New to the site…. Recently bought 1955 Chevy 3800. It’s got a GMC 270 with 4 speed “Granny Tranny”. I’ve trying to keep it as stock as possible but need some more speed since it tops out around 35mph with the 5.13 rear end. I saw a tech tip on here about swapping out pumpkin with 4.11 or 4.10 from 67-72 C20. Looking for some advice on where to start looking for pumpkin and then is this swap out something a DIY could handle?
Jeremy
Last edited by Peggy M; 05/24/20259:34 PM. Reason: more info in title ;)
Hi Jeremy, welcome to the ‘bolt. Nice looking truck. If you have some tools and the desire, there will be a lot of people on here that will be happy to help you tackle about anything.
Rammer - Welcome to Stovebolt! I'm guessing the info you mentioned reading about this swap came from this Legacy Driveline Tech Tip? To answer your question about where to begin looking for the sought after 4.10 center section, go back to the Tech Tip and scroll down to the section on "Finding the correct Donor". It details which trucks you want to be looking for as a donor source. When you do locate one of the potential donor trucks, look for the Service Parts Identification Decal (SPID) that should be on the inner face of the glove box door in the cab (see pic below for a sample). You are looking for a Rear Axle with code "HB8" as that designated the truck was equipped with the optional 4.10 rear axle. The same "HB8" code was used for 1968-72 C/K20 & 30 Chevy trucks for this option.
As an alternative to seeking a donor truck, you can scour your local Craigslist and FB Marketplace ads to see what may be offered. Or you could post an ad of your own in our Truck Parts Wanted forum and see if anyone has what you seek or can provide pointers to an offered item.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
Even with 28" tires, that 5.13 gear has your 270 turning 2200 RPM at 35 mph. Rated horsepower is around 3400 rpm which would be screaming down the road at 55 mph.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
The rear axle conversion to 4.10 in the Tech Tip sounds great but those axles are very scarce. My 1950 3600 with the 4 speed transmission (not sure how different your drive train would be) had a 4.56 rear axle and I gave up looking for an axle that could convert the original and instead installed a 1986 14 bolt rear axle with the 4.10 ratio. It was not a straight forward swap but there is a lot of information available here on what is required.
Dan, thank you … I may have found a pumpkin off a 1967 C20 Custom … they are saying it’s AT, V8, 10 bolt with 4.10. I’ve asked about the SPID info … awaiting to see a response. Yes that was the legacy driveline tech tip I was referencing.
WICrusier, thank you …. If I strike out on finding pumpkin and maybe even going with T5 for the OD… that may be the path I have to take. I’m trying to keep it as close to original/stock, but I at least need to get 45-55 mph out of it at some point.
If the U-joints and slip joint are in good shape, properly timed and the angles match, there shouldn't be any vibration to speak of. In that case, running along at 55 MPH will be a bit noisy, mostly because of the fan, but not unreasonable. Also, those trucks came with 32 or 33 inch tall tires in 7.00 or 7:50x17 size. The 7.50 size is still available from Coker and maybe others but not the 7.00 AFAIK.
By the last years of the task force trucks 19.5 wheels were offered. There are a couple of radial sizes of these available that might do for you. And you may be able to find steelies with hubcap clips. They will get you to 55 MPH at a more reasonable RPM, 3300 @ 60MPH as I recall. I"m sure there are many 4.57 punkins around that will drop your RPM by around 350 RPM at highway speed. They are found standard in 3/4 tons.
I would avoid a T5 in a 1-ton, especially for towing. There were Spicer 5-speed OD trans offered in the big trucks that have the correct bolt pattern. I have seen one installed behind a 302 GMC in an AD GMC PU. Except for the floor pan it was a bolt-in. Clark was also offered but I believe parts are NLA for those.
1951 3800 1-ton "Earning its keep from the get-go" In the DITY Gallery 1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
One of the things I invested in was a mini-tac. It is small, has an lcd display and runs on a watch battery. You just run one wire into the engine bay and wrap a couple of turns around a plug wire. It will give you a handy rpm readout. It is also helpful when setting the idle. I would guess they are under $20 to buy.
I'm guessing the info you mentioned reading about this swap came from this 
