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Old 02-24-2021, 09:41 AM   #1
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Thoughts on a Cat 3126 engine w/ a 6sp manual transmission

Im looking at a 2000 Frightliner/Thomas bus with a Cat 3126 turbo With a manual transmission. 168k miles. We will be using it in some hilly areas as well as 10 to 12 hour drive times. I don't know the rear axle ratio but he said it will do 65mph and has cruise control. Looking at some thoughts on this combo?

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Old 02-24-2021, 10:53 AM   #2
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Im looking at a 2000 Frightliner/Thomas bus with a Cat 3126 turbo With a manual transmission. 168k miles. We will be using it in some hilly areas as well as 10 to 12 hour drive times. I don't know the rear axle ratio but he said it will do 65mph and has cruise control. Looking at some thoughts on this combo?
Sounds AWESOME.
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Old 02-24-2021, 11:00 AM   #3
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Grab it asap, those are very hard to find with a manual transmission of any kind. Rear axle ratio can be changed later on if necessary

Hopefully it has air brakes!
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Old 02-24-2021, 11:12 AM   #4
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You don't have any location information in your profile so the usual warnings about rust in the rust belt. You didn't mention the length of the bus which would have an impact on weight.

I wouldn't be afraid of the 3126 in a bus. Not sure how realistic it is
regarding cruising at 65 on ANYTHING but dead level roads if you have one in a 11+/- window medium duty chassis.

I was looking at a short bus last year (essentially a 6 window) on a Freightliner medium duty chassis which had a 3126 with a 5 speed (Eaton-Fuller??, can't remember). I can't remember the model year but it was early 2000s. I did a lot of research into the 3126 and everything I saw suggested it was a good engine with the only questionable component being the HEUI (hydraulic electronic unit injection). I'll throw in here that the 3126 wasn't the only engine with HEUI, for example the 7.3 I currently have in my bus is also HEUI as well as others.

I happen to have a heavy equipment mechanic of 20 some years as a neighbor that I discussed it with and he didn't have anything negative to say about the engine either. I also found a YouTube channel by the Adept Ape who is a CAT mechanic and spent a lot of time watching his videos related to that engine. You could check that out.

The 3126 is isn't one of the bigger engines you'll find in a medium duty bus. The one in the bus I was looking at was rated at 190hp. In the short bus it had plenty of stuff but in, say, an 11 window I don't know.

In your position, I'd also look into the 6th gear ratio of that tranny, is it 1:1? I suspect it's an overdrive if that bus will cruise at 65, that or it's got a low rear end ratio. Either way I suspect you'd be doing a lot of shifting between 5th and 6th even on 'flat' roads.
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Old 02-24-2021, 11:14 AM   #5
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My bus can do 65 mph and has cruise control, but it has an awful 6.17 rear ratio. 65 is full out - it does 50 mph at about 1900 RPM.
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Old 02-24-2021, 11:21 AM   #6
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My bus can do 65 mph and has cruise control, but it has an awful 6.17 rear ratio. 65 is full out - it does 50 mph at about 1900 RPM.
6.17! Have you thought about swapping that out before you hit the road?
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Old 02-24-2021, 11:26 AM   #7
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Sounds like a good package, but just because the seller says it will do 65 doesn't mean doing 65 sustained is a good idea. I transported a member's recent purchase, seller said it would do 65... At 2400 rpm. Not exactly a great idea for what turned out to be a 2800-mile trip. I personally wouldn't push any diesel much past 2000-2200, and that's accounting for these smaller-displacement engines (I used to drive semis) being able to get away with a little more RPM.
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Old 02-24-2021, 11:37 AM   #8
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My bus cruises at 65 with a 3126. 1750 rpms @ 65 thanks to the 3.42 rear.

Unless its rusty GO GET IT!
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Old 02-24-2021, 12:59 PM   #9
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Thanks for the input, It a Mid-West bus with a little rust. Its an 11 window bus.
Has any one had to replace a clutch and how hard was it. I have worked on cars, truck and motorcycles for 40 years so knowledge is not the issue. Its how much of a pain in the ass is it. Can you just move the transmission back or does it need to come all the way out.
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Old 02-24-2021, 01:04 PM   #10
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How do you know the engine horse power?
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Old 02-24-2021, 01:05 PM   #11
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Not difficult to do at all.

Use a trans jack to support it and slide it backwards. No need to drop it per say, but by the time you have it slid back. it's just as easy to drop it if you wanted to.
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Old 02-24-2021, 01:06 PM   #12
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Engine rating will be on a tag on the valve cover or front gear cover.

Or you can connect to the diagnostic port and read the current hp level off the ecm.
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Old 02-24-2021, 01:07 PM   #13
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Not difficult to do at all.

Use a trans jack to support it and slide it backwards. No need to drop it per say, but by the time you have it slid back. it's just as easy to drop it if you wanted to.
So no different than a car, just heavier.
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Old 02-24-2021, 01:58 PM   #14
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My bus cruises at 65 with a 3126. 1750 rpms @ 65 thanks to the 3.42 rear.

Unless its rusty GO GET IT!
I think this might be interesting to the OP and maybe others. I found it while I was researching for the short bus I mentioned in my post above. I think it was designed to calculate ratios for race cars not heavy duty vehicles but I needed something to do some ball park calculations when I was considering buying the short bus and replacing the carrier with a lower ratio. I didn't mention it here at the time because I didn't want anyone chewing me a new one, now . . . I don't care.

EastCoast, you can see that my target ratio turned out to be 3.73 which, by this calculator, would have been about 60 mph at 2000 rpm. I was considering 3.42 as well but I was concerned that 1700 to 1800 rpm, which is what I recall it being at about 60 mph would have been too low for cruising. You can attribute that conclusion to the fact that I have no experience running diesel buses.
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Old 02-24-2021, 03:06 PM   #15
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Mine loves the high gearing. Its always in the torque curve. Rarely does it ever even see 2000 or above.
3.73 would be a good ratio on larger buses. Mine's only 5 windows.
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Old 02-24-2021, 04:37 PM   #16
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6.17! Have you thought about swapping that out before you hit the road?
I only think about it every time I drive. It's not actually a top priority since I'm intending this bus as more of a self-propelled tiny house than an active RV. I wouldn't mind doing it and I would probably jump on the opportunity if it was less than $2000, but around where I live nobody would do it that cheap.
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Old 02-24-2021, 04:59 PM   #17
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its another learning step.
changing the gears are the easy part.
some auto stores sell the clay for meshing the gears which you have to learn how to read and alot of places just use a feeler guage.
when the guage doesnt have enough thickness then do the math and flip some paddles out.
you can do it
research it and you can do it
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Old 02-24-2021, 05:05 PM   #18
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I think this might be interesting to the OP and maybe others. I found it while I was researching for the short bus I mentioned in my post above. I think it was designed to calculate ratios for race cars not heavy duty vehicles but I needed something to do some ball park calculations when I was considering buying the short bus and replacing the carrier with a lower ratio. I didn't mention it here at the time because I didn't want anyone chewing me a new one, now . . . I don't care.

EastCoast, you can see that my target ratio turned out to be 3.73 which, by this calculator, would have been about 60 mph at 2000 rpm. I was considering 3.42 as well but I was concerned that 1700 to 1800 rpm, which is what I recall it being at about 60 mph would have been too low for cruising. You can attribute that conclusion to the fact that I have no experience running diesel buses.

Very nice. BUT, You left out the only really critical information for those of us who'd like to try it for ourselves...... What's the site URL. This kind of resource never goes out of style, I can use it for my own and others who are looking for answers. I'll try to find it too but I like the look and layout of this site. The others that I've found are always lacking and/or just look bad. Thanks in advance.
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Old 03-03-2021, 05:08 PM   #19
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A cautionary note. The 3126 is a bit more to work on, the parts are expensive and I asked 2 CAT mechanics if the 3126 was a good idea for a school bus conversion. They looked at each other, chuckled and said pass.
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Old 03-03-2021, 05:20 PM   #20
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A cautionary note. The 3126 is a bit more to work on, the parts are expensive and I asked 2 CAT mechanics if the 3126 was a good idea for a school bus conversion. They looked at each other, chuckled and said pass.
Name a part and I'll look up that same part for Navistar or Cat. Not a lick of difference on parts.
I've owned both and the Cat parts myth kept me from lots of good buses... Wish I hadn't listened to all the internet "cat parts price" hype.
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