hogpuke

I'm a little confused about the LS1 flex plate vs. 700R4. Some sources say the whole flex plate needs to be replaced to run an earlier transmission on a Gen. III engine, others say just a spacer is needed. It looks like the .400" spacer and longer bolts would go on after the stock dished Gen. III flex plate in order to support the pilot on the 700R4 torque converter and everyone's happy. Is there more to it than that?

the_merv

iTrader: (88)

You can't use the LS1 Flexplate, you need the 6.0l Truck Flexplate with the Spacer & Bolts to bolt on the back of a LS1 and to use an older style Transmission.

billyflantos

iTrader: (6)

i just went to my local chevy dealership and bought a flexplate for a 2003 2500 series chevy truck with a 6.0 in it(gm ppart#12621399) for $85+tax and it came with the spacer and bolts with it...then i stuck in on the front of my 700's converter and took a magic marker and marked the area on the flexplate that needed to be ground off...it needs about a half inch ground off for the converter bolts to go thru the flexplate into the converter....in other words just oblong the flexplate converter holes about a half inch for it to fit....works like a charm and easy to do...hope this info helps...billy

the_merv

iTrader: (88)

I did this..

chuckd71

iTrader: (23)

I took mine to a machine shop and had them elongate the holes. The plate metal was too much for my pathetic tools to cut through.

hogpuke

Thank you guys for the information!

the_merv

iTrader: (88)

I used a drill press and just filed down the high spots.

ocp04

Will thae same go as far as bolting up a th350 or 400 to ls engine?

billyflantos

iTrader: (6)

it will work with the th350 400 and a powerglide also....

99VetteFRC

The flexplate number is 12621399, spacer 12563532, and 6 bolts 12563533. The flexplate has the same bolt battern for a th400 or th350 (no elongating neccessary). If using a 700r4 or early 4l60e you will need to elongate the holes as in the picture above. I used a high speed air grinder and a carbide cutting tip. worked perfect. All is well with my 700r4 to 4.8L.

the_merv

iTrader: (88)

I had to elongate the holes that I pictured to run my TH350...ATI 4500 Stall so that might be the case there.

hogpuke

I know I'm as dumb as mud, but if the rear face of the flex plate on the current 350 is .875" from the bell housing mating surface, and the rear face of the flex plate on the 5.3L is the same distance from its mating surface, why does the flex plate need to be replaced? The converter will wind up the same distance from the engine and will have the same oil pump engagement that it has now. The only difference is that the crankshaft flange, and so the torque converter alignment support, is .400" farther from the converter so a bushing is needed to make up the difference. I saw a .400" spacer from one of the suppliers that says it goes on after the flex plate, with the longer bolts, in order for the torque converter pilot to reach. The holes in the flex plate still need to be elongated. It looks like it would work to me - am I'm I missing something? Thanks again for all the help.

Bo185

The 350 flex plate will not bolt up to a Gen III/IV motor.

Why waste $85+ dollars this can be done for cheap!

You run a used dished 5.3L flex plate and aftermarket bushing. This supports the hub of the converter. Or get the more expensive GM spacer and longer bolts. Either way with the dished flex the spacer goes on after the flexplate. The bushing just rides in the crank if you go with it. Then elongate the holes a little.

GM spacer needs longer bolts $40+
http://www.jegs.com/i/GM%20Performan...0002/-1?CT=999

Or just the bushing for $20 bucks
http://paceperformance.com/i-6520249...nk-spacer.html



The flex plate merv posted is the flat 6.0L plate noted by the six holes for the verter. And uses a spacer first then the flex plate since its flat.



The Gen III/IV converter hole spacing is metric hence the need to elongate the holes.

What LS flex plate do you have now???

hogpuke

Thanks Bo185. I understand the 350 plate won't work. The plate on the new engine is the stock plate - 2001 5.3 out of a Silverado. It looks to me like the spacer that goes on afterward is the simple fix. But then again...

ZQ8LS1

iTrader: (3)

Does this apply to a 2003 4L6OE from a 4.3 ??

Bo185

Go good.

All you need is the $20 bushing I posted.

Now remove the dished flex plate. place $20 bushing into the converter hub, then hold dished 5.3L flex plate up to hub using the bushing to align it. Use this a guide to elongate your holes. Once holes line up reinstall dished flex plate then set bushing in crank. Then bolt up the trans.

Easy fix for $20 bucks!

Yes this apply to a 4.3L 4l60e and a 4l80e. They use the same setup as a Gen I motor and 4.3L motors use. Only the 4l80 bolts up with the flat flex plate.

ZQ8LS1

iTrader: (3)

Thank you !!!

I was totally confused till I read your explanation.

hoopty

I reused the stock LS1 flex plate. I just added the spacer and longer bolts. It worked fine with my th350 tranny. After I elongated the bolt holes for the torque converter.

Goldhawg

iTrader: (7)

If its useful, I have a flexplate off a 2010 LY6 that I'm not using (going to T56). I'll ship to you for $40. PM if interested.

Bo185

If its off a 4l80 used in the vans with the LY6 it would work perfect as well.

I am not sure the 6l90 one will work well with the early trans.