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Old 09-03-2018, 03:56 PM   #1
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Insane amongst the insane?

There is a 1974 Crown conversion in the classifieds that I'm interested in. Moreover, so's my wife, and that counts for a lot. I've sent the seller some questions, but I am completely deer-in-the-headloghts about taking on the care and feeding of a DD 2 stroke.



Is this the 6-71 that people say is a million mile engine, or the 6-71 that people say no mechanic alive will touch? I've rebuilt a couple of car engines in my time, and I've read up enough that I would be willing to take on an inframe on a wet-sleeve Cummins, but this is a horse of a different color.



There seems to be at least adequate aftermarket parts support, and lots of "good runner" cores to be had if that's how I need to get a usable <insert name of part here>. Would I be rolling $20K worth of dice if I bought this?


Also lots more questions about headroom, top speed, maintainability of transmission and other drivetrain, etc., but this is the engine room so i'll start with engine topics.
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Old 09-03-2018, 04:41 PM   #2
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Year: 2002
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Chassis: All American RE (A3RE)
Engine: Cummins ISC (8.3)
Rated Cap: 72
The two stroke Detroit is likely has the highest production numbers of any series of diesel engine.

The 6-71 was produced from 1938 until sometime in the '90s.

Parts are easy to find and there great interchangeably. A 12v71 uses 2 x 6-71 heads, the same pistons, connecting rods etc.

While the number of two stroke savvy mechanics is not what it was 20 years ago there are still enough that service at any Detroit shop and many independent shops is not a problem.

Million mile motor? Probably not without an inframe or two. My last 8v71 had over 900k miles on it when I sold it. It still ran great. It had 2 inframe rebuilds.

If I could pick my dream skoolie it would be a Crown with a two stroke Detroit, 10 speed Roadranger and a well done roof raise.

Rebuild cost is not cheap. I would take the bus to the local Detroit shop for inspection and if it gets a clean bill of health then GO FOR IT.
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Old 09-03-2018, 04:46 PM   #3
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Another note. The DD 2 strokes are wet sleeve motors. With the simplicity of these motors I would expect them to comparable or perhaps easier to rebuild than my Cummins 8.3.
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Old 09-03-2018, 04:53 PM   #4
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Originally Posted by PNW_Steve View Post
The two stroke Detroit is likely has the highest production numbers of any series of diesel engine.

The 6-71 was produced from 1938 until sometime in the '90s.

Parts are easy to find and there great interchangeably. A 12v71 uses 2 x 6-71 heads, the same pistons, connecting rods etc.

While the number of two stroke savvy mechanics is not what it was 20 years ago there are still enough that service at any Detroit shop and many independent shops is not a problem.

Million mile motor? Probably not without an inframe or two. My last 8v71 had over 900k miles on it when I sold it. It still ran great. It had 2 inframe rebuilds.

If I could pick my dream skoolie it would be a Crown with a two stroke Detroit, 10 speed Roadranger and a well done roof raise.

Rebuild cost is not cheap. I would take the bus to the local Detroit shop for inspection and if it gets a clean bill of health then GO FOR IT.
You and I have very similar tastes!
Although I'd rather have the 855 with 10 speed. Would have to have OD too!
A two stroke Detroit is stout and sure makes a cool sound!
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Old 09-03-2018, 05:04 PM   #5
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Join Date: May 2016
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You and I have very similar tastes!
Although I'd rather have the 855 with 10 speed. Would have to have OD too!
A two stroke Detroit is stout and sure makes a cool sound!
I do love the sound of a DD 2 stroke bangin'the governer @3450 rpm......

The 855 would be my second choice. No disrespect for the 855. I just have 2 strokes in my blood....


I built a few RD's. One ran a 10.2 quarter. Needed wheelie bars to do it
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Old 09-03-2018, 06:59 PM   #6
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Year: 1990
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Chassis: Crown Supercoach II (rear engine)
Engine: Detroit 6V92TAC, DDEC 2, Jake brake, Allison HT740
Rated Cap: 37,400 lbs GVWR
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Originally Posted by PNW_Steve View Post
I do love the sound of a DD 2 stroke bangin'the governer @3450 rpm......

The 855 would be my second choice. No disrespect for the 855. I just have 2 strokes in my blood....


I built a few RD's. One ran a 10.2 quarter. Needed wheelie bars to do it
If a Detroit, apart from maybe a 53, maybe, ever did almost 3500 RPM without becoming a big smoking pile of hot metal parts, it would be mortally wounded and good for nothing more. They may sound like they're doing 4000 RPM, but that's only when comparing their sound to a four-stroke's. Remember that there's twice as much of the Suck Squeeze Bang Blow as other engines.

And yes, I do also like the sound of big grunty Cummins 855s, especially with a three-stage Jake and a ten-speed Road Ranger. But it's always the howl of a 2-stroke that gets my juices flowing. I grew up in the days of Deltic locomotives hauling trains at 100 MPH between London and Edinburgh, and hearing the sound of 72 pistons inside 36 cylinders (and six crankshafts) at full welly is something you never forget, ever. I also have a soft spot for the venerable Commer TS3 two-strokes - 3 cylinders, 6 pistons and only one crankshaft (work that one out!). Even Foden had their own 2-stroke that was the smoooothest-sounding truck engine. So now having my own 2-stroke is just a natural progression.

I took my bus out for a short drive up Pacific Coast Highway this morning just to exercise it. There's nothing better than listening to a Detroit 2-stroke while driving past the beautiful Pacific ocean on a nice sunny day. Bliss!

John
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Old 09-03-2018, 07:05 PM   #7
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Quote:
Originally Posted by PNW_Steve View Post
Another note. The DD 2 strokes are wet sleeve motors. With the simplicity of these motors I would expect them to comparable or perhaps easier to rebuild than my Cummins 8.3.
Some are, some aren't. The 71s are dry-sleeve, while the 92s are wet-sleeve (at least, the upper few inches of the cylinders above their intake ports). All Detroits are sleeved, unlike some lesser throw-away engines. This makes a 92 a lot more sensitive to over-heating and use of incorrect coolant, but you can still bugger up a 71 if it gets too hot.

John
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Old 09-03-2018, 07:19 PM   #8
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Quote:
Originally Posted by Iceni John View Post
Some are, some aren't. The 71s are dry-sleeve, while the 92s are wet-sleeve (at least, the upper few inches of the cylinders above their intake ports). All Detroits are sleeved, unlike some lesser throw-away engines. This makes a 92 a lot more sensitive to over-heating and use of incorrect coolant, but you can still bugger up a 71 if it gets too hot.

John
Humbled.....

I was about to argue with you but decided to do my due diligence first.

I was wrong. John is right. 71 series are dry sleeve.

I apologize for the bad info.

All said.... I miss my 2 stroke Detroits.
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Old 09-03-2018, 07:24 PM   #9
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Join Date: Jun 2016
Location: Orange County, CA
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Year: 1990
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Chassis: Crown Supercoach II (rear engine)
Engine: Detroit 6V92TAC, DDEC 2, Jake brake, Allison HT740
Rated Cap: 37,400 lbs GVWR
Quote:
Originally Posted by dan-fox View Post
There is a 1974 Crown conversion in the classifieds that I'm interested in. Moreover, so's my wife, and that counts for a lot. I've sent the seller some questions, but I am completely deer-in-the-headloghts about taking on the care and feeding of a DD 2 stroke.



Is this the 6-71 that people say is a million mile engine, or the 6-71 that people say no mechanic alive will touch? I've rebuilt a couple of car engines in my time, and I've read up enough that I would be willing to take on an inframe on a wet-sleeve Cummins, but this is a horse of a different color.



There seems to be at least adequate aftermarket parts support, and lots of "good runner" cores to be had if that's how I need to get a usable <insert name of part here>. Would I be rolling $20K worth of dice if I bought this?


Also lots more questions about headroom, top speed, maintainability of transmission and other drivetrain, etc., but this is the engine room so i'll start with engine topics.
A 6-71 is not a million-mile engine, unless you don't count a few in-frames along the way! Their bottom end is very stout, but they'll still need in-frames every quarter- to half-million miles or so. They are however very reliable engines, maybe not as good in a heavy Crown tandem as a Cummins 855 because of their lower torque, but in a 35-foot Crown with a 5- or 10-speed they are more than quick enough. Even paired with a MT6xx transmission they will still give good performance. The turbo 6-71s make 270 HP, almost as much power as my engine (but a lot less torque).

Crown headroom is 77 inches, top speed with 12R22.5 tires and 4.1 axle is 67 MPH at 2100 RPM, and everything under the floor is pure Class-8 truck so no problem getting parts for brakes/steering/axles/etc. Transmissions are either 5- or 10-speed manual, or non-electronic Allison MT6xx with lockup in 3rd and 4th. Jakes are often fitted, very nice to have, and they do make a great sound!

John
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Old 09-03-2018, 07:52 PM   #10
Bus Nut
 
Join Date: Jun 2016
Location: Orange County, CA
Posts: 802
Year: 1990
Coachwork: integral
Chassis: Crown Supercoach II (rear engine)
Engine: Detroit 6V92TAC, DDEC 2, Jake brake, Allison HT740
Rated Cap: 37,400 lbs GVWR
If you want insane, my good friend Al has not just taken the muffler off his latest Crown like he did with his previous #13, but this time he paid someone to make it even louder! It's now the angriest-sounding Crown you'll ever hear, loud enough to get thumbs-ups from passing Harley-Davidson riders. Here he's driving his new #3 around his storage area, just to make some noise:

And to Compare & Contrast with his old #13 before he sold it:

Here we are at this year's Buses Gone Wild, making a nuisance of ourselves driving through town:
I drove his #3 for a short while, and it was FUN! It has a rare 5-speed Allison that keeps it in the power curve all the time, making it a quick bus.

Al, you're the best.

John
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