The Ford Barn

roadster36 · 03-28-2016, 02:09 PM

Does anyone know the difference between an 81A-7017 and a 8M-7017 main drive gear ? Both are 8.25 inches long and have the same tooth count. Thank you- Dennis

Bob C · 03-28-2016, 03:22 PM

Don't know the difference but the parts book say 81A-7017 replaced by 8M-7017.
(actually shows 8M-7071 but I assume that is a misprint)

Bob

35fordtn · 03-28-2016, 04:34 PM

To my knowledge they are the same and interchange with one another.

Lawrie · 03-28-2016, 04:50 PM

Why the different part number then.
lawrie

Mac VP · 03-28-2016, 05:30 PM

There is a subtle difference between the 81A-7017 and the 8M-7017 gears. Yes, the two are interchangeable, but here is why they differ. Please refer to the attached picture comparison, but pretend that the upper gear is not a 48-7017 but that it was the 81A gear, and that the lower gear is the 8M-7017 (which it really is).

You'll notice the section to the left of the bearing mounting surface has a longer "shoulder" just left of the snap ring groove on the lower picture (8m-7017). The reason for that was to "seal off" this area inside the front bearing retainer on the Mercury 1949-51 transmissions (which did NOT have a front grease seal). The Merc front retainer had that spiral groove to help retain any gear oil that might make it out the front of the gearbox.

The 8M part number was a 1948 introduction to the parts system for the 49 Mercury cars. So this main drive gear was the match to that bearing retainer. Ford simply mfr'd it so that it would also work on the earlier transmissions that used the 78-7050 bearing retainer with the 78-7052 oil seal. This way they didn't have to offer two separate parts for the same type applications.

One detail to keep in mind. Although the two gears are fundamentally interchangeable, you would not want to use the 81A-7017 gear in the 49-51 Merc trans (with the 8M-7050 front bearing retainer) as there would be a gap of space that might allow some gear oil to leak out the front. If you decide to use the 81A drive gear on the Merc trans, I would swap the 78-7050 front bearing retainer (with the 78-7052 oil seal installed) for the Merc retainer.
http://www.vanpeltsales.com/FH_web/F...comparison.jpg

Lawrie · 03-28-2016, 07:59 PM

Sponsored Links (Register now to hide all advertisements)

Mac, what would we do with out guys like you.
thanks
Lawrie

roadster36 · 03-28-2016, 09:20 PM

Thank you for input on the input !

03-28-2016, 02:09 PM	#1
roadster36 BANNED Join Date: May 2010 Location: Sonoma,Calif. Posts: 877	3 spd input shafts Does anyone know the difference between an 81A-7017 and a 8M-7017 main drive gear ? Both are 8.25 inches long and have the same tooth count. Thank you- Dennis

03-28-2016, 03:22 PM	#2
Bob C Senior Member Join Date: May 2010 Location: So Cal Posts: 8,757	Re: 3 spd input shafts Don't know the difference but the parts book say 81A-7017 replaced by 8M-7017. (actually shows 8M-7071 but I assume that is a misprint) Bob

03-28-2016, 04:34 PM	#3
35fordtn Senior Member Join Date: Aug 2010 Location: McMinnville, TN Posts: 2,327	Re: 3 spd input shafts To my knowledge they are the same and interchange with one another.

03-28-2016, 04:50 PM	#4
Lawrie Senior Member Join Date: May 2010 Location: Qld, Australia Posts: 4,223	Re: 3 spd input shafts Why the different part number then. lawrie

03-28-2016, 05:30 PM	#5
Mac VP Senior Member Join Date: May 2010 Location: Cincinnati, OH Posts: 1,361	Re: 3 spd input shafts There is a subtle difference between the 81A-7017 and the 8M-7017 gears. Yes, the two are interchangeable, but here is why they differ. Please refer to the attached picture comparison, but pretend that the upper gear is not a 48-7017 but that it was the 81A gear, and that the lower gear is the 8M-7017 (which it really is). You'll notice the section to the left of the bearing mounting surface has a longer "shoulder" just left of the snap ring groove on the lower picture (8m-7017). The reason for that was to "seal off" this area inside the front bearing retainer on the Mercury 1949-51 transmissions (which did NOT have a front grease seal). The Merc front retainer had that spiral groove to help retain any gear oil that might make it out the front of the gearbox. The 8M part number was a 1948 introduction to the parts system for the 49 Mercury cars. So this main drive gear was the match to that bearing retainer. Ford simply mfr'd it so that it would also work on the earlier transmissions that used the 78-7050 bearing retainer with the 78-7052 oil seal. This way they didn't have to offer two separate parts for the same type applications. One detail to keep in mind. Although the two gears are fundamentally interchangeable, you would not want to use the 81A-7017 gear in the 49-51 Merc trans (with the 8M-7050 front bearing retainer) as there would be a gap of space that might allow some gear oil to leak out the front. If you decide to use the 81A drive gear on the Merc trans, I would swap the 78-7050 front bearing retainer (with the 78-7052 oil seal installed) for the Merc retainer. http://www.vanpeltsales.com/FH_web/F...comparison.jpg __________________ VANPELT SALES LLC Cincinnati, Ohio Office: 513-724-9486 www.vanpeltsales.com www.classictransmission.com Last edited by Mac VP; 03-28-2016 at 07:46 PM.

03-28-2016, 07:59 PM	#6
Lawrie Senior Member Join Date: May 2010 Location: Qld, Australia Posts: 4,223	Re: 3 spd input shafts Sponsored Links (Register now to hide all advertisements) Mac, what would we do with out guys like you. thanks Lawrie

03-28-2016, 09:20 PM	#7
roadster36 BANNED Join Date: May 2010 Location: Sonoma,Calif. Posts: 877	Re: 3 spd input shafts Thank you for input on the input !