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258mm TC compatibility?

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Old 12-26-2014, 10:49 PM
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Default 258mm TC compatibility?
As I understand it, all of the 4T65E-HD productions use 258mm torque converters that required a special flexplate balanced for the SC 3800. However, I was looking through the 1998 tag listings and noticed that the 1998 Park Avenue also uses a 258mm flexplate on the non-SC build.

What is the actual facts about flexplate compatibility?

Thanks!
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Old 12-27-2014, 07:42 AM
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I've ran into this twice during L36/L67 swaps. The bolt holes on the L36 flexplates for the TC are closer to the crank vs the L67 flexplate.

As far as mating to the crank, they are both the same. How do I know? I discovered once you install the engine in and attempt to connect the flex plate to the TC and it don't fit, you have to pull the engine back out and swap flexplates. LMAO.
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Old 12-27-2014, 11:07 PM
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why do so many people talk about flexplate needing to be rebalanced?
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Old 12-28-2014, 08:08 AM
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Last I knew, the flexplate is balanced to the engine, not the TC. But I haven't had any issues yet... yet. LOL.
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Old 12-28-2014, 09:25 AM
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Thats correct. I guess the Park Avenue flex plate is special for N/A 3.8 with the 258mm pattern

But rockauto shows pretty wide interchange for the FRA484 flexplate

BUICK ALLURE (2005 - 2007)
BUICK LACROSSE (2005 - 2009)
BUICK LESABRE (1995 - 2005)
BUICK LUCERNE (2006 - 200
BUICK PARK AVENUE (1995 - 2005)
BUICK REGAL (1995 - 2004)
BUICK RIVIERA (1995 - 1997)
CHEVROLET IMPALA (2000 - 2005)
CHEVROLET LUMINA (1998 - 1999)
CHEVROLET LUMINA APV 1995
CHEVROLET MONTE CARLO (1998 - 2005)
OLDSMOBILE 88 (1995 - 1999)
OLDSMOBILE 98 (1995 - 1996)
OLDSMOBILE INTRIGUE (1998 - 1999)
OLDSMOBILE LSS (1996 - 1999)
OLDSMOBILE REGENCY (1997 - 199
OLDSMOBILE SILHOUETTE 1995
PONTIAC BONNEVILLE (1995 - 2005)
PONTIAC GRAND PRIX (1997 - 200
PONTIAC TRANS SPORT 1995
It doesn't show which of those are NA vs SC, but if they are like the NA 3.8 Park Avenue then they would all be NA with the larger bolt pattern which would be odd for such a high number
Last edited by ehall; 12-28-2014 at 09:27 AM.
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Old 12-28-2014, 12:51 PM
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Well, you can narrow it down some. Some of those models didn't come with a L67 to begin with.
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Old 12-28-2014, 01:55 PM
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Okay that'* a piece of puzzle then

Is it also correct that the larger TC will not install on the non-HD?

So the logical conclusion is that 258mm TC on a car that did not have SC means that some of NA 3.8 platforms had the HD, and had a special flexplate (the one above).
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Old 12-28-2014, 02:24 PM
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Again, last I knew, the only difference between non-HD and HD was the internal clutches and differential. Cases are the same either way. With that said, a larger TC should fit in a non-HD trans. And vice versa.
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Old 12-28-2014, 02:49 PM
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Do you think the Park Avenue with the 3.8 NA may have used a regular 4T65E with the larger TC (presumably for the larger clutch)?

What I am looking at is that trans has the gearing I want, but it has the large TC, and I was under the impression that large TC meant HD. If it has a non-HD with the large TC then that would be a good match for me.

I need to take a box of donuts to the dealer parts counter
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Old 12-28-2014, 03:05 PM
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I think everything after 97 got the 65-E. The L67 models got the 65-HD. I doubt any L36 got the larger flexplate.

The TC does not make the trans HD or non-hd. It'* the internals. The flexplate size has an effect of how much torque is applied to the trans.
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