Bellhousing gasket that determines idler gear end float and seals end of engine and gearbox.
We recommend a trial fit if the gearbox is off the motor to check idler gear endfloat... which is approx .004" - .006".
If the engine is already installed to the gearbox it is difficult without good measuring equipment to determine the clearance, and the only guess !! is to measure the old gasket pieces and compare with the new one.
Too little clearance and the gear train SEIZES not a good look !!!