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Old 02-12-2018, 09:53 PM   #1
superhondaz50
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New flexplate help

Ok, I've never installed a flexplate before, or an auto trans. So I have a new boss hog TC, new ATP flexplate, th400 to 350sbc. When I went to bolt the flexplate to the TC, I noticed a huge variation between the distance between the TC tabs and the flexplate mounting surface. One tab prob had a 3/16 gap and the other two around 1/8, maybe 1/16. Is this normal or is the flywheel a POS? Thanks
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Old 02-12-2018, 10:18 PM   #2
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Re: New flexplate help

Not fully understanding the description - got a picture?
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Old 02-12-2018, 10:50 PM   #3
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Re: New flexplate help

check this out https://www.youtube.com/watch?v=qdCJXjElkoE

jump to around minute 5:30
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Old 02-12-2018, 11:15 PM   #4
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Re: New flexplate help

After looking it over, the dished portion of the flexplate was too small to fit the larger non concentric crank end causing the flywheel to sit caddywompus. I need to order a shim I think to push the flywheel out to clear properly.
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Old 02-12-2018, 11:24 PM   #5
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Re: New flexplate help

Here's my crank end

And here's where it was hitting, right under the sharpie line
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Old 02-12-2018, 11:44 PM   #6
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Re: New flexplate help

can u show the whole flexplate?
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Old 02-13-2018, 12:24 AM   #7
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Re: New flexplate help

It's this one

https://www.rockauto.com/en/parts/at...flexplate,8608
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Old 02-13-2018, 12:30 AM   #8
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Re: New flexplate help

for reference, the face shown on rock auto is the face that goes toward the trans. I believe you may have yours on backwards(?) (was kinda hoping on a zoom out of your flexplate to confirm, but if you look at the pic in the link you just posted, you'll see what I mean).

EDIT: Bottom line - the raised pads for the converter go toward the converter, i.e. face rearward.

Last edited by jocko; 02-13-2018 at 12:37 AM.
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Old 02-13-2018, 12:48 AM   #9
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Re: New flexplate help

Ya, I had the raised pads towards the convertor. The center section is also raised, that raised portion also is towards the convertor, so the crank end sits in the dished side, but doesn't fit right, hence needing a shim.
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Old 02-13-2018, 12:52 AM   #10
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Re: New flexplate help

Hmmm. Sorry, was hoping that was maybe all it was. I'm stumped. Shouldn't have to shim it.
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Old 02-13-2018, 01:00 AM   #11
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Re: New flexplate help

Ya, I read somewhere that these old style crank ends need a shim sometimes. The circumference of the crank end is just too big to fit into the dished flexplate, and I'd prefer not to buy a different brand when the shim is only $4.
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Old 02-13-2018, 08:39 AM   #12
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Re: New flexplate help

This is a result of the crude casting and the factory short cutting the machining on your GM crate crankshaft. A die grinder and some patience will fix it. Some flex plates will clear most of these misfits but be sure to run a dial indicator as some of the China made crap will have 1/8" runout on the ring gear even when fitting tight to the crankshaft flange. The one pictured was hitting mostly on the balance weight but also on the flange. I did one last week with exactly the same issue as yours. Check the hole for the bolt for the passenger side spark plug heat shield as it is probably not there on yours either.
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Last edited by CaptRMW; 02-13-2018 at 09:53 AM. Reason: add photo
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Old 02-13-2018, 09:50 AM   #13
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Re: New flexplate help

Quote:
Originally Posted by CaptRMW View Post
This is a result of the crude casting and the factory short cutting the machining on your GM crate crankshaft. A die grinder and some patience will fix it. Some flex plates will clear most of these misfits but be sure to run a dial indicator as some of the China made crap will have 1/8" runout on the ring gear even when fitting tight to the crankshaft flange.
That's what I figured. The white mark on the machined surface of your crank pic is where mine hits. Gonna try the shim first for $4. If that's a no go, I'll bust out the grinder.
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Old 02-13-2018, 10:28 AM   #14
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Re: New flexplate help

Good info. Is the shim just an annular ring with all the bolt holes drilled out? A crate is in my future so I’m gonna have to remember all this!
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Old 02-13-2018, 11:27 AM   #15
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Re: New flexplate help

Quote:
Originally Posted by jocko View Post
Good info. Is the shim just an annular ring with all the bolt holes drilled out? A crate is in my future so Iím gonna have to remember all this!
Here is the shim, two are about the thickness of a washer. This will work good I think for me because, even if the flexplate fit, I'd still need to add shims in between the TC and flexplate due to the gap. This will solve two problems at once hopefully.

https://www.summitracing.com/parts/fcn-62320
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Old 02-13-2018, 11:47 AM   #16
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Re: New flexplate help

I had the exact same problem with my GM crate engine, Gen 1 style with 2 piece rear seal. I relieved the casting area with a die grinder and burr bit while still in the truck. I still had a bad wobble on the third new flex plate I tried. I pulled the engine and checked run out on the flex plate at the starter ring and had .050 run out and .005 run out on the crank flange itself. I pulled the crank and bought a new Eagle one from Summit and have it at the machine shop getting balanced right now. The GM crank was cast in Mexico and I suspect they missed a machining step on the rear flange. I've been fighting a vibration in the engine for a year and I hope this takes care of it.
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Old 02-13-2018, 11:53 AM   #17
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Re: New flexplate help

The card board template in the pic is one I took off a good crank at the machine shop.
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Old 02-13-2018, 12:09 PM   #18
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Re: New flexplate help

If this is a crate engine, I would assume that it was balanced? Taking a grinder to one side of the crank shafts will disrupt the balancing that took place. Or, are these crate engines being sold un-balanced? If I took a grinder to any part of my rotating assembly, I'd eat the cost of a set of gaskets and have the rotating assembly balanced after all clearancing. I know that is the last thing anyone wants to do that's trying to button up an engine install/swap, but man, you don't want that thing way out of balance, beating it's bearings up. You may end up doing the job again a lot sooner than you want!
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Old 02-13-2018, 12:15 PM   #19
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Re: New flexplate help

Ya, these crates are internally balanced. That's why I'm gonna try the shim thing first. Seems like the safest easiest fix. Either that or find a flexplate that is not "dished" out in the center, or has a larger dished portion. But for $4 the shims are really worth a shot.
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Old 02-13-2018, 12:55 PM   #20
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Re: New flexplate help

Man, that's rough. I wish that I could remember if the one that I bought from Summit was dished. I will crawl under the truck later and check. If it's not dished, I could get a part number for you.

Good luck!
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Old 02-13-2018, 01:05 PM   #21
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Re: New flexplate help

The odds of me buying a low budget crate engine and tearing it down and sending the pistons, rods and a $200.00 Eagle crank to the machine shop for a pricey balance job is about the same as the odds of winning the lottery. There are plenty of engine builders doing a quality job and not skipping critical steps with good customer reviews and customer service. Buy a rebuilt from any of the big name engine rebuilders like Ja$purr and get an engine built with a mis-matched set of rods and a crankshaft that is not matched to anything, built with plenty of Chinese parts.
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Old 02-13-2018, 01:12 PM   #22
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Re: New flexplate help

I've had four new flex plates, one from Auto Zone, one a B&M from Summit and one from NAPA and the one I just bought, Summits heavy duty and they are all dished. I'm keeping my fingers crossed that the balancing of the reciprocating assembly with the new crank will solve the dilemma.
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Old 02-13-2018, 01:26 PM   #23
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Re: New flexplate help

Should add, this is a brand new Chevrolet Performance Mexican crate. I'm thinking my flexplate dish is smaller than others. I may order a high end flexplate just because I'm curious if the center dish will be bigger.
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Old 02-13-2018, 01:30 PM   #24
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Re: New flexplate help

See, this Meziere is not dished. But what a waste of a flywheel on a 250hp truck lol
https://www.summitracing.com/parts/mez-fp300
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Old 02-13-2018, 01:50 PM   #25
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Re: New flexplate help

Comparing pictures I believe the center dish is slightly bigger on mine. It's installed now and I can't get to it to measure. Are you lacking just a fraction of an inch clearance? https://www.amazon.com/gp/product/B000C8RDCS
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