In spite of their importance, the analysis of the effects of gas leakage and crevices on engine performance has not received much attention so far. There have been a few attempts made to analyze blowby in engines [2,3]. The performance of the ring pack is influenced by (a) the movements and deflections of the rings under me action of gas pressure, inertia and friction forces, (b) the hydrodynamics of the oil film on the liner, and (c) the finish of the various surfaces moving relative to one another. In view of these complications, it is unlikely that a single comprehensive model that can predict the performance of ring packs of all engines at all operating conditions can be developed. This paper presents a simple model to estimate gas leakage and crevice effects on the engine cycle. Since cold starting at low ambient temperatures is of concern, the model is described as it applies to a motored engine; its extension to an operating engine is straightforward.