Replacing 6.9 with a 7.3 - Diesel Forum

gtuplin · 05-27-2007, 02:03 PM

OK so I am new to this form and hope I can work through the bugs but here goes!
I figure this is one of the largest pools of knowledge when it comes to 6.9/7.3 issues so I could use a little assistance!
I have a new 7.3 that I would like to install in my truck that has a 6.9 c-6 trans can I place these two units together without balancing issues or pricey upgrades?
Question # 2 I have a second truck with a 6.9 that I would love to install a ZF into but I have some confusion regarding flywheel, clutch pressure plate! Why can I not use the 6.9 flywheel and clutch assembly in a ZF, I mean seriously is there a problem with this or is it a myth that these don't mate? I have people tell me they put the 7.3 in-front of the old 4 speed with the 6.9 flywheel and it works fine so what the deal?
So let the process begin!

DevoK12RS · 05-27-2007, 02:16 PM

Here are
three threads
for you.
I think that will be a good start for you....
Devo

Fozz · 05-27-2007, 08:09 PM

Previous poster has some good reading there. I think the 6.9 is externally balanced. I do know the 7.3 is internally balanced. Don't know about your second question. Good luck - Foz

heinzboz · 05-28-2007, 12:49 PM

I do know for a fact that there is an issue of some sort with the flywheel....I just can't remember for sure what the deal was. My uncle did this same swap in his 89 F250 (except it went the other way, from 7.3 to a 6.9). They swapped the motors, buttoned everything up, and the thing shimmied, shook and vibrated all over the place. They ended up having to drop the transmission and change flywheels, but I can't remember if they had put the 7.3 flywheel on the 6.9 before, or if they had left the 6.9 flywheel on the motor?? Anyway, there is an issue, so I'd find out before dropping the other motor in.

85Hauler · 05-28-2007, 05:01 PM

I've been running a 6.9 flywheel on a 7.3 for a few years now with no noticable problem. Many people have done it without problem. I have never spoke to anyone who has had a problem just hearsay of other peoples problems. Ford says not to interchange flywheels and flexplates between 6.9 and 7.3s. However it appears Ford themselves did it in the late '80s during the change over from 6.9 to 7.3/ c-6 to e40d/ T-19 to Zf5. [img]/ubbthreads/images/graemlins/shrug.gif[/img]

heinzboz · 05-28-2007, 06:00 PM

[ QUOTE ]
I have never spoke to anyone who has had a problem just hearsay of other peoples problems.

[/ QUOTE ]

So does this mean I'm number 1 on your list!!!! [img]/ubbthreads/images/graemlins/grin.gif[/img]

85Hauler · 05-29-2007, 04:27 PM

[ QUOTE ]
[ QUOTE ]
I have never spoke to anyone who has had a problem just hearsay of other peoples problems.

[/ QUOTE ]

So does this mean I'm number 1 on your list!!!! [img]/ubbthreads/images/graemlins/grin.gif[/img]

[/ QUOTE ]

Well....... You say it was your uncle and you're not sure which flywheel they used. So I'll put you on the hearsay list for now but you can be number one on that list [img]/ubbthreads/images/graemlins/grin.gif[/img]

gtuplin · 05-29-2007, 07:52 PM

OK so here is a bit of weird stuff unearthed from catalogues and verified by clutch and pressure plate master lists!
In 1987 Ford did use the combination of a 6.9 and a ZF trans with a non dual mass flywheel. It also states that this clutch kit is used in 7.3 applications with factory non dual mass fly wheel.
So here is the thing! If I must not mix the flywheels from 6.9-7.3 why does the 87 6.9 with ZF trans use the same flywheel, clutch and pressure plate as the 87 7.3 ZF combo with non dual mass ? Pioneer clutch kit part number 10065 shows this usage as does Fen-co so I think that in the proper combination of parts the 6.9 or 7.3 non dual mass flywheel will switch back and forth (clutch and plate to match ZF application 1 1/4") I could see a viberation if a t19 was used behind a 7.3 and the 1 1/4" spline was used on the 1 1/16" input shaft unknowingly!
So I am going with urban myth on this one!! Ford did it and perhaps it was not a perfect match but it worked well enough to sell trucks!!
I will go ahead with my projects unless there is someone with 100% irrefutable evidence stating otherwise!
Thanks again for the tips and info as it has better help me with the search!
Grant

gtuplin · 07-24-2007, 12:23 AM

So as promised here is a simple straight forward observation of the 6.9 / 7.3 conversion!
I have removed the 6.9 that had an auto behind it and replaced it with a 7.3 ZF combo but here is the good stuff at least so far! I used a 6.9 flywheel for a std. purchased a new clutch pack for a 1987 6.9 ZF combo ($212.00) put it all together and flashed her up today. Idles like a rock!! no shaking shimmying, vibrating or any of that stuff, just purrs like a new 7.3 should! Even at higher RPM, still nothing! The clutch works like a hot damn and I only moved it around the farm to see but so far I have no related issues.
Once the truck is all back together and I get it out on the road I will report!
Thanks everyone for the help with this!
Grant