Is a SMF bad for transmission?

vegas39

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After posting a few days ago about converting to SMF, I'm now reading all over the place that it can actually be harmful to the transmission.
I would like to hear some opinions on this matter, as I'm just a couple days away from buying a kit.
If DMF is the way to stay, should I be concerned with replacing mine, or keeping it? I have no idea what shape mine is in, aside of the fact that it seems fine at the moment but I dont want to go through the hassle of having a new clutch put in, only to have my DMF fall apart in two months.

Anyone know of a good place to buy a new DMF?
 

lindstromjd

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I'm not completely familiar with the DMF vs SMF battle... but isn't there something called a "lucky mod" that you can do to the DMF to fix the problems it has of vibrating itself apart?
 

IDIDieselJohn

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Subscribed to this.

I to am not decided what to do on mine once I put everything back together. I know for a fact my DMF is tired though.
 

Fordsandguns

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I have been told by a knowledgeable member that the gear roll over can wear the gears in the zf transmissions. Over time it wears the teeth of the gears to a thin razor edge that can break.
They were designed for use with the DMF. The T19 transmissions that were behind the 6.9's were designed to handle the engine vibrations and didn't have this issue.
Wish they would have made the T19 with an added overdrive. Best of both worlds.
 

The Warden

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I'm not completely familiar with the DMF vs SMF battle... but isn't there something called a "lucky mod" that you can do to the DMF to fix the problems it has of vibrating itself apart?
The Lucky mod fuses the two halves of a dual-mass flywheel together and makes it into a single-mass flywheel. So, if having an SMF really will ruin a transmission, a Lucky'ed DMF would have the same problem.

I haven't disassembled my ZF, but I've been running a LuK SMF for about 50K miles and haven't had a single problem. That doesn't mean that a problem won't crop up down the road, but that's been my experience to date :dunno
 

franklin2

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The diesel engine is not very smooth when it runs. When the crankshaft goes around, it a series of very hard hits that make it turn instead of a smooth shove from a gasoline engine. These hard hits are what slap the gears together and make all the noise in the zf. The dual mass flywheel has clutches and springs that cushion and soften these hard hits coming from the diesel engine, which makes the gear slapping and the noise go away.

I don't think the t19 was "designed" for the diesel engine and a SMF. It's a standard design that has looser tolerances and runs the heavier gear oil. This combination along with the heavier cast iron case dampens the noise, though the gears are still trying to slap together in this tranny also. I have never driven one of these trucks with the 4 speed, but I bet it's noisier than one behind a 460.

I did the lucky mod myself, and have had no trouble so far. If I had the money, I would definitely like to put a DMF back in. The noise it makes gets a little aggravating after awhile.

I am like the others on the gear wear, I don't know, time will tell. I have my doubts though. Gears slapping together with no load, versus gears pressed together under load when pulling a trailer or something? I don't see the gear slapping causing that much trouble myself.
 

Fordsandguns

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Maybe "designed" was poor wording on my part. I don't totally understand the differences but I believe the gears are made differently in the zf vs the T19.
Softer maybe? Like we have been told that it isn't good to shift the zf without the clutch.
 

NO_SPRK

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any trans that has syncros should not be shifted without a clutch...

a fellow member here i met in vegas had a issue with a new SMF being drilled off center and ruining his ZF5.

i have driven both SMF and DMF trucks. i slightly favor a DMF because i can be a little harder on it without it bucking

the main issue is the transfer of force. the SMF clutch hub still isnt up to par.
 

Mulochico

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how could you even shift a light duty tranny without a clutch
Very lightly and carefully:sly. I had to do it to get home when my clutch hydraulics went out. I am a professional truck driver (30+ yrs) and rarely use the clutch in the semi except to stop/start. The syncros in the light duty trans allow for you to push in the clutch and just slip it from one gear to the next. If it is done w/o the clutch you need to wait for the rpms of the input and output shafts to match and you risk messing up the syncros if done to soon/late or to hard.

heavy duty trucks can shift without it why?
See above: On a heavy duty truck they are not a syncromesh trans so if you shift with the clutch you need to "double clutch" (push in the clutch to get it out of gear, let out the clutch to get the input/output shafts to even out their rpms as the engine slows, then push in the clutch to put it back in gear) That can be a pain if you don't watch your rpms carefully. So what we do is let off the throttle to ease the tension on the trans to slip it out of gear, let the rpms drop while holding a very light pressure (no grinding) on the shifter to the gear we want, then when the input/output shaft rpms line up the trans goes right into gear. If done properly this does no damage to the trans and saves the clutch (I have gone 2 1/2 years on the old style clutches w/o need of any adjustment [Local in LA to boot :D]). If done improperly it can grind the gears and cause damage
 

IDIDieselJohn

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You shift into neutral as you still have your foot on the gas, so the tranny doesn't have a load on it, it'll just slide out of gear without effort, then as the RPM's drop, put a light load on the shifter in the next gear, as soon as the RPM'***** the spot, it'll slide right in.


For downshift, I would slide in N around 1000RPM, give it a rev while putting a slight pressure on the shifter, again, when the spot hits, it'll slide right in.


It takes practice to get it right :D You really have to know the truck to.



I was upshifting around 1800RPM in that video.
 

Mulochico

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well how do you know when they match or is it a certain rpm?

There is a certain rpm spread between the gears. Not sure exactly on the ZF as I have just gotten good at the "feel" after all these years. On the ZF I know the spread between 4th and 5th is less than the rest as it is an overdrive.

The above video proves it can be done. I do not suggest it as the trans is not designed for it. In an emergency (i.e., my clutch slave cyl going out) fine. To show off, whatever, it is your trans :dunno . But it is still better to use the clutch to avoid unnecessary problems.
 
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