Fan clutch install 72 GMC C15 350

[huntznsam]

In the hopes of just a tad more MPG, I installed a fan clutch to replace the fixed 7 blade fan on our 72 GMC C15 350. Yowza! MPG takes a back seat to quiet. At idle it is world of difference; on the highway I’m no longer hesitant because of the old jet-engine roar at 65mph. It sounded like I was turning 6000 RPM. I was blaming the 3.73 gears. Heck no – it was the fan!

Now I hear a rattle that needs attention, and the exhaust leak that was trivial is now more prominent.

I used the following:
Fan clutch=AC Delco 15-80245 (Amazon)
Fan blade=Dynacorn M1017 7 Blade (Ebay)
Water pump studs=Dorman 675-001 (Summit – correct 5/16-24 fine thread on the short end)
I had the balance of the hardware in my bins.

The fan sits just right, about one inch out of the shroud. I haven’t driven long enough to hear it kick in, but I don’t expect trouble.

Thanks to the experts here for all the good info – I got the part numbers from you. I hope this is useful to the next guy.

I’m a serious tracker of MPG, more for vehicle condition/efficiency than cost. I’ll see if it really makes a difference.

[wpavlis]

Thanks for posting this. I have been trying to find a source for those studs for a while. That really helped. Glad the clutch is working out for you.

[Steeveedee]

You should see an improvement. Please be sure to let us know. I don't think that it will be a lot, though.

[gmc684x4]

Do you have a pic of your setup in the C10 ?

[54blackhornet]

Where they not fitted originally ? My 71 GMC 1500 350 has one installed ?

[huntznsam]

Blackhornet,
Better experts than me, but I reckon fan clutch was an option or part of an option.

Steveedee - I will. I track tank to tank as well as 10 trailing fill up average.

[72c20customcamper]

Originally Posted by huntznsam

In the hopes of just a tad more MPG, I installed a fan clutch to replace the fixed 7 blade fan on our 72 GMC C15 350. Yowza! MPG takes a back seat to quiet. At idle it is world of difference; on the highway I’m no longer hesitant because of the old jet-engine roar at 65mph. It sounded like I was turning 6000 RPM. I was blaming the 3.73 gears. Heck no – it was the fan!.

I'm thinking a lot of people mistake the fan for engine revs. So they want to change the ratios or get an overdrive trans. My truck turbo 350 with 4:11 gears will comfortably run like this and I can still hear the radio ....

[Steeveedee]

Originally Posted by 54blackhornet

Where they not fitted originally ? My 71 GMC 1500 350 has one installed ?

Generally only came with AC, but if you knew to ask, there was a heavy duty cooling package.

[typerr]

I just ordered parts to do the same thing. Buying a 50 year old truck sure is an adventure. My C-10 currently has a fixed 4 blade fan that roars. She runs very cool but I suspect the previous owner may have removed the thermostat. He also installed an aluminum 3 pass radiator but the fitment is very poor. There is a large gap between the radiator and the shroud and the radiator is sitting crooked in the mounts. I have to drive out to Brothers Truck this weekend to pickup some back ordered parts so I'll get some replacement rubbers for the mounts and see if I can get it all straightened out.

[zicc1835]

Originally Posted by huntznsam

In the hopes of just a tad more MPG, I installed a fan clutch to replace the fixed 7 blade fan on our 72 GMC C15 350. Yowza! MPG takes a back seat to quiet. At idle it is world of difference; on the highway I’m no longer hesitant because of the old jet-engine roar at 65mph. It sounded like I was turning 6000 RPM. I was blaming the 3.73 gears. Heck no – it was the fan!

Now I hear a rattle that needs attention, and the exhaust leak that was trivial is now more prominent.

I used the following:
Fan clutch=AC Delco 15-80245 (Amazon)
Fan blade=Dynacorn M1017 7 Blade (Ebay)
Water pump studs=Dorman 675-001 (Summit – correct 5/16-24 fine thread on the short end)
I had the balance of the hardware in my bins

The fan sits just right, about one inch out of the shroud. I haven’t driven long enough to hear it kick in, but I don’t expect trouble.

Thanks to the experts here for all the good info – I got the part numbers from you. I hope this is useful to the next guy.

I’m a serious tracker of MPG, more for vehicle condition/efficiency than cost. I’ll see if it really makes a difference.

I’m that next guy .....Thanks very much for posting I had everything laying on the shelves minus the studs

[typerr]

Originally Posted by wpavlis

Thanks for posting this. I have been trying to find a source for those studs for a while. That really helped. Glad the clutch is working out for you.

Glad I read this thread again. Did not realize I needed the studs for my fan install. On the way from Summit! Thanks!

[Stocker]

..... And I have not seen studs used in that application. My K20's fan clutch is attached with 4 bolts, which is how it left the factory.

[Minkota]

Originally Posted by 72c20customcamper

I'm thinking a lot of people mistake the fan for engine revs. So they want to change the ratios or get an overdrive trans. My truck turbo 350 with 4:11 gears will comfortably run like this and I can still hear the radio ....

just out of curiosity, what is your RPM at around 60, what size tires? Stock Exhaust?

Mine has 3.73 and turns about 2900 at 60mph, with the Flowmasters it is very difficult to hear anything.

[Markuli]

Before my 283 blew up, I swapped the existing direct drive fan for a seven- blade clutch fan. What a treat it was to not sound like a jet dryer truck past 3k rpm! Thinking about it now, my new- found inclination to go a little higher in the revs may have been the silver bullet for that tired old engine...

[slikside]

Sub'd!

Thanks for posting this up. The testimonials on noise reduction alone make this upgrade very appealing to me.

Won't be right away, but this is definitely on the list!

[MARKDTN]

I have not looked lately, but used to be those studs with the different threads were available in a kit of 4 in the HELP section of the parts stores.

[huntznsam]

Another post about fan clutches prompted this replay. 2-1/2 months, 1250 miles, 7 fill ups. Old truck mods don’t occur in vacuum, however. I did play with the timing curve, and replaced a bad plug with a full new set. I installed a cheap manual choke conversion. The divorced choke was perfect all winter, but for the life of me I couldn’t get it to perform well in summer. After 2 weeks, I trashed it, bought a good cable, made solid brackets & linkage and it works perfectly.

So anyway, my tank-to-tank mileage has increased consistently, up 10%. Ten trailing is up 5%. My truck does like summer, but it has never shown such a regular increase.

How much is the fan clutch responsible for? Some, but exactly I can’t say. It is irrelevant. The free and easy acceleration and cruising more than compensate. At first, I was really enjoying the throttle, but as mileage increased every fill from 12.5mpg to 13.8 mpg, I’m sure some psychological factor came into play. My commute is consistent, averaging 45mph, anticipating lights for fewest stops possible. The truck is so much more enjoyable, I’d highly recommend the fan clutch conversion.

Stocker – the studs make aligning WP, pulley, and clutch a breeze. We sold them all the time back in the ‘real’ auto parts store. Many pumps even included them. Bolts are a pain in the dupa with limited clearance and a pump & pulley that want to rotate freely.

Minkota – TH350, 3.73 rear, 235/75R/15 Michelin Defender LTX, no tach but by the calculator about 2700 at 60 mph. Without that fan noise, we enjoy the revs coming so easy.

[Ironangel]

Good info! Thank you for sharing!

[Stocker]

Originally Posted by huntznsam

Stocker – the studs make aligning WP, pulley, and clutch a breeze. We sold them all the time back in the ‘real’ auto parts store. Many pumps even included them. Bolts are a pain in the dupa with limited clearance and a pump & pulley that want to rotate freely.

Agreed, having studs there certainly would make life easier! Maybe I'll remember to get some next time I have to take it apart. Thanks for the tip.

[Grumpy old man]

O'rielly's has the studs for .79 each

[Ol Blue K20]

I've been wanting to do this. Now I know where to find th fan. Thanks for posting part# and the brand names.

[GASoline71]

I did this swap in my 1972 GMC C1500 as well. My truck came stock with a 7 blade 1:1 fixed fan. I swapped it out for a Hayden fan clutch and a Derale 6 blade clutch fan. My rig runs more in the heat zone it should and no more belt squeal when I wind up the RPM's.

Best 70 bucks I've spent on my truck yet.

Gary

[Grumpy old man]

Hayden 2705 with 6 blade fan .

https://www.summitracing.com/parts/hda-2705