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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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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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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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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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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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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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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
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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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Old 09-04-2018, 07:34 PM   #11
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Quote:
Originally Posted by Iceni John View Post
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
This is described as having a 5 speed manual. So maybe a RR? If so, can the brownie-type mechanism be sourced in a junkyard and retrofitted?



No idea if this is turbo'ed. Described as a '74; can you make any assumptions from that? Did your speed vs rpm specs come from a turbo or an NA model?


This may all come to naught; the seller has not so far responded to message in thread, PM, or email. Maybe sold by seller who has moved on.
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Old 09-04-2018, 07:50 PM   #12
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Not scared of an inframe rebuild; glad it's possible! WTF is a "dry sleeve" engine?



How's the access in the case the rebuild is needed? He says at one place that it has 300K and in another 350K so I have to plan for it. Maybe on-condition; maybe pre-need (fishing for a word here).
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Old 09-04-2018, 08:43 PM   #13
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Quote:
Originally Posted by dan-fox View Post
This is described as having a 5 speed manual. So maybe a RR? If so, can the brownie-type mechanism be sourced in a junkyard and retrofitted?



No idea if this is turbo'ed. Described as a '74; can you make any assumptions from that? Did your speed vs rpm specs come from a turbo or an NA model?


This may all come to naught; the seller has not so far responded to message in thread, PM, or email. Maybe sold by seller who has moved on.
Spicer 5 speed most likely. Would be my LAST choice in a Crown.

They're strong enough. I just don't like the wide ratios and lack of OD.
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Old 09-04-2018, 11:03 PM   #14
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Year: 1989
Coachwork: Crown Coach
Chassis: 40ft 3-axle 10spd O/D, Factory A/C
Engine: 300hp Cummins 855
Rated Cap: 91
Crown 5-spd manual transmissions were most commonly Fuller 905A/B for anything not specifically requested differently by the buyer. Common as grass and in most all Crowns I've driven. Spicers were not common at all. It's actually pretty simple to replace a 5 spd with a 10 spd RR, either with, or without OD. An added support for the weight and extra length and you're done. Fuller has a TO-905 (Overdrive) available, but I've never seen one in a Crown. Just the 10 speeds, straight 1:1 and Overdrive.

I believe the Detroit engines started around the mid-1960's or thereabouts, using a newly designed block with "water below the ports" for extra cylinder cooling, especially for turbocharged builds. These are noticeable by the square air box cover as opposed to the early style rounded ones. This was additional water cooling jackets for cooling more of the cylinder area. The water jackets are cast into the block and don't allow any contact for the water with the cylinder liners themselves. This means the cylinder liners slide into the block with no gaskets needed, or any interface with the coolant at all. Hence the term Dry Sleeves. Also more reliability and less potential for coolant leaks. All coolant stays within the blocks' cast-in passages.

Detroit 2-stroke engines are a simple and easy to re-build engine. They were preferred by fleet operators due to low parts costs, and reliable operations while being abused by drivers. As long as drivers didn't lug them, or get them hot, they could drive them like they were mad at it and the engine loved it. Today it's a little harder to find mechanics only because there aren't as many on the road, but it's not impossible and I'm pretty sure you can't put enough miles on a Crown conversion with a solid performing engine to compare with what they did in daily revenue service clocking 80k miles a year or more. Even if this Crown has 300k miles on it, there may be no way to know when any work was done on the engine. Only a good DD mechanic can check it out and say if anything is trying to go bad. It may easily last another 100k miles from now with nothing going wrong.....or it could take a dump next time it starts. That's just the way it is with these things. An experienced ear can tell a lot about the overall condition. Driving it and noting smoke, throttle response, and overall sound while underway can say a lot. I think I noticed on the ID plate it was 210 HP, that makes it a naturally aspirated, non-turbo for sure, unless it's been re-built and upgraded along the way, which I seriously doubt.

Engine access is very good since it lays on it's side and you can open left and right service panels and get to the top and bottom of the engine in place. An in-frame is very simple and can be done on a concrete slab without any fancy equipment.
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Old 09-05-2018, 01:32 PM   #15
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210 hp is correct, so no turbo. What can I expect as to acceleration, top speed, and mountain performance? BTW, thanks for chiming in; I was hoping to hear from you on this.



What are the odds of being able to swap 22.5's onto this bus? I looked up 11.00R20's and people are getting quite a bit for them.



Doable thing to backfit a turbo? Easier to get an engine from a donor? Easier thing to just buy the bus I want in the first place?



TIA
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Old 09-09-2018, 04:09 PM   #16
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Chassis: 40ft 3-axle 10spd O/D, Factory A/C
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Rated Cap: 91
Adding a turbocharger to a non-turbo factory build is difficult due to all the parts that need to be replaced. All pistons, Rods, blower speed/gears, Camshaft, maybe exhaust manifold unless it already has the turbo one, which most did around that time. Many other things great and small which requires an almost complete tear down and rebuild, possibly even having to be removed from the vehicle to reach everything easier than trying it while hanging from the frame. Nutshell is it's easier and cheaper to find another engine already set up with a turbo ready to go. And good luck with that. There aren't many available since they get destroyed along with the buses, fork lift tangs through the blocks to completely destroy them, Sinful and Despicable.

I drive a Crown with a 210hp engine setup with a 5-spd and it does just fine in all kinds of driving. I've had it in the mountains and on the highways, and after re-gearing the rear-end it'll do an honest 72 mph sitting on the governor....all day long. Most school bus gearing tops out at about 60mph at governed engine speed. You can easily have larger injectors installed and kick the power up to around 238 HP which will give a little more torque and pulling power, but remember the basic 6-71 just doesn't have all that much pure raw torque as most other 4-stroke engines. It's entirely adequate and you'll never notice any real performance loss, unless you have experience with other similar Crowns having more horsepower. It's all rather incremental and noticeable only on grades and when under heavy loads, which as a conversion vehicle not likely to ever see. Passengers and baggage had a whole lot of weight that the normal conversions probably can't approach. The removed seats alone account for about 2k pounds.

I suppose if you aren't concerned about doing the conversion yourself the way YOU want it done, you should probably just look for a solid vehicle to build upon and get it the way you want it, and it should be lots less than any already done conversion. You never know about all the little things that could be wrong in someones conversion. You should have the privilege of doing it yourself and making all your own mistakes, at least then you'll know what needs to be fixed, and how to do it.

The tires, if they are still 11x20s should probably be changed, BUT that requires all new wheels as well. Not a small thing. All wheel and tire items are not cheap. Six tires and wheels at about $4-500 per set, times six, 7 with a spare, you can do the math. Most are running 12RX22's with 22.5's an expensive extra not really worth the cost. Large Fleet Greyhound type operators use the 22.5s but they also lease the tires and get better value that way. I run 12Rx22s on my Crown and they're just fine. More available when needed, which is a key variable when on the road in the middle of the Great American Outdoors.

With the supply of good quality running condition Crowns drying up fast, you should be prepared to go for around $15K for a Crown worth having, and one that won't suddenly have something expense break, right after you take delivery. Even if you pay that much for a good runner, you'll be ahead of the game because once something is fixed properly on a Crown and kept in shape with regular maintenance you shouldn't have many worries about getting stranded on the road somewhere. Crowns have a well deserved reputation of always getting you home, and that's a fact. If you think you might want to go that route I can help since I have my eye on a few around and about worth having but needing work to get them back up and running, all for around the $15K when all said and done.
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Old 09-12-2018, 03:31 PM   #17
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Join Date: Feb 2015
Location: Akron, Ohio
Posts: 17
Year: 1974
Coachwork: Crown
Chassis: Supercoach
Engine: cummins 250
Our 74' crown has a 250 Cummins with a 5 speed and the hills of Ohio slow us down. Also, our top speed is only 55/60. We make it work though. Good luck.
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Old 09-13-2018, 01:23 AM   #18
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If you have 20" tube type tires then swap them out for 24.5" tubeless tires. The 11x24.5 are very common, relatively inexpenseive, available everywhere, and will raise the top speed about 5 MPH.

I did that to my three axle Crown with a Cummins small cam 262, my two axle Crown with a 6-71T/5-speed, and the Crown at church that has a 6-71TAC/MT643. All of them were able to get up to 72 MPH and would cruise very easily at 68 MPH.

As it has been said earlier, if the engine is running well you won't be able to wear it out using it as a moho. Without a turbo it is going to be a bit pokey on the hills but it will maintain highway speed on the flat with no problem.

The only difficult thing about a DD 2-cycle is finding the correct Delo 100 40 wt oil. Using multi-weight or the newer oils without the ZDDP in it will trash a 2-cycle DD in about 1000 miles.
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Old 09-13-2018, 07:35 AM   #19
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Join Date: Dec 2017
Location: Cincinnati
Posts: 19
Year: 1973
Coachwork: MCI
Chassis: MC-7
Engine: 8V92 DD 400hp
Rated Cap: 39k
I own an 8V92Ta DD, a 1973 MCI. Although daunting at first, not as scary after a year of ownership. There is a large aftermarket out there, an entire forum dedicated to that series of busses, and you just cant beat the sound. There are said to do a million miles, and mine has been rebuilt 3 times and has over 2.1 million. Just feed it oil and crappy diesel and you will climb right past other diesel pushers in the moutains. Ask away!
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Old 09-13-2018, 04:00 PM   #20
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two stroke motorcycles

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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

yep I had an r-5 350, RD 200, RD 400, and a RZ 350, you could say i like two strokes too-but a diesel? i work on 8.3 blue birds all day, never have i seen or heard a two stroke diesel, now i am interested....
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