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!

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.

why do so many people talk about flexplate needing to be rebalanced?

Last I knew, the flexplate is balanced to the engine, not the TC. But I haven't had any issues yet... yet. LOL.

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

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

Well, you can narrow it down some. Some of those models didn't come with a L67 to begin with.

Okay that's 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).

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.

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

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's the internals. The flexplate size has an effect of how much torque is applied to the trans.

I got that, what I was hung up on was the larger TC being mandatory for the HD and I was under the impression (until your previous post) that it was mandatory both ways, that only the HD could use the larger TC. I see cars with 258mm TC but without SC so that is what had me confused.

I actually don't want an HD, I don't need it for this build and its just additional cost and complexity (having to drill the flexplate, different axle inners, etc). Looking at trans tags for 1998, the two best matches for subset of [comfort, gears, tire size, weight, stall] are probably the 8FFB for the 3.8 NA Park Avenue and the 8TNB for the 3.8 NA Regal. The Park Avenue is probably the better fit, but it shows the large TC, hence my questions--is this an HD trans on an NA motor, or is it the stronger TC on a regular 4T65E.

I think its just a larger TC. Strength doesn't play a factor, but probably stall speed would.

According to this page the 8FFB from the Park Avenue is non-HD (text search for 8FFB). That would mean the TC is the only difference between the luxury Park Avenue and the cheaper cars that used the 8TNB. Might be comfort factor--longer spoolup time, different stall, more PWM slipping programmed into the TCU requiring larger TCC clutch or improved heat shedding or something, etc.