1977 gmc s6000

[Rackettytam]

Hello all! Long time lurker and dreamer, but my wife and I have finally taken the plunge and purchased a school bus. It’s a ‘77 GMC s6000, with an automatic transmission and Chevy 327 (industrial?) v8. We have affectionately nick-named it The Crusty Musty Bus, at least until we can make it a little prettier. Engine runs wonderfully, but there are definitely some drivetrain issues to be figured out (for instance a loud howling when underway at about 45 mph, but only when I touch the gas pedal).

My question for you is: where do I find resources for this particular bus? I’ve scoured Google and this website for any info or resources but have come up pretty much empty handed, especially when it comes to the engine/trans combo I have. Does anyone have any hidden watering holes of old-heads and info, or am I doomed to getting my degree in bus-ology through the school of hard knocks?

Thanks in advance!

[Jolly Roger bus 223]

the drivetrain will be comparable to the medium duty trucks of that time period.
the roaring noise leads me to the rear end.
welcome
and the school of hard knocks is how we remember things.

[DeMac]

Is the 327 absolute, or might it be a 427? Upload a photo, if you can.

In the 70s, I believe GMC offered their school bus chassis with Chevy engines, including the 250 inline-6 (later 292 inline-6), 366 V8, and 427 V8. (Industrial tall blocks) Both Chevy & GMC school buses used Allison AT475 3-speeds automatic transmission. Some with single or two-speed rear axles...

Just suggesting likely possibilities.

[Rackettytam]

I only have this photo of the engine bay currently, which isn’t too helpful. Will try and take a better one once I’m back where the bus is being stored.
I know it is a v8
I also have this photo of the data tag, which might help:

[cadillackid]

Originally Posted by DeMac

Is the 327 absolute, or might it be a 427? Upload a photo, if you can.

In the 70s, I believe GMC offered their school bus chassis with Chevy engines, including the 250 inline-6 (later 292 inline-6), 366 V8, and 427 V8. (Industrial tall blocks) Both Chevy & GMC school buses used Allison AT475 3-speeds automatic transmission. Some with single or two-speed rear axles...

Just suggesting likely possibilities.

by 77 many of the school busses had AT540 4 speed automatics in them.. its most likely likely a C50 / C60 GMC truck.. and theres still lots of parts and such for those... its possible someone swapped in a 327... I dont remember them in school busses but not saying it didnt happen..a lot of things were built..lots of combos

[Jolly Roger bus 223]

find the engine serial block numbers.
usually on the drivers side.
love an old chevy so please share. but i have old fords as well.
the block serial numbers will help us help you.
my 77 truck is stamped on the frame rail and not block casting.
block casting is exactly that? the vin is on the frame.

[Rackettytam]

I am currently out of town but will try and get block serials when I get back on Monday.

the roaring noise leads me to the rear end

When you say rear end, what specifically did you have in mind? After talking to some mechanic buddies I was questioning the driveshaft support bearings, but should I be poking my nose around the rear diff?

It doesn't seem like wheel bearings, since the noise stops when I'm not touching the gas pedal.

[s2mikon]

By 1977 the 327 was out of production. It is more than likely a 366 or 427 with a at 545.

[s2mikon]

After looking at the picture closer it looks like a 350. Cant tell for sure. Most of the 366 and 427 industrial engines had a velocity governor under the carburetor that has a q jet. The op wont have any problems finding parts.

[DeMac]

As Jolly Roger mentioned, check the Big Block Chevy casting numbers...

BBC Engine Block Casting Numbers



The third digit in the VIN is the engine code (GM V8 Gasoline) 🤷

[Kentucky Dreaming]

I can't tell anything from the photo through all the hoses, but the 327 had been gone for almost a decade by 1977. Just going by memory but I think the last year of production was 1969.

In that photo I can't even tell if it's a small block or big block.

[Rackettytam]

Alright, got home and got some more 'beauty' shots of the engine bay, including block casting number. (Yes, I know, an air filter is coming soon). Appears to be '3970010', what would that indicate? I don't see it on the list linked previously.

Also, when I was looking at the driveshaft for any clear noise culprits, I found this section (pictured) Just forward of the 3rd driveshaft segment, is this cause for some serious concern? Looks like it may be coming apart?

Thank you all, you're all so helpful!

[s2mikon]

Looks like a 350 to me. They sold those in the c-60 Chevy trucks back then. It was a budget option to the 366 / 427 gasoline engines and an upgrade from the 292 6 cylinder engine. I have driven the c-50 and c-60 trucks with the 292 and it was slower than the speed of smell. Anyway you can get parts anywhere and at good prices.

[Booyah45828]

Looks like a 350 to me too.

I'm surprised by the 4bbl carb, as all the ones I've seen had real small 2bbl carbs on them. IDK if you could swap the manifold off a car/pickup 350 onto one of the truck engines, something in my gut tells me they were different.

Your 3rd pic is the slip yoke in the drive shaft. When the bus goes up and down, the driveshaft will have to shorten and lengthen to accommodate the movement, and the slip yoke does that. So long as there is no up-down or twist play in it, it's likely fine.

The howl coming and going with throttle input says rear end to me as well. Some of them are known to be noisier then others. I'd have to hear it to condemn it.

[Kentucky Dreaming]

Valve covers and exhaust manifolds say it's a small block. In 1977 that would almost certainly be a 350

[Rackettytam]

Originally Posted by Booyah45828

Looks like a 350 to me

Seems like this is the consensus, good to know! Debating whether or not to try and rebuild the carb, or just buy something new that will bolt on no problem. I know the current carb was at one point governed, but the governor has been 'deleted' according to the previous owner (unsure what in actuality that means, there is a pipe sticking out the back base of the carb that has been flattened shut, he pointed to it when mentioning the governor)

Originally Posted by Booyah45828

Your 3rd pic is the slip yoke in the drive shaft. When the bus goes up and down, the driveshaft will have to shorten and lengthen to accommodate the movement, and the slip yoke does that. So long as there is no up-down or twist play in it, it's likely fine.

Phew, that's good. I'm super green when it comes to heavier-duty truck / bus systems, most all my experience is airplanes and fiddling around with personal cars, so the carburetor was a joy to behold but everything aft of the engine still requires some learning on my part.

I'll have to do some more research on the rear end and see if I can figure out if I'll need to replace some things.

Before that, though, me and the missus are going to be gutting the bus and raising the roof! Wish us luck

[s2mikon]

Before you go to the trouble of raising the roof check the insurance threads. They are becoming a PITA now.

[Ronnie]

On the rearend, a good starting point is to check the oil in it, there is a plug about a third of the way up on the back cover. Oil should be up to that. Also grab the driveshaft where it goes into the rearend and see if it has any play up and down. You do need to have it in nuatral and if equipped with a driveshaft brake release it. do Block the wheels first though.

[cadillackid]

im in the 350 camp.. looks like a standerd rochester Q-jet carb on there... most often the bus models had a governer on the carb, that one doesnt look like it has it..


casting numbers are hard to use as chevy used them on multiple engines.. interestingly is that casting number could be on a 327 but it was also on a 350 as well.. its entirely possible someone swapped the engine at ;east once in a 46 year old bus.. although a 327 wouldnve likely gone in a hotrod and not in a bus.. my bet is the original 350 got swapped out and in went this one from ?? pickup truck...



id pull the carb, rebuild it.. cap rotor, plugs, and wires.. and time it up.. and install a tach if it doesnt have one although with an automatic you arent likely to over-rev it.. if its geared really low in the rear you could over-rev thew transmission.. you dont want to go much above 4000-4300 with an auto even in top gear... the allisons of the era didnt like spinning.. assuming it has an allison

[Jolly Roger bus 223]

as stated that is the block casting number for a 350 small block and could have been 2-bolt or 4-bolt main.
yep a quadrajunk carb.
a holly 650 would be nice on there.
the driveshaft section looks fine wouldnt hurt to change the carrier bearing and check u joints but i can almost guarantee the noise is in the rear end.