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Upgrade 1976 727 with parts from a 1999 A-518?
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Author:  Reed [ Wed Aug 20, 2025 1:59 pm ]
Post subject:  Upgrade 1976 727 with parts from a 1999 A-518?
I am getting ready to do a quick and dirty refresh of the drivetrain in my 1976 D100. Ultimately, I want to switch to an A-500 overdrive transmission, but that isn't in the cards right now.

My truck has (I think) the original 1976 727 transmission. It makes noise and slips so I will be tearing it down and going through it. I happen to have a 1999 A-518 transmission in parts under my workbench.

QUESTION: I know the A518 is basically a 727 with the overdrive unit on the back. Are there any parts worth switching from the 518 to the 727?

I suspect the 727 is not lockup, but I need to pull the converter to be sure. The A518 is electronically controlled and a 4wd version. Really, I guess the only things that might interchange would be clutch packs and planetary gears.
Author:  SlantSixDan [ Wed Aug 20, 2025 8:07 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
I can't answer with specifics, but I will say the A500 and A518 do not have nearly the sterling/bulletproof reputation of the A904 and A727.
Author:  Reed [ Wed Aug 20, 2025 10:58 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Interesting. My research strongly suggests that the A500 and A518 are essentially A904s and A727s, respectively, with the tailhousing overdrive unit and a few fluid passages added. Even the clutches and bands are identical between the units.

I guess I will be a guinea pig and spend some time playing with parts and measuring.
Author:  Reed [ Wed Aug 20, 2025 11:00 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
I DO know the deeper oil pan from the A518 will bolt onto the 727 and I should be able to use the factory steel/rubber pan gasket off the 518.
Author:  SlantSixDan [ Thu Aug 21, 2025 10:06 am ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Designwise, yes, the A500 and A518 are essentially A904 / A727 plus overdrive. But a good bit more than the design of a thing goes into determining how well and dependably it works, for how long.
Author:  Reed [ Thu Aug 21, 2025 1:32 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Thanks, Dan. At this point I will most likely be tearing the 727 apart, seeing exactly what needs to be repaired/replaced, and then looking at the guts of the 99 A518 and comparing.
Author:  eric22t [ Thu Aug 21, 2025 1:37 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
why not just swap in the 518 and not do any of the od/lockup wiring yet?
Author:  SlantSixDan [ Thu Aug 21, 2025 4:15 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Quote:
Thanks, Dan. At this point I will most likely be tearing the 727 apart, seeing exactly what needs to be repaired/replaced, and then looking at the guts of the 99 A518 and comparing.
Sounds like a fun exercise. But let me also ask this: what does the 727 do that you wish it wouldn't, or not-do that you wish it would? The 727 is at the high-water mark for 3-speed rear-drive automatic transmissions by whatever measure: efficiency, durability, shift quality, driveability…what needs upgrading?
Author:  Michael [ Thu Aug 21, 2025 6:17 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Hey Reed,

Hit me up when you get to the torque convertor for the 727. I have an NOS military application unit from 76-77 sitting on the shelf in Seattle (I believe that it is marked "low stall".) Trade or very reasonable $$'s. Cheers - M
Author:  Reed [ Thu Aug 21, 2025 6:47 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Quote:
Hey Reed,

Hit me up when you get to the torque convertor for the 727. I have an NOS military application unit from 76-77 sitting on the shelf in Seattle (I believe that it is marked "low stall".) Trade or very reasonable $$'s. Cheers - M
Hey, thanks! I wasn't planning on replacing the torque converter, but if you are willing to let it go cheap, maybe I will!


Quote:
what does the 727 do that you wish it wouldn't, or not-do that you wish it would? The 727 is at the high-water mark for 3-speed rear-drive automatic transmissions by whatever measure: efficiency, durability, shift quality, driveability…what needs upgrading?
It slips, despite my adjusting the bands and kickdown it lags on initial acceleration and kickdown is delayed, either the planetary or the rear sprag howl pretty bad at speed, its heavy, it takes more power to operate than a 904, and it doesn't have overdrive. It is just plain worn out.

Plus, I get a kick out of mixing and matching factory parts never intended to go together and making them work.

My short term thoughts were since I have a 727 that will be rebuilt and I already have a disassembled 1999 4x4 A518 in a big plastic tub under my workbench, I could see if there was anything worth swapping from the 99 to the 76. Clutch pack, planetary gearset, something along those lines. I know the 727 is hydraulically controlled and the 99 is electronic, so not much will swpa over. At a minimum, I know I can use the deeper fluid pan and better pan gasket from the 518 on the 727.

Long term, my plan is to rebuild the A-500 I have and install it using the slant six to smallblock adapter. Short term, I need to get it operable ASAP and repairing what I have is faster and cheaper than all the driveshaft shortening, engine mount futzing, trans bracket futzing, starter motor clearance grinding, etc... that will come with the A500 swap.
Author:  SlantSixDan [ Thu Aug 21, 2025 11:25 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Quote:
Quote:
what does the 727 do that you wish it wouldn't, or not-do that you wish it would? The 727 is at the high-water mark for 3-speed rear-drive automatic transmissions by whatever measure: efficiency, durability, shift quality, driveability…what needs upgrading?
It slips
Yeah, I get that this particular 727 needs rebuilt, and that you'd like a 4th gear; I meant the 727 per se; what would you want upgraded about this kind of transmission, that you would go fishing in an A518 for?
Author:  Reed [ Fri Aug 22, 2025 12:47 am ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Quote:
Quote:
Quote:
what does the 727 do that you wish it wouldn't, or not-do that you wish it would? The 727 is at the high-water mark for 3-speed rear-drive automatic transmissions by whatever measure: efficiency, durability, shift quality, driveability…what needs upgrading?
It slips
Yeah, I get that this particular 727 needs rebuilt, and that you'd like a 4th gear; I meant the 727 per se; what would you want upgraded about this kind of transmission, that you would go fishing in an A518 for?
Oh. Well, that's why I was asking. My motivation for asking the question was my experience upgrading a 1973 slant six 904 by swapping in the guts of a 1968 318 904. More clutches, four planetary gears instead of three, things like that. I thought perhaps the 23 year gap between the manufacture of the 1976 727 and the 1999 A518 might bring improvements to the internals (other than computer control and overdrive).

If the 518 has stronger clutch packs or planetary gears, I would happily put those in the 727. I don't know that the 518 does have any more durable parts than the 727, but I am game to find out.

I have no complaints about the 727 in general, it is a fine and robust design. Perhaps a bit too unnecessarily robust for most slant six applications, which is why I still intend to switch to the A500. However, since I will be refurbishing the 727 for continued use and I already have a disassembled A518, I thought it would be a good time to install any improved parts, if there were any.
Author:  Charrlie_S [ Fri Aug 22, 2025 2:32 am ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Quote:




I have no complaints about the 727 in general, it is a fine and robust design. Perhaps a bit too unnecessarily robust for most slant six applications, which is why I still intend to switch to the A500. However, since I will be refurbishing the 727 for continued use and I already have a disassembled A518, I thought it would be a good time to install any improved parts, if there were any.
I'm really not up on the A-500 or A-518 But from what I understand, there are several upgrades that need to be done on the OD units. Those and the converter clutch are the weak points
PS: If I ever get several other projects completed, I have plans to do an A-500 build/upgrade for one of my slant cars. I have a core trans, and a master rebuild kit on hand. Also have a A-518 core in stock. I'm thinking the 518 might be a good canditate for my 88 D-150. It is a 360 4bbl with a 727, and gets horrible fuel milage.
Author:  Reed [ Fri Aug 22, 2025 10:21 am ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Thanks, Charlie, I will keep those points in mind. I haven't gotten too far into the reasearch about A500s and A518s, other than confirming that that share the same clutches and bands as the 904 and 727. When the time come to finally rebuild the A500, I will get much more in-depth about weak points and upgrades. I do know the A500 and A518 share the same Borg Warner overdrive unit and disassembling the overdrive unit for service can be dangerous because there is a spring that is compress with over 300 ft-lbs of force. Or somthing like that. Definitely a job to be done with a press.

I agree the A518 would be a good candidate for your truck. My 76 D100 has 3.9 rear gears so even with no load or trailer the 225 spins around 3300 RPM to keep up with highway traffic. The overdrive lets you run a lower reaar axle ratio for improved pulling power but keep the engine RPMs down once you get moving.
Author:  SlantSixDan [ Fri Aug 22, 2025 6:23 pm ]
Post subject:  Re: Upgrade 1976 727 with parts from a 1999 A-518?
Quote:
I do know the A500 and A518 share the same Borg Warner overdrive unit
Mmm…no, sir. There is no Borg-Warner overdrive in either of those transmissions. They were in-house designed, in-house built by Chrysler.
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