The newer cars have a wider battery slot because the newer nimh cells are longer then when cars like the xxx & t-4 1st came out. In order to make them fit into something like a t-4 you have to dremmel the sides down some to make them fit, if you go with a 2p pack. Cars such as the b44 & new tcs already have a very wide battery slot so the a123s will fit. Cars such as the xxx-cr & k car that have the wider battery slot may not require dremeling.