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Old 09-23-2018, 12:36 PM   #1
tdangle
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Replacing a tired 350 with a L31 Vortec 350

Just spend some time searching and really have not found what I am looking for. My goal is to install a L31 in place of my Gen1 350. Looking to get some sort of parts list together and have everything ready for the swap when the time comes.

I know I need:

electric fuel pump
different (melonized) for HEI dist.
vortec intake manifold (not sure of the brand yet. square or spread bore ??)
new flex plate (153 tooth??)
rear AC bracket that mounts to the manifold
Holley carb with electric choke (like the quadajet but don't want to loose choke)

I want to change the timing cover out and just reuse mine off of my 350 including the harmonic balance. (going to run the original v belts and have a short water pump)

Am I missing anything else? the original motor has factory AC (working) and power steering
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Old 09-23-2018, 09:41 PM   #2
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Re: Replacing a tired 350 with a L31 Vortec 350

Which intake you get will depend on the carb you plan to use.

The new flex plate is due to the change from a 2-piece to a 1-piece rear main seal.

If you want to reuse your harmonic balancer (which I don't recommend) you need to remove the reluctor behind the timing gear on the crankshaft. You might as well install a double roller timing set. The Vortec balancer is shorter to accommodate the reluctor.
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Old 09-23-2018, 11:45 PM   #3
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Re: Replacing a tired 350 with a L31 Vortec 350

why do you not recommend using an earlier harmonic balancer?
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Old 09-23-2018, 11:57 PM   #4
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Re: Replacing a tired 350 with a L31 Vortec 350

You can use your quadrajet if you use an electric choke conversion.
With your electric pump you should set up a return style fuel regulator. Electric pumps don’t like to be deadheaded.
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Old 09-24-2018, 01:01 PM   #5
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Re: Replacing a tired 350 with a L31 Vortec 350

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why do you not recommend using an earlier harmonic balancer?
That's not what I said. I said I wouldn't re-use your harmonic balancer. The rubber ring is 45+ years old. It's time for a new one. The old style is fine; that's what I have on mine.
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Old 09-24-2018, 02:20 PM   #6
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Re: Replacing a tired 350 with a L31 Vortec 350

you must have missed this thread . http://67-72chevytrucks.com/vboard/s...d.php?t=763121

i have not got mine fired yet but dropped right in place and bolted up good . http://67-72chevytrucks.com/vboard/s...d.php?t=755514

even used the cam sensor block off plug in the link provided by another member . it was 20 bucks . https://www.eficonnection.com/home/p...illet-aluminum

fit n finish of the edelbrock intake for square bore carbs 1450 series was o.k. for my needs going holley sniper unit .

make sure to give radliy chevy a call were i got mine from . they were the best price i found for the 4 bolt main version shipped to my work . https://www.ebay.com/itm/GM-OEM-NEW-...4AAOSwysRas9qX
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Old 09-30-2018, 12:11 AM   #7
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Re: Replacing a tired 350 with a L31 Vortec 350

Did your gen1 350 have a 153 or 168 tooth flexplate ?
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Old 09-30-2018, 02:09 PM   #8
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Re: Replacing a tired 350 with a L31 Vortec 350

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Did your gen1 350 have a 153 or 168 tooth flexplate ?
Not sure. I do remember dad having a lot of problems with the starter when new and something about Chevy replacing the flexplate. Never thought to identify what I had when I swapped a 200r4 trans for the th350
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Old 09-30-2018, 03:42 PM   #9
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Re: Replacing a tired 350 with a L31 Vortec 350

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Did your gen1 350 have a 153 or 168 tooth flexplate ?
If I remember when I installed my L31 it was not so much the tooth count , its the fact of the L31 has a one piece seal and the old 350 have 2 piece seal and the bolt hole will not be correct also , you will need a flexplate for the 96-99 vortec engine,but also specify if you have a 2 bolt or 4 bolt vortec engine , I believe the 1500 series trucks had 2 bolts and the 2500 series had 4 bolts . my L31 is a 4 bolt ,So I ordered a flexplate to fit a 1998 5.7 2500 series truck , I had a TH 400 and then put in a 200 4r with no issues .
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Old 09-30-2018, 11:17 PM   #10
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Re: Replacing a tired 350 with a L31 Vortec 350

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Not sure. I do remember dad having a lot of problems with the starter when new and something about Chevy replacing the flexplate. Never thought to identify what I had when I swapped a 200r4 trans for the th350
Just going off your first post on items you may need for the activity you are trying to perform, the starter maybe something to look at as well....
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Old 10-01-2018, 03:41 PM   #11
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Re: Replacing a tired 350 with a L31 Vortec 350

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my L31 is a 4 bolt ,So I ordered a flexplate to fit a 1998 5.7 2500 series truck ,
Ummmm yeah, the number of bolts holding the main caps to the block have absolutely nothing to do with a 1 or 2pc rear main seal, which is the only difference. Any flexplate from 1pc rear main truck would work, which is what 1986 or so?
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Old 10-01-2018, 03:48 PM   #12
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Re: Replacing a tired 350 with a L31 Vortec 350

Friendly reminder. A lot of folks forget that that trans behind the "tired" engine is ALSO "tired". Then you get everything going and a few thousand miles later the trans craps behind all that new found power. I'm just saying be ready and not surprised when it happens.
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Old 10-02-2018, 02:47 PM   #13
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Re: Replacing a tired 350 with a L31 Vortec 350

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Friendly reminder. A lot of folks forget that that trans behind the "tired" engine is ALSO "tired". Then you get everything going and a few thousand miles later the trans craps behind all that new found power. I'm just saying be ready and not surprised when it happens.
Trans just got rebuilt prior to the trans swap. Maybe 600 miles and I have a 2 year warranty on it. Engine is running OK now, but I'm looking at being prepared and would like to spread the cost out over time.

I didn't know that I would need to run a fuel return line, so that's something else I need to look into.

Basically looks like my list is fuel delivery system, vortec intake manifold, probably a new carb, flexplate plugging the crank sensor hole, timing marks, and long water pump (I have a short one now, which is new) and making a rear AC bracket.
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Old 10-02-2018, 05:54 PM   #14
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Re: Replacing a tired 350 with a L31 Vortec 350

On my 78 it had a gas tank vent line I didn't use anymore, it was like a 1/4" and with a return style regulator not regulating anything it gave me 6psi at the carb due to the return limiting flow lol. You can run the pump dead head/not return style, but it will last longer and run cooler with a return style regulator.
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Old 10-02-2018, 08:00 PM   #15
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Re: Replacing a tired 350 with a L31 Vortec 350

Early Subarus with electric fuel pumps had no return pump and those pumps tended to last 100,000 miles. It might shorten the pump's life some, but you might not ever notice it.
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Old 10-03-2018, 09:10 AM   #16
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Re: Replacing a tired 350 with a L31 Vortec 350

Which would be a good dependable fuel pump
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Old 10-03-2018, 07:04 PM   #17
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Re: Replacing a tired 350 with a L31 Vortec 350

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Early Subarus with electric fuel pumps had no return pump and those pumps tended to last 100,000 miles. It might shorten the pump's life some, but you might not ever notice it.
Every car made today uses a returnless regulator for emissions reasons. The pumps are built to handle it just like starters on vehicles with start/stop tech. Doesn't mean the old tech will hold up to that use.
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Old 10-03-2018, 09:38 PM   #18
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Re: Replacing a tired 350 with a L31 Vortec 350

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Which would be a good dependable fuel pump
I've had good luck with Wagner.
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