1986 Sierra motor/tranny swap - The 1947 - Present Chevrolet & GMC Truck Message Board Network

1helluva86 · 05-16-2010, 02:13 PM

hey,
I got a 1986 sierra short bed 2wd with a 305/700r4 combo from the factory. I want to swap a built 350 and th350 tranny in its place. Does anyone one how difficult this will be with the different transmission. I know the motors are a direct swap, but was wondering is my best bet to get the drive shaft lengthend for the th350, get a longer tail shaft, or just put a higher stall converter on the 700r4. I need a 2600 stall to go with the engine combo.
appreciate the help.

STOCKISH · 05-16-2010, 03:13 PM

A 'vette converter for a 700r4 will be just about right for your swap. I would keep the 700r4 if I could. Then everything will be a direct swap with no driveshaft mods needed.

cableguy0 · 05-16-2010, 03:17 PM

A th350 with a 9 inch tailshaft will be a direct swap. there will be no mods needed to do the swap. theres only 1/16 of an inch difference between the 700r4 and the th350 with the 9 inch shaft. If you have a healthy 350 and are running a stall converter the stock 700r4 is not going to hold up.

1helluva86 · 05-16-2010, 03:21 PM

thanks guys,
yeah its makin about 420 hp. so if i finda 9 inch tail shaft ill be set. wheres my best bet for finding the tail shaft. I was hoping the th350 converter would swap over but i dont think so. appreciate the help.

cableguy0 · 05-16-2010, 03:25 PM

Th350 converter will not fit in the 700r4. th350/th400 interchange with each other. A lot of the big cars and trucks in the late 70's/early 80's had the 9 inch tail shaft. If your going to have the th350 rebuilt I would however have a heavy duty sprag put in as well as a decent shift kit just for insurance. Its still gonna be a lot cheaper than beefing up the 700r4

bigdnew · 05-16-2010, 03:30 PM

just ordered some hard to find 4L80E parts, try WITTRANS.Com.

1helluva86 · 05-16-2010, 03:37 PM

I could also just get the driveshaft lengthend. But what does it entail to swap in the th350? just move the crossmember forward? It looks like there are pre drilled holes on the mount for the crossmember to be moved.

cableguy0 · 05-16-2010, 03:44 PM

slide the crossmember up. thats about it. There should be holes in the frame already. If not just drill the holes.Otherwise theres really no difference.

1helluva86 · 05-16-2010, 04:08 PM

awesome, i really appreciate the help guys. the th350 was already rebuilt. i had the combo set up for an 80 malibu i built and just sold as a rolling chassis, thats why i figured id have fitment probs. Ill just throw the th350 in and lengthen the shaft, seems to be the cheapest route.

05-16-2010, 02:13 PM	#1
1helluva86 Registered User Join Date: May 2010 Location: Webster, MA Posts: 17	1986 Sierra motor/tranny swap hey, I got a 1986 sierra short bed 2wd with a 305/700r4 combo from the factory. I want to swap a built 350 and th350 tranny in its place. Does anyone one how difficult this will be with the different transmission. I know the motors are a direct swap, but was wondering is my best bet to get the drive shaft lengthend for the th350, get a longer tail shaft, or just put a higher stall converter on the 700r4. I need a 2600 stall to go with the engine combo. appreciate the help.

05-16-2010, 03:13 PM	#2
STOCKISH Formerly- 1972SuperCheyenne Join Date: Oct 2008 Location: Wake Forest NC Posts: 5,782	Re: 1986 Sierra motor/tranny swap A 'vette converter for a 700r4 will be just about right for your swap. I would keep the 700r4 if I could. Then everything will be a direct swap with no driveshaft mods needed. __________________ --Josh My Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=799218 A good crossthread is better than Loctite any day. Life is not about what you have, but who you have to share it with.

05-16-2010, 03:17 PM	#3
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	Re: 1986 Sierra motor/tranny swap A th350 with a 9 inch tailshaft will be a direct swap. there will be no mods needed to do the swap. theres only 1/16 of an inch difference between the 700r4 and the th350 with the 9 inch shaft. If you have a healthy 350 and are running a stall converter the stock 700r4 is not going to hold up. __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

05-16-2010, 03:21 PM	#4
1helluva86 Registered User Join Date: May 2010 Location: Webster, MA Posts: 17	Re: 1986 Sierra motor/tranny swap thanks guys, yeah its makin about 420 hp. so if i finda 9 inch tail shaft ill be set. wheres my best bet for finding the tail shaft. I was hoping the th350 converter would swap over but i dont think so. appreciate the help.

05-16-2010, 03:25 PM	#5
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	Re: 1986 Sierra motor/tranny swap Th350 converter will not fit in the 700r4. th350/th400 interchange with each other. A lot of the big cars and trucks in the late 70's/early 80's had the 9 inch tail shaft. If your going to have the th350 rebuilt I would however have a heavy duty sprag put in as well as a decent shift kit just for insurance. Its still gonna be a lot cheaper than beefing up the 700r4 __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

05-16-2010, 03:30 PM	#6
bigdnew Never Satisfied Join Date: Sep 2009 Location: Sherman, Texas Posts: 360	Re: 1986 Sierra motor/tranny swap just ordered some hard to find 4L80E parts, try WITTRANS.Com. __________________ 1973 4x4 Blazer- Parting Out 1986 LWB Silverado Undergoing LQ4/4L80E Swap '86 Build Thread:http://67-72chevytrucks.com/vboard/showthread.php?t=390566

05-16-2010, 03:37 PM	#7
1helluva86 Registered User Join Date: May 2010 Location: Webster, MA Posts: 17	Re: 1986 Sierra motor/tranny swap I could also just get the driveshaft lengthend. But what does it entail to swap in the th350? just move the crossmember forward? It looks like there are pre drilled holes on the mount for the crossmember to be moved.

05-16-2010, 03:44 PM	#8
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,950	Re: 1986 Sierra motor/tranny swap slide the crossmember up. thats about it. There should be holes in the frame already. If not just drill the holes.Otherwise theres really no difference. __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

05-16-2010, 04:08 PM	#9
1helluva86 Registered User Join Date: May 2010 Location: Webster, MA Posts: 17	Re: 1986 Sierra motor/tranny swap awesome, i really appreciate the help guys. the th350 was already rebuilt. i had the combo set up for an 80 malibu i built and just sold as a rolling chassis, thats why i figured id have fitment probs. Ill just throw the th350 in and lengthen the shaft, seems to be the cheapest route.



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