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49/50 3600 SBC Conversion
#1434860 Mon Dec 27 2021 05:51 PM
Joined: Nov 2021
Posts: 159
W
'Bolter
I am at the beginning of my 3600 restoration project that includes plans to convert to a SBC and I am looking for input/feedback regarding the approach others have taken. Overall project goal is to have a great looking truck that I can drive and show that will be comfortable to drive on country roads at 55/60 mph. May do some limited hauling/towing but not really a priority.

First some basic thoughts (nothing set in stone):
I plan to keep the 3/4 ton 8 bolt wheel pattern and original axles/suspension, I have read a bit about rear axle ratio changes to reduce the original 4.56 ratio to 4.10 and am anticipating going to a NV3500 or 4500 transmission to get overdrive, I am anticipating a disk brake conversion for the front and possibly the rear, I anticipate converting the steering to power assist, and I have a 350 SBC that spun a rod bearing so needs a complete rebuild. Not sure what wheel/tire size to shoot for but clearly that interacts with the rear axle ratio discussion.

I am considering building the SBC as a 383 given that I need to replace all the rotating parts anyway but realize either a 350 or the 383 will have a lot more power than really needed. The engine was from a TBI version, 1991 Chevy 2500 but I anticipate utilizing a carburetor set-up rather than fuel injection to keep it simple. I do not anticipate using the 1991 cylinder heads due to poor performance so probably Vortec heads with the correct intake manifold to utilize a carburetor. Cam selection to be matched to the displacement/heads/compression ratio etc. to provide a reasonable idle and low end torque appropriate for a truck engine.

I have seen several options for SBC engine mounting (bolt in and weld in) but I am a little unclear about the need to modify the firewall for bell housing clearance and/or rear engine mount changes required. The original in-line 6 has a 4 speed SM 420 transmission which just hung off the back of the bell housing but I needed to remove the transmission separate from the engine because the transmission would not fit between the cab and the rear cross brace. Ideally I would not modify the firewall for clearance but as I need to make cab floor repairs as part of the project I would prefer to know if modifications are needed before I get too far with the cab floor repairs. Also curious about drive shafts as the original was a torque tube from the transmission to the intermediate bearing with conventional from the intermediate bearing to the rear axle. I have read somewhere that a single shaft from the transmission to the rea axle may be too long.

I have read about exhaust manifold to steering gear clearance issues with various solutions including: no problem with original gear, spacing out the original gear with a modified drag link, newer gear (manual or power) mounted outside the frame and forward of the axle, and probably a few others but curious of pros and cons for various options given that I prefer to go to power steering anyway.

I anticipate utilizing a new radiator with either an engine fan/shroud or an electric fan/shroud set up.

I have also read about fuel tank options for under the bed near the rear with the fill either located in the bed floor or possibly out the side. My first thought is to go out the side but would appreciate input from others regarding what tank and fill set-up was chosen and any pros/cons.

Thanks for reading.


1949/50 3600 Project
Re: 49/50 3600 SBC Conversion
WICruiser #1434879 Tue Dec 28 2021 12:36 AM
Joined: Mar 2010
Posts: 10,074
5
'Bolter
If you are using the original frame and leaf springs, I recommend that you stick to a 350 in its original configuration without modifications.
These trucks are not nimble regardless of disc brakes. Emergency stops combined with avoiding contact with obstacles is not the strongest suit of these trucks.

Re: 49/50 3600 SBC Conversion
WICruiser #1434883 Tue Dec 28 2021 01:28 AM
Joined: Dec 2018
Posts: 1,444
F
'Bolter
WICruiser First done a SBC in 1967 in a 53 3100 older hot rod pals said you couldn't do it. Don't have to bother the firewall,have the thickness of your palm between the corner of a tin valve cover and the firewall. Next use a 66big car manifold on the steering side,shim steering box out 5/8 inch. end up with your carb mount -up level. stay centered in the frame.used the short water pump radiator in stock location'was a 265 bored to 283 2 2121s carter AFB's 098 cam.worked good !!

Re: 49/50 3600 SBC Conversion
52Carl #1434884 Tue Dec 28 2021 01:32 AM
Joined: Dec 2018
Posts: 1,444
F
'Bolter
52 Carl your right ,don't handle like a stingray !!

Re: 49/50 3600 SBC Conversion
WICruiser #1434899 Tue Dec 28 2021 04:23 AM
Joined: Sep 2018
Posts: 46
7
'Bolter
The first thing you mentioned was the rear end gear. Using a 410 rear end gear is going to get burn a lot of gas. A buddy of mine is running a 370 gear with a 350 Chevy trans and is running 3500 rpms at 60 mph. You might want consider a 350 gear with an overdrive trans to lower your rpms. If you don’t modify the firewall and you run an HEI distributor it will be close to your firewall. I just modified my 46 firewall because my distributor was too close. Good luck with your truck.

Last edited by 7045george; Tue Dec 28 2021 04:25 AM.
Re: 49/50 3600 SBC Conversion
WICruiser #1434908 Tue Dec 28 2021 05:40 AM
Joined: Feb 2004
Posts: 24,250
H
Kettle Custodian (pot stirrer)
There are no new 4.10 gears available for the original rear axle- - - -the supply of OEM gears dried up decades ago from us dirt track racers grabbing every set we could find. A late 1960's through mid-70's 3/4 ton Suburban with an automatic transmission, and a few of the pickups with automatics ran the 4.10's that people are searching for now, and those rear ends are very scarce. A better choice of a complete 8 bolt rear axle assembly with better gear ratio selections available would be a Dana Type 60 from a 3/4 ton GMC, Ford, or Dodge box van. The pickup rear ends are a bit too wide to fit your truck without fender modifications.
Jerry


"Freedom's just another word for nothing left to lose"
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Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Re: 49/50 3600 SBC Conversion
WICruiser #1434909 Tue Dec 28 2021 05:48 AM
Joined: Jul 2014
Posts: 317
1
'Bolter
With 5.13 gears and 32.5 inch tires my 51 turns 3300 at 60. 2800 in overdrive (17%). Running empty my warmed over 235 could easily pull a 4.10 axle. Not so, loaded to the gunnels as it usually is. I'm following along because I've considered eventually installing a SBC so I don't have to shift gears so much.


51 3800 PU, 55 235 (w/cam, headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.
Re: 49/50 3600 SBC Conversion
WICruiser #1434915 Tue Dec 28 2021 12:57 PM
Joined: Nov 2021
Posts: 159
W
'Bolter
Ton_tommy, thanks for your information, my 4.56 gears (original) would run about 10% lower rpms and if I find 4.10 gears it would run another 10% lower. What size tires are you running that provides the 32.5" diameter?

I have read some about converting the rear axle to something more current and the overall width being a problem but how much wider are they and what fender modifications are typically done to accommodate them?

I did a similar project a few years back to put a SBC400 in a 1992 Jeep Wrangler. I built the engine with a short water pump and had a home made shroud with electric fan that had good clearance at the front of the engine. Long story about the 400 selection but I understand the firewall clearance concerns for the distributor, etc. just wasn't sure about bell housing clearance as I have read that the firewall may need to be cut out for clearance.

Fixite7 - did you modify or replace the rear engine mount? My impression is the original rear mount needs to be removed and a new rear mount installed in the proper location for the transmission mount.


1949/50 3600 Project
Re: 49/50 3600 SBC Conversion
WICruiser #1434916 Tue Dec 28 2021 01:05 PM
Joined: Aug 2020
Posts: 86
A
'Bolter
Originally Posted by WICruiser
I have read some about converting the rear axle to something more current and the overall width being a problem but how much wider are they and what fender modifications are typically done to accommodate them?.

I have a 3600 running 30” tires, I’m looking to go to a 3.73 FD to raise my cruise speed. You can swap to a 14bolt for pre 85 trucks pretty easy. The hub to hob with will be the same 72” bit the wheel mounting surface differs between axle versions. You’d want to find one with no wider than a 67” wms-wms. Then any fine tuning to tuck the tires can be done with wheel back spacing.


Pat
1940’s tech was great in the 40’s
Re: 49/50 3600 SBC Conversion
WICruiser #1434967 Tue Dec 28 2021 09:38 PM
Joined: Jun 2011
Posts: 4,453
E
'Bolter
A SF 14 bolt has 6 lugs, is an option for half ton trucks, FF 14 bolt from c and c truck is narrower.

Ed


'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.
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