We present a double sided, single pass Michelson heterodyne interferometer for dimensional stability measurements. In preliminary measurements, the double deadpath configuration (no sample) showed better than ±1.5 nm (2/) over 13 hours. A 30 mm stainless gauge block was then measured with a stability of ±1.2 nm (2/) over 9 hours. The interferometer was then moved to a facility capable of measuring in vacuum. In a pressure sealed environment, but not vacuum, the interferometer stability was better than ±0.6 nm (2/) over 23 hours. Using a Fourier analysis on this drift measurement, the limiting factor is the slight spatial gradients in the refractive index. With relatively large air paths greater than 400 mm, refractive index fluctuations on the order of parts in 10 9 are needed to cause this drift.