OK, so I finally found a used ZF 5 for my project truck. Came out of an early 1990's F350 IDI truck. Came with the BW 1356 T-case, original dual mass flywheel and clutch parts, hydraulic cylinders etc.,

The flywheel "looks" fine, but I have read that the dual mass flywheels are a ticking time bomb. Should I:
A)use the DMF that came with the transmission
B)use the SMF that's currently between my 6.9L and T19
C)buy a different flywheel all together (suggestions on brand type???)

Unfortunately this transmission is the close ratio type, and I don't really like the gear spread as much as the wide ratio type where one can bypass the low first gear. I've been thinking about switching from 4.10's to 3.55's to get that first gear a little taller. Only thing is, I will be running a gear vendor overdrive behind the ZF, and in "double overdrive" the final output would go from 2.46 to 2.13. My original intent was to get something right around 2.5, and I'm wondering if 2.13 might be too much. Any ideas? I mostly don't like the 16.97 first gear, and second is too high to start out in....

 

The 6.9 and 7.3 are both externally balanced. Both have different flywheels/ dampeners. So a 6.9 and 7.3 flywheel should not be interchanged. I personally if i had the coin would replace it with a SMF. If i couldnt afford it, id stick with the DMF it came with and run it till the clutch dies and hope that i have the coin to replace with a SMF at that time. Southbend or LUK sell SMF IDI clutches i believe, havent actually researched it myself.

As far as gear ratio, i would run 3.55s, throw some 285's (33's) under it and save the money on the GV. If you already have the GV, keep the 4.10s and start in 2nd. Just my .02 hope it helps.

 

Can I use the SMF flywheel that is on my 6.9L right now, provided I use the clutch disc and pressure plate for the ZF? I've seen aftermarket SMF "conversion" flywheels at rockauto starting at $112 on up. Don't know how these would differ from the SMF for my T19...

I already have the Gear Vendor unit, and plan to use it in conjunction with the ZF. It puts my tallest gear right about where I want it, but I don't like the narrow range 1st and 2nd gears. I can't start out in second like the old wide range T18's that I'm used to, and 1st is way lower than needed most of the time. Perhaps starting out in 2nd will be possible once I cut the truck down from a CC long bed to a single cab short bed, add the Banks and put in the torque cam, but in its current configuration it does not work to start out in 2nd. That's why I was thinking about the 3.55's (to move 1st and 2nd up a bit and start out in 1st) but I'm worried that my high end will be too much. It would not be too expensive to do 3.55's if I got a JY rear axle that already had them, and only had to do the R&P swap on the D60 front axle. I know the GV with the T19 and 4.10's (3.20 final output) isn't high enough, but 2.13 might be over the top??? Probably should just be happy with 4.10's and the ZF plus GV (2.46 final output).

 

A 87 with the 6.9 and zf5 supposedly has the same flywheel part number as the later 7.3's. The 93 and 94 turbo flywheels are the different ones as far as I know.

 

You can use the 6.9 flywheel, but you can't use the original 7.3 clutch plate and pp with it. The 6.9 pressure plate is mounted with 6 bolts, and the 7.3s is mounted with 8. the 6.9 flywheel does not have the mounting holes for 8. The dual mass clutch plate does not have the dampner springs is it.
Do a search on the dual mass conversion. A few years ago a member by the handle of CheaperJeeper posted the parts numbers for the stuff he used to do this.

 

Found the thread https://www.ford-trucks.com/forums/6...uestion-3.html . The part numbers are listed in post #39.

 

Clayton is correct ...

The 6.9 and 7.3 N/A's are the same, The IDI Factory Turbo motor is different.

-Enjoy
fh : )_~

 

Thanks, this thread is extremely helpful... just to be clear, these are the parts that work with the existing flywheel that came with the 6.9L and T19 correct? There was some discussion about buying a new flywheel, but there was more than one set-up discussed.

Also, it sounds like this is an 11" clutch set up. Does anyone know of 12" clutch that will work with the ZF and 6.9 flywheel? Not critical, but if I'm building it up from scratch I'd rather have the HD clutch if there is one to be had...

 

Yes Cheaper used the original 6.9 flywheel, and the parts listed, from what I understand. When I put the ZF in the 85, I used the single mass conversion for a 90 7.3 from NAPA. It had the 12 inch clutch disc, worked great until I wrecked the truck.

 

Great, thanks! I think you saved me a lot of headaches, returned parts, frustration etc.... BTW, how did you like the ZF with 3.55's? (I see in your signature that's what you had in your '85). That would be a 2.73 final output in 5th. Just curious how it drove, if it felt like it still had power in high gear, how it revved at freeway speed etc. My 4.10's and GV (3.20 final output) still feels waaaay to low for highway driving. I'm in OD by 35 mph, and it still screams above 60-65. My new set up will be 2.46... just wondered how you liked 2.73?

 

The 85 was used mostly as a chore truck, and it never saw the freeway. Driving on the highway, it saw 2100 rps at 60-65 if I remember right. I never pulled a trailer much with it, but when I did it handled it pretty well. I had a 7.3 turboed pump on it, and shortened the wastegate rod so it was getting around 8 lbs. boost max. It got around 15-16 mpg on the highway, and always had enough power for me.

 

A ZF5 with 3.55 is sweet as long as you don't need to back trailers up, then you are screwed!

It's a clutch burning process, the ZF has to high of a reverse.

-Enjoy
fh : )_~

 

x2 with what festus is saying, I have 3.55's I think in my 1990 with the ZF...it backs up WAY too fast with my trailer and i keep having to pull forward and start over...

 

On the plus side, if you're unloaded, 3.55's back up nice and fast The way my driveway is setup its the dead end of one street on a corner lot. Coming home I pull in straight. In the morning back right out straight so the reverse speed is nice for that.

Most of the time if I have a trailer, I just reach down and pop the transfer case in low to back up. Otherwise I end up straining my foot playing with the clutch continually.

My truck sometimes has issues with steep hills while empty, but thats more dependent on the speed I'm going at the time (or the jackhole thats going too slow in front of me). Its been better since I put the turbo on and replacing the injectors and IP. When towing up hill its sometimes an issue, but not usually to the point where I'm actually going too slow for traffic. I also don't tend to rev up the engine much, its usually just to the point the next gear will work, so that might make some difference.

For the small amount of time that I'm towing something, I'd much rather have the fuel economy of the 3.55's and deal with taking a little longer to get there.

 

I've got a ZF5 & 3.55 w/ 33" tires, I like the combo. 1st and reverse are pretty high, but you can always use low range on the t-case. (assuming a 4wd, of course)

However, if you have a GV box as well 3.55 might be too much. I'd stay with the 4.10 or maybe try a 3.73. 5th with the 3.55 works pretty well for me, I'm somewhere right around 1900 rpm at 65-70 which works well. I use 5th when towing light (3-4000# trailer) on the flat but you don't have much power if you hit a light incline or something. I do find that the spacing between gears is wider than I'd like which is where a GV would help a bit. I have a US Gear underdrive I'm going to install one of these days which will hopefully help that problem.

I'm using the LUK single mass flywheel conversion setup, it works well and I recommend it. The clutch is much heavier duty and smoother than the stock 7.3 setup, and no DM worries, and it's really not much more than a new clutch alone would be. Something like $550-600 with shipping?

Yes, the 6.9 & 7.3 are differently balanced, but the conclusion we reached after talking to several machinists and checking things out was that the difference is minimal. I've been running a 7.3 SM flywheel on a 6.9 for probably close to 50k miles now and never had any issues, or evidence of problems. I had the engine apart after 10-20k with the SM and saw no adnormal wear on the bearings or evidence of vibration problems. If you're building a race motor and planning to redline it forever and a day, then maybe you should be worried, but for general use? It's close enough.