TH350 / NP205 - ATF or 80w-90? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

DarkFlounder · 06-03-2017, 07:26 PM

Every manual I've found (from Haynes to original shop) say to use GL-5 80w90 gear lube.

Doing some extensive maintenance on my '72 and found ATF in the transfer case. It didn't look like a mix of ATF/gear oil, so the PO probably put that in.

Should I replace with 80w90?

And if I have/get a trans rear seal leak, will the ATF affect the GL-5?

**kwmech** · 06-03-2017, 07:30 PM

You more than likely have bad seals in the adapter between the 2 boxes. You'll have to pull the 205 off the back, then the adapter. Change the seals and reassemble.

bhayes1 · 06-03-2017, 08:07 PM

When I purchased my truck it had 100% ATF in the np205 as well. My transmission would get low from time to time and id keep adding to it. ATF was slowly moving from the th350 to the 205.

Didn't hurt the transfer case a single bit, but it was sure nice to replace the seal and stop the leak.

DarkFlounder · 06-03-2017, 10:08 PM

Quote:

Originally Posted by bhayes1

but it was sure nice to replace the seal and stop the leak.

This is what I'm currently thinking. Replace the seals, and not worry about it.

How hard was it to drop the transfer case?

**truckster** · 06-03-2017, 10:31 PM

Quote:

Originally Posted by DarkFlounder

This is what I'm currently thinking. Replace the seals, and not worry about it.

How hard was it to drop the transfer case?

They're heavy. Really, really heavy. A transmission jack will help a lot.

**kwmech** · 06-04-2017, 12:56 AM

They are 158 pounds and basically round without a lot to grab onto.

**special-K** · 06-04-2017, 11:28 AM

A lot of people started using ATF in their NP205s once newer ones started calling for it.

Money_pit · 06-04-2017, 11:52 AM

If I remember correct iron cases get the gear oil. Aluminium ones get the atf.

**special-K** · 06-04-2017, 01:23 PM

I still run gear oil. I was wondering since this one has pure ATF if that's what was used, not a bad seal.

**Stocker** · 06-04-2017, 01:44 PM

I run gear oil too. When my seal was bad, the transmission would run low on fluid as it was pumped into the t-case. If your transmission fluid level remains normal, the seal should be good.

jjzepplin · 06-04-2017, 02:32 PM

I forget where, probably the service station I worked at when I was young, but I read that mine took ATF. I have run that for years and had no problem. I did have a leak between the two as well but fixed that. I have since been corrected that I should run gear oil. I dunno. Seem it would have less rotating resistance with the ATF. I have run this for 3000 mile trips and had no issues...? Now the 205 I took off the back of a SM465 had gear oil in it. Maybe it had a leak too.

Alex V. · 06-04-2017, 02:50 PM

In researching this for the '74 K10 I had a few years ago (TH350/205 combo like yours) the best consensus I could come to is that the 205 is so impervious to any dismal matter like lubrication that if it's behind a stick shift, use gear oil, and if it's behind an automatic, use ATF. Either is very unlikely to be the reason a 205 fails.

**special-K** · 06-05-2017, 07:05 AM

They only seem to fail when they lose too much "whatever". As long as they have adequate "whatever" they are fine. The synthetic 80w90, I use Royal Purple, has a much thinner consistency similar to ATF

06-03-2017, 07:26 PM	#1
DarkFlounder Registered User Join Date: Nov 2014 Location: Austin, TX Posts: 281	TH350 / NP205 - ATF or 80w-90? Every manual I've found (from Haynes to original shop) say to use GL-5 80w90 gear lube. Doing some extensive maintenance on my '72 and found ATF in the transfer case. It didn't look like a mix of ATF/gear oil, so the PO probably put that in. Should I replace with 80w90? And if I have/get a trans rear seal leak, will the ATF affect the GL-5? __________________ 72 K2500 Sierra Longbed Wideside 350/TH350/NP205 66 Plymouth Barracuda 225 Slant6

06-03-2017, 07:30 PM	#2
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,397	Re: TH350 / NP205 - ATF or 80w-90? You more than likely have bad seals in the adapter between the 2 boxes. You'll have to pull the 205 off the back, then the adapter. Change the seals and reassemble.

06-03-2017, 08:07 PM	#3
bhayes1 Registered User Join Date: Jun 2016 Location: Central California Posts: 229	Re: TH350 / NP205 - ATF or 80w-90? When I purchased my truck it had 100% ATF in the np205 as well. My transmission would get low from time to time and id keep adding to it. ATF was slowly moving from the th350 to the 205. Didn't hurt the transfer case a single bit, but it was sure nice to replace the seal and stop the leak. __________________ 1969 GMC K2500

06-04-2017, 12:56 AM	#6
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,397	Re: TH350 / NP205 - ATF or 80w-90? They are 158 pounds and basically round without a lot to grab onto.

06-04-2017, 11:28 AM	#7
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,863	Re: TH350 / NP205 - ATF or 80w-90? A lot of people started using ATF in their NP205s once newer ones started calling for it. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

06-04-2017, 11:52 AM	#8
Money_pit Registered User Join Date: Jan 2014 Location: Illinois Posts: 131	Re: TH350 / NP205 - ATF or 80w-90? If I remember correct iron cases get the gear oil. Aluminium ones get the atf. __________________ 71 C10 Finally took a leap and did the 5.3 4l80e swap. Running the Holley Terminator Xmax. Still working bugs out.

06-04-2017, 01:23 PM	#9
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,863	Re: TH350 / NP205 - ATF or 80w-90? I still run gear oil. I was wondering since this one has pure ATF if that's what was used, not a bad seal. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

06-04-2017, 01:44 PM	#10
Stocker 20' Daredevil (Ret) Join Date: May 2003 Location: Jefferson State Posts: 13,506	Re: TH350 / NP205 - ATF or 80w-90? I run gear oil too. When my seal was bad, the transmission would run low on fluid as it was pumped into the t-case. If your transmission fluid level remains normal, the seal should be good. __________________ - Mike - 1972 K20 LWB 350/350/205 RIP El Jay

06-04-2017, 02:32 PM	#11
jjzepplin Registered User Join Date: Feb 2012 Location: Ruskin Florida Posts: 4,545	Re: TH350 / NP205 - ATF or 80w-90? I forget where, probably the service station I worked at when I was young, but I read that mine took ATF. I have run that for years and had no problem. I did have a leak between the two as well but fixed that. I have since been corrected that I should run gear oil. I dunno. Seem it would have less rotating resistance with the ATF. I have run this for 3000 mile trips and had no issues...? Now the 205 I took off the back of a SM465 had gear oil in it. Maybe it had a leak too. __________________ 70 swb 4x4 406sbc 700r4 203/205 d60/14blt locker yadda yadda http://67-72chevytrucks.com/vboard/s...88#post6935688 Yeller 72 Blazer 2wd conversion project "No Daggum Money" http://67-72chevytrucks.com/vboard/s...d.php?t=550804 LS1-T56 3.73 LSD super budget build Blanco-2014 Sierra SWB https://67-72chevytrucks.com/vboard/...d.php?t=810350

06-04-2017, 02:50 PM	#12
Alex V. Registered User Join Date: Mar 2013 Location: Campbellsville, KY Posts: 888	Re: TH350 / NP205 - ATF or 80w-90? In researching this for the '74 K10 I had a few years ago (TH350/205 combo like yours) the best consensus I could come to is that the 205 is so impervious to any dismal matter like lubrication that if it's behind a stick shift, use gear oil, and if it's behind an automatic, use ATF. Either is very unlikely to be the reason a 205 fails. __________________ Alex V. ------ 1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs. 1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue.

06-05-2017, 07:05 AM	#13
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,863	Re: TH350 / NP205 - ATF or 80w-90? They only seem to fail when they lose too much "whatever". As long as they have adequate "whatever" they are fine. The synthetic 80w90, I use Royal Purple, has a much thinner consistency similar to ATF __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~



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