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1 Ton GMC 248 to 302 swap #1294366
Fri Jan 11 2019 08:56 AM
Fri Jan 11 2019 08:56 AM
T
TrustyOldIron  Offline OP
New Guy
Joined: Jan 2019
Posts: 3
Oregon
I have a 1949 gmc 9' bed with the Muncie 4 speed and a 248 (I think. could be a 228) it runs and drives ok but it's really low on power. I've rebuilt the carburetor, replaced the points, condenser, ignition coil and spark plug wires, replaced the intake and exhaust manifold gaskets, removed the exhaust riser butterfly, adjusted the timing, valve adjustment and probably a few other things I can't remember right now. It has 98 psi compression across the board +/- 4 psi and runs good oil pressure. Is there anything else I should try to get more power out of it? Because recently a military 302 showed up near me on CraigsList and I was considering a swap. Are the engine mounting points bellhousing etc for the 248 the same as a military 302? Thank you for your time!


I have a lot of interests in old machinery and electronics. My parents bought me a 1949 GMC 1 ton 9 foot bed project truck for my 11 birthday and I've loved it ever since.
Re: 1 Ton GMC 248 to 302 swap [Re: TrustyOldIron] #1294406
Fri Jan 11 2019 05:48 PM
Fri Jan 11 2019 05:48 PM
E
EdPruss  Offline
Shop Shark
Joined: Jun 2011
Posts: 2,580
Longmont, CO
The ser. no. Starts with the displacement , I. e., 228xxxx. Seems like military front mounts are different, however, center bottom, front mount could be used, or make your own front side mounts. Your bellhousing is probably too small for a, 11" clutch, might need to get a later bellhousing, or get an adapter plate to use a modern bellhousing. Your existing flywheel is probably 4 bolt, 302's have 6 bolt flywheels, so use the 302 flywheel. Chevy flywheels can also be redrilled to the 302 pattern. Military 302 oil pans are extra deep, OK to use unless too high. Only 270 and 302 oil pans work on 302's, longer stroke requires sides of pan to be wider.

Makes your truck more useful. Easy swap to do if not in a hurry.

Ed

Last edited by EdPruss; Fri Jan 11 2019 05:49 PM.

'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.
Re: 1 Ton GMC 248 to 302 swap [Re: TrustyOldIron] #1294419
Fri Jan 11 2019 06:50 PM
Fri Jan 11 2019 06:50 PM
G
Grigg  Offline
.
Joined: May 2005
Posts: 8,828
Lexington, VA
Welcome,

Do you have 5.13 axle gears or something faster?

A 228 is plenty enough power for slow gears, and your engine sounds like it's in good tune, or should be given your work on it unless something is off.
I have 4.10 gears in mine and a somewhat worn 228, it drives nicely, enough power still. (though more would be fun)

A 302 and faster gears would be great, but first find out why you're not satisfied with it as is?
What are you comparing the truck to, why do you think it is "low on power"?


1951 GMC 250 in the Project Journals
1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-
Re: 1 Ton GMC 248 to 302 swap [Re: Grigg] #1294492
Sat Jan 12 2019 03:20 AM
Sat Jan 12 2019 03:20 AM
T
TrustyOldIron  Offline OP
New Guy
Joined: Jan 2019
Posts: 3
Oregon
I believe mine is a 5.12 or 5.13 rear but I get passed by semi trucks when pulling away from a light. I don't know how quick these trucks are meant to be as i've never driven another one. But it seems like it should be faster. What do you guys think?


I have a lot of interests in old machinery and electronics. My parents bought me a 1949 GMC 1 ton 9 foot bed project truck for my 11 birthday and I've loved it ever since.
Re: 1 Ton GMC 248 to 302 swap [Re: EdPruss] #1294494
Sat Jan 12 2019 03:32 AM
Sat Jan 12 2019 03:32 AM
T
TrustyOldIron  Offline OP
New Guy
Joined: Jan 2019
Posts: 3
Oregon
I appreciate the response! Yea it's a complete engine with all accessories. Would it be ok to use the front center mount hold the 302 engine up? I have a craigslist add link are you allowed to post those here? I'll check my engine's serial # here soon to check for sure what size mine is.


I have a lot of interests in old machinery and electronics. My parents bought me a 1949 GMC 1 ton 9 foot bed project truck for my 11 birthday and I've loved it ever since.
Re: 1 Ton GMC 248 to 302 swap [Re: TrustyOldIron] #1294511
Sat Jan 12 2019 06:14 AM
Sat Jan 12 2019 06:14 AM
P
pdlightning  Offline
Shop Shark
Joined: Aug 2011
Posts: 101
Santa Clarita, Ca.
I have a heavy 51 with a 228 and converted rear end from 5:14 to 4:10s. The engine is strong with good torque but limited HP. It will do 50 MPH with a good run on flat ground, but takes a while to get there. Hills are a different story! I think the stock 228 engine is maxed out with our heavy trucks.

Re: 1 Ton GMC 248 to 302 swap [Re: TrustyOldIron] #1294528
Sat Jan 12 2019 01:09 PM
Sat Jan 12 2019 01:09 PM
T
tom moore  Offline
Shop Shark
Joined: May 2006
Posts: 521
New Hampshire
Check out Inliners International, OldGMCtrucks, and research this site for the list of tasks needed to convert a military 302 to civilian application. Currently doing the same and wrapping up this project just today. I found a Craigslist a 24V 302 that was surplused and completely rebuilt but never run. I still disassembled it to do some block work for full pressure oil conversion, dip stick hole drilling, cutting the opening for the mechanical fuel pump and grinding down a large boss that was part of the block casting. Added the standard front mount timing plate and cover, did not use the 302 flywheel because it was for a completely different automatic military hydro transmission, but acquired a trashed 270 for swap parts such as oil line for rocker arm, oil pan, fan, bellhousing, flywheel, starter, distributor, oil pump and sump, and other miscellaneous parts. A 270 oil pan is the same width but deeper and heavier than the 228 oil pans I have - seemed like a better choice.

You can do the same with your 248 and switch over everything that attaches to the block, head, or crank. Your 248 timing plate and cover, oil pan with pump, distributor, water pump, thermostat housing, all will be a direct replacement.


Last edited by tom moore; Sat Feb 02 2019 05:17 PM.
Re: 1 Ton GMC 248 to 302 swap [Re: TrustyOldIron] #1297233
Sat Feb 02 2019 03:18 AM
Sat Feb 02 2019 03:18 AM
E
EdPruss  Offline
Shop Shark
Joined: Jun 2011
Posts: 2,580
Longmont, CO
248, etc., pans will not fit 270's, etc., because of the longer throw.

Ed

Last edited by EdPruss; Sun Feb 03 2019 01:53 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.
Re: 1 Ton GMC 248 to 302 swap [Re: EdPruss] #1297248
Sat Feb 02 2019 05:40 AM
Sat Feb 02 2019 05:40 AM
tclederman  Offline
Boltergeist
Joined: Sep 2001
Posts: 27,501
Jarvisburg, NC
Originally Posted by EdPruss
248, etc., pans will not fit 270's, etc., because of the longer trove.
Ed

Excuse my ignorance, but, what is a "trove"?


Tim
1954Advance-Design.com
1954 3106 Carryall Suburban - part of the family for 50 years
1954 3104 5-window pickup w/Hydra-Matic - part of the family for 14 years
Z-series (54/55) GMC 350 (2-ton) COE - now part of Dave's family
- If you have to stomp on your foot-pedal starter, either you, or your starter, or your engine, has a problem.
- The 216 and early 235 engine are not "splash oilers" - this is a splash oiler.
Re: 1 Ton GMC 248 to 302 swap [Re: TrustyOldIron] #1297256
Sat Feb 02 2019 12:40 PM
Sat Feb 02 2019 12:40 PM
G
Grigg  Offline
.
Joined: May 2005
Posts: 8,828
Lexington, VA
Probably misspelling of “throw”

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