I have a sort of back burner plan to put a wide ratio big block ZF behind my 6.9 (I know about the bell pattern ) And thought it might be worth asking after finding pretty minimal info via google and forum searches.

So I am thinking that since the SMF 7.3 kits are so popular, using them might be a better idea than using the obscure kit for the rare 6.9/ZF combo- especially since most of them are inherently more durable. I'm looking at a 12.25" AMS clutch with sprung hub for solid flywheel and 1.25" 10 spline ZF input for $230. Since this will use an 8 bolt pattern rather than the 6 bolt 6.9 pattern I'll need to re-drill. I also plan to resurface and tend to the pilot port should it need something different or special.

I figure that a s542 should be fine behind a 6.9, but if I happen on a s547 for cheap I'll probably snag it. I haven't made up my mind on the bell holes, but am thinking either overbore offset and press fit steel sleeves, or MIG them in, mill the surface and redrill the pattern. And of course address the rear trans mount issue.

I know someone will ask why wide ratio: Because the final in first and reverse with 3.55's is about the same as going to 4.88's in the axles, but with long legs for the freeway.

Thoughts? Am I missing something obvious that would make this not work?

 

You can modify the 6.9 flywheel or just buy the NA 7.3 SMF conversion kit that includes the flywheel. There is supposed to be a 6.9 NA vs 7.3 NA difference in balance, but the consensus is that it is negligible if there even is a difference. There are no more 6 bolt SMF clutch sets being made for a 6.9 solid flywheel. I looked really hard and all the ones I had reference to were discontinued. The 8 bolt kits say they are for 6.9 or 7.3 NA applications.

As far as the wide ratio ZF, I will be putting one in my truck. I have a worn out close ratio diesel ZF so I think I will just swap the main case and set the pre-load on the input shaft bearing instead of modifying the holes. Filling in and re-drilling is a good option, if you have the ability to weld aluminum. If you enlarge the top two holes for the diesel bolt size, then bolt the inch thick adapter plate from the back of the engine onto the bell you have a really solid jig to guide the rest of your hole drilling.

Unless you tow very, very heavy all the time, the close ZF (or close T19) have the gears too close together for my taste. This is especially true when a turbo is added, which tends to extend the power band higher in rpm's compared to NA. Wide ratio may not be for everyone but for my set up it will be a very worthy upgrade.

 

Right on, thanks for the input. I was hoping to reuse the 6.9 flywheel just to be a cheapskate. It's just hard to want to toss a perfectly good single mass. I guess the 6.9/ZF kit is even more obscure than I had guessed!

IMO the wide ratio is a better option anyway for heavy towing. Wider shift spacing is merely a nuisance, tall first is a genuine liability for towing. *Especially* with tall axle gears.

 

Heck yeah, if you have the machining capability to drill and tap the 6.9 flywheel then go to town. I am actually buying NMB2's 6.9 flywheel that he drilled for a 13" Southbend SMF clutch.

I like the idea of having the crawl gear on tap without having to switch to low range on the transfer case. My rig is a little different, I have 4.10's and a Gear Vendor overdrive. I will have 2nd gear start outs 99% of the time. I am curious how you will like it on 3.55's... post back when you get the swap done and let me know what you think.

 

I did this to my truck just this summer, the c6 just sucked, didn't like it at all, anyways, I got a 4x4 zf for the 460 for a steal of a price, I used a stepper bit to enlarge the top 2 holes in the case, then bolted the 1" addapter plate to the trans and centered it as best as I could, then I drilled the dowl holes and suck them in to prevent walking of the plate, then proceeded to drill the holes, now I did this with a 4 flute end mill and wasnt very pleased with the results, it worked but the holes were still a bit funky to say, but keep in mind that 3 of the 4 holes are ontop of the big block holes so there a bit more fun, the other one is by itself.

If you look closely at the casting of the bell housing youll see the outlines for the diesel holes btw.


I got 3.55's and it drives like a new truck to me compared to the c6, now it can actualy move out of it's own way I start off in 2nd all the time, first is useless for dd, reverse is nice, doesn't take any throttle to get it going either.

I found with the few truck that I have that have autos is that there is a trans tunnel plate under the carpet, but its different then the one for the zf, also its tack welded on in a few places and not cut out under it, very minor detail, just watch your windsheild with grinder sparks! they will burn in as I nicely found out.

Also because the trans is 4x4 I put a tcase behind it and cut the hole out for the shifter, hooked it all up, now I got 4lo if I ever need it and in the future if I go 4x4 I just need the axle.

Any more info just ask

 

Thanks for sharing your experience with the wide ratio ZF... That is very cool to know. I did not expect that 2nd gear start out would work with 3.55's, but it's nice to know that it does. I was even concerned that 2nd gear starts with 4.10's might be pushing it... but another member confirmed that it was a sweet combination. Glad to hear another opinion on that with even higher gearing

That is odd that you had to grind out the tunnel for your auto to manual conversion. The 7the generation truck that I just did that to did not require any modification except cutting the 4x4 shifter hole into the taller 8th generation floor pan.

 

Ifrythings- I'm curious, what size tires to you run on the wide ratio truck?