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Old 02-27-2021, 09:41 AM   #1
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Engine and tranny questions!

Okay, hereís the gist. Iím actually a writer and im doing this for research. Maybe one day Iíll get around to converting my own Skoolie ó my mom and I really want one.

Anyways, so the story centres around a bus. The first portion of the story is fixing it and converting it and other shtuff.

But Iím pretty much clueless. I know Cummins diesel engines (in dodges and Freightliners) and thatís pretty much it. I also know some mechanics, but again, on the above mentioned vehicles (and garbage trucks).

The bus in the story is a 1983 GMC (I think most buses are Bluebirds based on Google searches), but I can easily change stuff. Itís not set in stone. But I think most had gas engines (I was reading through this site but figured it would be easier for me to make my own post).

I was going to make it a diesel, manual with air brakes. Do they even have airbrakes? I can take that out too if need be.

Anyways, the bus has engine and tranny problems, so says the mechanic. Head gasket is leaking really bad, engine sounds tired. Tranny shifts horribly.

How easy of a fix is any of that? How pricey? Is it expensive to do an engine swap, if need be?

Pls help this desperate writer ! Thank you 🙏
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Old 02-27-2021, 09:50 AM   #2
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best advice.. buying a bus that has the drivetrain you want will always be more economical and practical esp if you arent super mechanical.. 83 GMC most likely has a 366 Gasoline engine and any number of transmissions..



diesel engines are heavier than gas so if you convert to diesel be ready to also need to beef the front suspension up with some aded leaves in the springs.. and of course rework the fuel tank piping to include return lines..



head gaskets on a gas 366 arent too bad to do assuming it hasnt rusted out the cyklinder walls or steamed the pistons.. then you should do a full rebuild of the motor..
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Old 02-27-2021, 01:38 PM   #3
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Originally Posted by Maddog View Post
Okay, here’s the gist. I’m actually a writer and im doing this for research. Maybe one day I’ll get around to converting my own Skoolie — my mom and I really want one.

Anyways, so the story centres around a bus. The first portion of the story is fixing it and converting it and other shtuff.

The bus in the story is a 1983 GMC (I think most buses are Bluebirds based on Google searches), but I can easily change stuff. It’s not set in stone. But I think most had gas engines (I was reading through this site but figured it would be easier for me to make my own post).

I was going to make it a diesel, manual with air brakes. Do they even have airbrakes? I can take that out too if need be.

Anyways, the bus has engine and tranny problems, so says the mechanic. Head gasket is leaking really bad, engine sounds tired. Tranny shifts horribly.

How easy of a fix is any of that? How pricey? Is it expensive to do an engine swap, if need be?

Pls help this desperate writer ! Thank you ��
First and foremost, hi from a fellow writer! (Debut work was "Lucky Number Thirteen" under the pen name Colyer Jameson).

Depends on the '83 GMC, and whether we are talking about a skoolie here, or a transit bus. There were also cutaways just like there are today. But since you're here, that says skoolie.

Another member mentioned the 366/427 tall-deck big-block V-8 that was common in these, but these engines are damn near bulletproof, so while it may be possible to kill one of these tall-deck engines, you'd have to do something extremely stupid, they were actually industrial engines adapted to highway use, something GM did often back then.

I was riding one that couldn't have been any newer than 1972 to school in 1994, with only a puff of oil smoke from valve seals when shifting. Our county still had at least 30 of these in 1994. I even remember the first school bus I ever rode being one of the old Dodges.

But I digress. Since you mention air brakes, diesel, tranny problems, engine problems, specifically head gasket, the perfect scenario for an '83 GMC to have such problems, would be an 8.2 Detroit with an AT545 trans, and such a bus could have air brakes. All were available in the '83 GMC 5500/6500/7500 series back then. The AT545 is a base Allison automatic, which were becoming a bit more common back then.

However, the AT545 Allison is an automatic, which would require some adjustment in your story, but most manual boxes in these were Spicer granny-low 4-speeds, and very tough. I don't remember ever hearing of one lunching a tranny. I suppose the clutch could go out, but for reasons that will soon be clear, you'd have to be extremely stupid to overspeed an 8.2 simply trying to move with a bad clutch. So the AT545 lends itself to this better, methinks, and with a lot more believable cause.

As for how the problem occurred, too low an axle gear ratio with too high a highway cruising speed. 8.2s are known for head and gasket issues, and the AT545 will overheat and burn up or puke if pushed too hard. Most school buses are geared low and not meant for interstate cruising, especially at today's speeds.

An 8.2's RPM range is fairly high for a diesel, in marine applications they are said to run best at about 2300-2500 rpm, and some are set to run as high as 3200. A marine application will typically use internals that allow a bit more RPM, so I would say a highway version (these were essentially marine/industrial engines adapted to highway use) would likely be in the danger zone at around 2400-2600, and most comfortable at around 2100, similar to the Navistar 7.3 / T444(E).

An 8.2 Detroit running 65 mph with a 6.13 rear gear (common in these) and an AT545 Allison would be pushing 3500-3700 rpm, depending on the tire height (most tires of that type these days are about 37 inches tall). And 3500-3700 rpm would certainly fry any 8.2, as well as an AT545, it cannot take the heat and would burn up fairly quickly, especially if the fluid hadn't been changed in awhile.

So that opens up a few foreshadowing options, as well as the mechanic knowing the driver doesn't know beans about what they're doing, or any passengers saying, "See? I told you you were pushing it too hard." That sort of thing...

Anyway, 8.2 Detroit Diesel, Allison AT545 trans, 5.29 / 6.13 rear axle gear ratio, 65 mph... It will overheat the 8.2 and AT545, fry the AT545 in short order, and blow the 8.2's head gaskets, which are all common and believable failures in that scenario.

The 8.2 is also a quirky beast -- its fuel pincher system makes its exhaust stink to high heaven, and it is also getting harder to find replacement engines or parts for them. Also, most diesel mechanics today do not understand them, as they are becoming increasingly rare, but vehicles with them can be bought for a song, owing to the exhaust smell.

As for fixing, replacement would be easier on both counts. The 8.2 isn't an easy beastie to find these days, but let's suppose the mechanic has a wrecked or abandoned truck or bus sitting around the back 40 that has an 8.2 and AT545 trans (these were common in GM medium duty chassis, which were the basis for many a school bus, box truck and medium-duty wrecker back then).

Of course, being that the 8.2 is pure mechanical, perhaps something better could be swapped in its place. Maybe even a different engine/trans combo swap, say a 7.3 / DT408 / DT466 / DT530 Navistar with an MT643 Allison? I suppose the 8.2's head gaskets could be replaced in a day or so, but there's no guarantee that 3200-3500 rpm won't do other internal damage, and there's still the overheated and fried transmission. Might also have the mechanic suggest swapping for the donor vehicle's rear axle for higher gearing (3.91-4.11-4.33), or a 2-speed rear axle (perhaps the bus already has one and the driver didn't shift it to high range, causing the failure at a much lower speed).

Coachwork options back in '83 included Thomas, Carpenter, Ward and Blue Bird, as far as I know. Superior may have still been in business in '83. Others may chime in on this.
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Old 02-27-2021, 01:48 PM   #4
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you are right on CW about the plot but my 86 ford thomas 8.2 fuel pincher 545 tranny and a 6.50 rear gear hits 49mph with a broom handle as cruise control.
it doesnt slow down on mild grades but of course i am postal sorry i meen coastal
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Old 02-27-2021, 01:59 PM   #5
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Originally Posted by Jolly Roger bus 223 View Post
you are right on CW about the plot but my 86 ford thomas 8.2 fuel pincher 545 tranny and a 6.50 rear gear hits 49mph with a broom handle as cruise control.
it doesnt slow down on mild grades but of course i am postal sorry i meen coastal
Marine 8.2s were turbocharged, perhaps this one could have been swapped for one in the past. And there are MPH governors, and there are RPM governors. Also, you have the 6.50 rear gear, which makes a fair bit of difference. With 37-inch tires and the AT545's 1:1 top gear, that's about 3000 rpm, a likely setting for an RPM governor. But 3500-3700 would fry even a marine 8.2 after awhile. Perhaps in the story, someone did something dumb in the process of conversion and disabled the governor.
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Old 02-27-2021, 02:00 PM   #6
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and as far as cost?
in this day and age if you have the 8.2?
the harder part will be finding a mechanic that knows how to work on it and still has the specialty tools to do so.
aint many left unless you are stranded around an older agricultural area where the old timers were more likely to teach and pass there knowledge along.
its usually cheaper quicker and easier to swap to a different powertrane
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Old 02-27-2021, 02:02 PM   #7
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83 almost all were still running 9r20 dayton tubed spoked rims.
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Old 02-27-2021, 02:18 PM   #8
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83 almost all were still running 9r20 dayton tubed spoked rims.
Yes, but 20/22 is the rim size, not the tire height. Also, I attended school from 1981-1994, and none of the buses in my county had Daytons as far as I know. Furthermore, none of them were newer than about 1979 for the Fords, and 1972 for GMs. We got a few newer ones after around 1987, but that was it. Every single last one, even the ones that were pre-1983, had the five-handhold wheels more common on later models.

Perhaps the non-spoke wheels just weren't that common back then, or ours were ordered that way or upgraded for safety. Then again, it may just vary depending on policy in the locality in question. Only thing I saw spoked Daytons on back then growing up were semi tractors. Never saw them on a school bus until I joined this site around 2006.

However, to account for this, there are more modern non-split upgrades for these, and what would be a typical tire height with or without such upgrades?
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Old 02-27-2021, 05:01 PM   #9
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Great stuff, posted below...these guys are thorough! You might also see if you can find a thread from a month or two ago, by a guy doing a podcast. There were lots of replies there, which might be helpful to you and worth skimming through. Unfortunately, I can't find it or I'd post a link for ya, Maddog.
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Old 02-28-2021, 01:46 AM   #10
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Mechanically, I'm a neophyte compared to the first three responders. But, that puts me in a position more aligned with the OP.

In my experience:
1) Really old vehicles are much harder to find parts for.
2) They require a lot of knowledge and time to understand how the systems work, and as one poster said, finding someone to work on some of the old iron can be tough.
3) Picking your battles in converting a bus is important, especially if you're not skilled in that area of battle.

Could I remove my tranny? Yes. Could I get it back in....ahhh???

I removed my tranny modulator today and was not prepared at all for transmission fluid to come pouring out. I thought since it was so much higher than the pan there would be no issue. But, I knew enough to push it back in and it stopped leaking.

I also drilled into my exhaust manifold and installed my EGT gauge today, but with my friend who is very mechanically wise and had installed the same gauge on his truck a month ago. It would have been pretty risky for me to attempted this alone.

I replaced my air filter a week ago. Today, I opened the engine bay and my friend said, hey, your air filter fell out. Upon closer inspection, the clamp has to line up with an ~1/2" tab to seat properly. Who knew? Obviously not me. Thankfully, there's a platform below the air cleaner. I've changed all sorts of air cleaners, but not on a bus.

I've worked on cars and motorcycles, built sheds, played with house electricity, done plumbing, etc. I've studied diesel technology, drive a diesel for work and my friend is a diesel gearhead. But my bus, it's pushed my limits.

Nothing is square, nothing is level, almost every plan has been dynamic.

Based on the OPs admission of lacking knowledge and skill set in this arena, it seems like a newer (like 20 years old) bus in solid running condition would better fit his desire to convert a bus versus rebuild a bus.

I apologize if this sounds a bit heavy and discouraging. I want the OP to be successful and the above has been my journey. I hope it helps him with his decision.

Best of luck.
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