I believe the 48-52 Chevy two speed axles(as I recall made by Timken) do not default to low. On my '50 COE there are two vacuum hoses going to the rear axle, and through the action of the diaphragm, pull the axle into high or low depending on the switch selection. My '47 Diamond T has a single vacuum hose going to the rear axle (as I recall made by Eaton)and it does default to low.
Warning: If you have such a rear axle that defaults to low, AND a drum type transmission mounted parking brake, DO NOT leave it in high gear when parked and you are depending on the parking brake to keep the truck in position. As the vacuum bleeds down, the rear axle will find a "neutral position" and your truck may roll away!
I drive my two larger trucks at 6250 feet elevation and up and down low mountain hills. My engineer friends and family tell me that you lose 3% of your horsepower for every 1000 feet elevation. My COE weighs 7500 lbs and is powered by a later 235 (1958) and my Diamond T weighs 6500 lbs and is powered by the stock Hercules JXC 282 cubic inch that is supposed to get 100 horses. Needless to say, I need all the gears available and enjoy having the two speed rear axles, even though they are truly low and lower. Especially the night I was the designated driver and went at 2:30 am to Whisky Row with the COE to pick up my two sons and 25 of their friends. I had a thousand foot climb over 5 miles back home. So I would try to maintain the function of your two speed axles if possible.
Kent
Last edited by Lightholder's Dad; 11/08/2015 7:13 PM.
