How different are the HO24/29 in the 3100 and the HO 52/72 Eatons in the 3600? Is it possible to modify the 3.55 pinion available for the 3100 to fit the 3600 rear end?
VERY different! The 4.10 gear found behind automatic transmissions in 3/4 ton trucks and Suburbans in the late 1960s is about the highest gear that's available for the HO52/72, I believe. I don't think even the aftermarket gear places sell higher ratios. The rock crawler bunch LOVES the 4.56 and lower gears, and they will swap 4.10s for the granny gear ratios if they have them. You might do a little window shopping on swap meets on the off roaders' websites. 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!
But why doesn't a 3.55 ring and pinion fit in an HO52/72? Is it the bolt pattern on the ring gear? Or is the shaft on the pinion too long/short/ugly? Just wondering if there's a mod a machine shop could do.
With enough money, a machine shop could probably fit a 3.55 R&P in an HO52/72 differential. But you'd need to give them money by the truckload if you could even find someone to attempt it. The pinion on a 1/2 ton axle is overhung, with both bearings forward of the gear, while the HO52/72 pinion is what's called straddle mounted with bearings both forward and aft of the gear. Take a look at the attached pic clipped from the 1948-1951 Truck Shop Manual and you'll see that they're significantly different.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
Or, you could just run a different axle housing like a GM corporate 14 bolt, or a Dana type 60 or 70 with a 3.55 ratio, and let somebody else do all the engineering and manufacturing for you. There's literally millions of them out there. 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 see the Dana 70 has 8 bolt wheels and used ones from Dodge Rams in the 2000's aren't all that expensive. Is there a particular application to be looking for that fits with fewer modifications?
The main issue with newer options for rear axle conversions is the overall width.
There is a great write up related to the conversion to the 14 olt GM axle that I bleieve includes information about the various options and what the trade offs are for the different axles. I bleieve it is a Tech Tip but I need to find it and will update this post with that information when I do.
Consider a Gear Vendor overdrive. Thats what I did to my 3600. I have the SM420 four speed, and I was able to find a 4:10 out of a 71 Chevy c20. I end up with a final ratio of 3.2:1, and with my 235 I have plenty of power to cruse hwy speeds with ease. I average between 17-20 mpg Hwy and I am very happy with it. I think if you go too high rear end gearing you will suffer in starting in second gear. 1st gear is the creeper gear and not practical to use except for parades. I run about 2100rpm at 55 mph and it is a pleasure to drive. I can go faster if I need to, but I don't live where the speed limit is higher than 65mph on the four-lane highway. So I use 2nd, 3rd, 4th, and 4th OD for my driving. Around town under 30 mph, I don't use the OD.
