I just acquired a 1954 GMC 450 fire truck minus cab and front end. 302 eng. and 5 speed trans. with an electric 2 speed rear axle. My question is was electric 2 speeds available in 54 or was this axle swapped in at some point?
If it is an older axle it is an Eaton brand, GM corporate axles were vacuum. It wouldn't surprise me that a GMC/fire truck would have the Eaton. My 1957 Chevrolet shop manual has tear-down for both. I checked the 1954 Chevrolet Truck specs on the GM heritage site, they list only one 2 speed axle, it being a vacuum shift, that doesn't mean that GMC didn't use the Eaton.
Last edited by 78buckshot; 01/21/20256:21 PM.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
Most of the 2 speed axles have two ratios- - - -"Slow"- - - -and "Really slow!" Chances are that the coach builder that made the fire body swapped in a 2 speed to get the rig moving when it had a tank full of water onboard. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I swapped in from a 6.5 single to a 5 something(I never counted the teeth) high two speed in my ‘54 630 with a 503. Milled the head .092”, has adequate power at 5200’ elevation.
Ed
Last edited by EdPruss; 01/24/20251:41 AM.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires. '47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle. '54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed. '55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
I thought I struck gold when I was able to remove the single speed 7.20 :1 and installed the 6.40/8.70 or whatever the ratios are. It changed my road speed by 10 mph in high range. Not that I expected highway speeds but I can now keep from upsetting folks on the 55 mph two lane, it'll do about 62 mph wound out at 4000 rpm.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
78buckshot, what is that 6.40/8.70 rear axle out of?
A day without laughter is a day wasted- Charlie Chaplin When wrestling a grizzly bear, you have to keep at it until the bear gets tired, not when you get tired. 1948 Chevy 2-Ton
78buckshot, what is that 6.40/8.70 rear axle out of?
The 2 speed pig came out of a 1956 Chevrolet 2 ton, one of our Big Bolt members, DougH6100 offered it to me. We live about 30 miles from each other so it was a "no brainer". I measured my axle housing and bolt pattern, measured the doner truck axle, looked like a fit. I had to use the axle shafts from the doner truck due to the length difference but it all was a "bolt in" process. I also salvaged the 2 speed shift valve and knob/cable from the doner truck, it fit like a glove. I ended up making gaskets for the pig and rear cover, I couldn't find any 13 bolt gaskets, I also rebuilt the vacuum actuator that mounts on the pig. I plumbed it with 3/8" soft copper 'cuz I have a good stock of it left over from my HVAC career. The 2 speed works great.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
In my admittedly limited experience with 2 speed axles, I've found that a vacuum shift unit is a lot smoother shifting than an electric type. Having a good sized reservoir tank with a check valve in it is a necessity, because when a shift is needed on an uphill pull, the intake manifold vacuum is going to be pretty close to zero. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
In my admittedly limited experience with 2 speed axles, I've found that a vacuum shift unit is a lot smoother shifting than an electric type. Having a good sized reservoir tank with a check valve in it is a necessity, because when a shift is needed on an uphill pull, the intake manifold vacuum is going to be pretty close to zero. Jerry
I've driven trucks with electric shift and trucks with vacuum shift, there is a learning curve to both. Once you have some miles on either one it becomes second nature, just like learning the little quirks of each transmission. I added a reservoir to the vacuum system when I added the 2 speed pig, the original vacuum system on these trucks has a check valve already installed on the firewall, that valve is the first fitting in line from the manifold. If you install a check valve at the reservoir, it won't allow vacuum into the tank or won't allow vacuum out of the tank depending on the orientation of the valve, the factory check valve holds whatever vacuum is in the system and keeps it from being pulled back into the manifold.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
Your 450 would either have a 16600 or 1700 Eaton. They were very good rears and Eaton built them for many years. Parts are still available. All truck manufacturers that offered a two speed used them. I always found more Eaton parts at Ford dealers than anywhere else. International dealers also stocked a lot of parts for them. Even while they were offered, GMC didn't stock many parts for Eaton rears. The main thing that goes wrong with them is the shift box. There is a pair of limiting switches that corrode over and there is a small ball bearing that corrodes if water gets in the box. But these parts are still readily available. Here is a parts catalogue
