engine swap? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

kamikazegmc · 03-22-2005, 11:14 PM

i'm going to do an egine swap in the near future and i have some questions here i go, i'm swaping out my 305e v6 for a 350 v8 what parts do i did besides the motor mounts and tranny mount?, also i changing out my sm-420 for a th400. what wiring would i need to do? and would i have to get a driveline made or from what year can i get one from?

thanx ahead of time for any info you guys can give me.

thanx, Jim

60-66 Nut · 03-23-2005, 01:45 PM

Jim, you will definatley need motor mounts and a rear trans crossmember. Also, depending on how you position your engine/trans, the old crossmember under the bell housing may need to be trimmed or removed for clearance, but I have heard of some guys leaving them in with no problems. As for the TH400 trans, if you are using one from a truck then you should be able to shorten your driveshaft to make it work. Nearly all of the truck 400's used a bolt-on rear yoke like your SM420, and I think the u-joint is the same, but the 400 is much longer than your SM420. Car 400's used a slip-in yoke, but the yoke is bigger than the TH350 and 3 spd yokes. The only wiring on a 400 is the electronic detent, and all you need for that is a switch mounted on the carb or on the accelerator pedal. By the way, the 350 is a couple hundred pounds lighter than the V6, so your front end is going to sit up higher with the V6 springs. Don't know if that's good or bad for you, but just wanted to let you know. Best of luck.

60-66 Nut

kamikazegmc · 03-23-2005, 07:50 PM

the 350 is about 300lbs lighter, and i want to bring the front up because i'm making it in to a prerunner, that and more parts for the 350 and better preformance. thanx for the info.

also what wiring needs to be changed to go from v6 to v8? and to find out gear ratio what do i need to do .

thanx, Jim

60-66 Nut · 03-24-2005, 12:13 PM

Wiring should be pretty simple. If your gauges(or lights) work now, then just swap the temp and oil senders over to the 350. Your same alternator should fit on the 350, but depending on which style brackets you use the alternator could end up on the opposite side so you may have to extend the wires. If you use an HEI distibutor(which I highly recommend) make sure to abandon the old resistor wire to the coil and the yellow wire from the coil to the starter, then run a 12 or 14 gauge wire from the "ignition unfused" terminal in the fusebox directly to the distributor. HEI requires a full 12V, whereas points only used 7 or 8 volts. Do you mean how to check your existing rear end ratio? If so, it's very easy. Just jack up one rear wheel, then mark the wheel and the driveshaft. Turn the wheel two full revolutions and count the driveshaft revolutions. Three full revolutions = 3.00:1, three and a half revs = 3.50:1, etc. If you have a posi-trac axle, then you have to jack up both wheels and turn them one full revolution. I hope that's what you're asking.

60-66 Nut

kamikazegmc · 03-24-2005, 06:21 PM

yes, you have hepled and that is what i was asking.

kamikazegmc · 04-02-2005, 04:56 AM

i found out the ratio kind of( i half a$$ed it) it some thing like 3:50 some thing

03-22-2005, 11:14 PM	#1
kamikazegmc Registered Smarta$$ Join Date: Mar 2004 Location: vancouver, wa Posts: 214	engine swap? i'm going to do an egine swap in the near future and i have some questions here i go, i'm swaping out my 305e v6 for a 350 v8 what parts do i did besides the motor mounts and tranny mount?, also i changing out my sm-420 for a th400. what wiring would i need to do? and would i have to get a driveline made or from what year can i get one from? thanx ahead of time for any info you guys can give me. thanx, Jim __________________ 63 GMC custom going to be slammed

03-23-2005, 01:45 PM	#2
60-66 Nut Professional Beer Tester Join Date: Aug 2004 Location: Louisiana Posts: 453	Jim, you will definatley need motor mounts and a rear trans crossmember. Also, depending on how you position your engine/trans, the old crossmember under the bell housing may need to be trimmed or removed for clearance, but I have heard of some guys leaving them in with no problems. As for the TH400 trans, if you are using one from a truck then you should be able to shorten your driveshaft to make it work. Nearly all of the truck 400's used a bolt-on rear yoke like your SM420, and I think the u-joint is the same, but the 400 is much longer than your SM420. Car 400's used a slip-in yoke, but the yoke is bigger than the TH350 and 3 spd yokes. The only wiring on a 400 is the electronic detent, and all you need for that is a switch mounted on the carb or on the accelerator pedal. By the way, the 350 is a couple hundred pounds lighter than the V6, so your front end is going to sit up higher with the V6 springs. Don't know if that's good or bad for you, but just wanted to let you know. Best of luck. 60-66 Nut __________________ 60 Ford F100 Custom Cab 61 Apache Short Fleetside 64 C10 Short Stepside 65 C80 409 V8 5spd(X-Firetruck) 83 GMC 350 Olds Diesel 85 GMC Short Bed 86 GMC Short Bed 90 GMC R3500 Crew Cab 92 GMC 1500

03-23-2005, 07:50 PM	#3
kamikazegmc Registered Smarta$$ Join Date: Mar 2004 Location: vancouver, wa Posts: 214	the 350 is about 300lbs lighter, and i want to bring the front up because i'm making it in to a prerunner, that and more parts for the 350 and better preformance. thanx for the info. also what wiring needs to be changed to go from v6 to v8? and to find out gear ratio what do i need to do . thanx, Jim __________________ 63 GMC custom going to be slammed Last edited by kamikazegmc; 03-23-2005 at 07:51 PM. Reason: grammer

03-24-2005, 12:13 PM	#4
60-66 Nut Professional Beer Tester Join Date: Aug 2004 Location: Louisiana Posts: 453	Wiring should be pretty simple. If your gauges(or lights) work now, then just swap the temp and oil senders over to the 350. Your same alternator should fit on the 350, but depending on which style brackets you use the alternator could end up on the opposite side so you may have to extend the wires. If you use an HEI distibutor(which I highly recommend) make sure to abandon the old resistor wire to the coil and the yellow wire from the coil to the starter, then run a 12 or 14 gauge wire from the "ignition unfused" terminal in the fusebox directly to the distributor. HEI requires a full 12V, whereas points only used 7 or 8 volts. Do you mean how to check your existing rear end ratio? If so, it's very easy. Just jack up one rear wheel, then mark the wheel and the driveshaft. Turn the wheel two full revolutions and count the driveshaft revolutions. Three full revolutions = 3.00:1, three and a half revs = 3.50:1, etc. If you have a posi-trac axle, then you have to jack up both wheels and turn them one full revolution. I hope that's what you're asking. 60-66 Nut __________________ 60 Ford F100 Custom Cab 61 Apache Short Fleetside 64 C10 Short Stepside 65 C80 409 V8 5spd(X-Firetruck) 83 GMC 350 Olds Diesel 85 GMC Short Bed 86 GMC Short Bed 90 GMC R3500 Crew Cab 92 GMC 1500

03-24-2005, 06:21 PM	#5
kamikazegmc Registered Smarta$$ Join Date: Mar 2004 Location: vancouver, wa Posts: 214	yes, you have hepled and that is what i was asking. __________________ 63 GMC custom going to be slammed

04-02-2005, 04:56 AM	#6
kamikazegmc Registered Smarta$$ Join Date: Mar 2004 Location: vancouver, wa Posts: 214	i found out the ratio kind of( i half a$$ed it) it some thing like 3:50 some thing __________________ 63 GMC custom going to be slammed



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