View Full Version : detroit 6-71 help

05-26-2010, 10:57 PM
six months ago we pulled a an engine from an old bus at our local shopping emporium otherwise known as a junk yard. we went to install it in on a piece of equipment and found out it runs backwards, have any of you heard of that and what is involved in changing the rotation of it. its a good running low miles detroit and we have a grand invested into it. Im not sure if we bought an expensive boat anchor or if it can be changed for a reasonable price. I hate detroits and the only reason we bought was we were hoping it would do the job until we could afford something quieter and less drippy I dont want to get a lot invested into it. any help would be appreciated

05-27-2010, 01:25 AM
It'll take more than you'd probably care to get into.

You'd have to pull the flywheel housing to move the gear train idler to the opposite side and I can't remember for sure but I think the the oil pump can be reconfigured for opposite rotation which requires pulling it off the front of the engine. Those are the first things that come to mind.

05-27-2010, 02:31 AM
Howdy, Idacal and DuaneB :waving:

I'm trying to get a grip on the Jimmy's age.

Idacal, you're not the first guy who paddled up this creek.
I'm presuming that this is an inline 71 and pulled from a GM bus, in which case you have most of the right parts, but in the wrong places.

At the top of this block there are two shafts (cam-shaft and balance-shaft) driven by a gear train at the flywheel end of the engine. As DuaneB mentioned, most basically -not all inclusive- the engine is reversed by changing the idler gear to drive the opposite cam-shaft or balance-shaft.

I'm almost sure that the idler gear and crankshaft drive gear are rotation specific and will need to be replaced, the other gears can stay. The oil pump also needs to be changed. There are probably a few other minor details too; I'm wondering about the crank seals.

If this were a truck or industrial engine I'd say you're ready to go. But... the bus engines had a queer front and rear cover. This engine has no front crank pulley, and an engine mount cast in the front cover, right? And since GM wouldn't have expected field rotation swaps with a bus application, I wonder if the bus cover has room for the gear swap.

So how far along is the coupling, and what sort of drive did you whip up?
Would it be possible to drive your load from the fan-end of the engine? Although I'm not sure how much twist that end of the crank is rated for.

I probably have parts books that may cover the bus engine, I'll try to take a look tomorrow and see if there's any clue as to possibility of the idler swap.

Good Luck

05-27-2010, 09:00 PM
I looked, the bus engine seems to have a rear housing that's unique to bus only.

The parts book drawing is artist's conception, but it seems to show that there is no room for an alternate idler location. There is no mention of an optional idler gear.

At this point would say the bus engine would require a different rear housing group to reverse its rotation.

Pictures at 11:00, dinner at 9:00 ;)

What's it turning? Could that be reversed?

Good Luck

05-28-2010, 12:23 AM
thanks guys its a v engine not an inline we are trying to run a 450cfm 300psi piston compressor with it. it looks like it will probably end up back were it came from. Im going to do a little more checking on it though. the scrapyard has another one in an old excavator I think its an 8v 71 hopefully he will swap with us with just a little extra thrown in.

05-28-2010, 12:34 AM
I just reread your post denrep the compressor had a detroit powering it and it was mounted directly to the flywheel I dont think it even had a clutch on it. the owner wanted 5000.00 extra for the engine because he had just rebuilt it and remembered the pain of paying for that. but at the time I had the attitude it would be a cold day in hell before I spend that kind of money on a detroit. now we just threw away 600 dollars and halve a day pulling it, but it should bring about 400 in scrap. we will see I guess

05-28-2010, 01:56 AM
Hope you can get a deal worked out with the yard.

For the record, same rotation change issues apply whether it's an inline or vee.

05-28-2010, 08:08 AM
The bus V-engine is a little more peculiar yet, because they were installed sideways -for lack of a better term- with one valve cover positioned nearly at the top and the other down low. There will be lube oil pick-up and drain-back issues if this engine is flipped.

But... a piston compressor may not be rotation sensitive, except for maybe a fan or lube pump.
Some compressor lube pumps build pressure regardless of rotation direction; something that was probably engineered to protect a pump in the event that an electric motor is unintentionally reversed.

Anyway, as cheap as omplete and running large compressors are selling for now, it's probably not worth too much more fight to resurrect this one.

Good Luck

05-28-2010, 08:37 AM
Meanwhile, we may as well look at the pictures:



05-28-2010, 01:19 PM

Suspect you're on the right track with looking at different engine options.

The 6v71 never enjoyed the reliability (especially in marine applications) the the inline engine did.

Shaft rotation was handled by hydraulic gears rather than reversing the engine rotation as was frequently done with the gas engines (RH, LH rotation).

The old 6-71N's were known for their longivity. I've seen them go for 18,000 hrs in sawmill applications. When they pumped them up (turbocharging/aftercooling) to 485HP for marine applications they were lucky to get 2000 hrs BMOH.

Penske greatly inproved the engine after he took over Detroit Diesel in about 1986, but the engine was always know as a leaker. Old saying was, "when a Detroit stops leaking, add some oil cause she ain't got none.":laugh:

Old haul/buy boat I grew up on had a 12-71 (1948 vintage) in her. She leaked a gallon of oil and burned one quart every 9 hours of operation. I used to run her from Hampton,VA to Baltimore, MD hauling oysters in the winter months. While my deckhand was offloading cargo, I'd go to the engine room. Pump oil from the catch pan, thru cheesecloth into a jug, dump the jug back in the engine, add a quart of oil, and I was ready for the run home.:cool:

05-29-2010, 12:02 AM
i listened to one of those 12 v71s on a drill rig for a year and several other smaller ones over the few years I have been working it got to the point that for me, hearing a detroit in full rev its like listening to someone scratch their nails on a chalk board it drives me nuttier than I am normally, but they are a good cheap engine that will get the job done as long as you got a good oil supplier.

Charley Davidson
05-29-2010, 09:04 AM
That engine according to what bus it came out of had a PTO drive running off one of the front pulleys turning a huge AC compressor, since it is a reverse rotation I would assume that the bus was setup the same. If it was out of a 4905 (buffalo) it would definitely have that setup. Go back and get that unit and go from there, then at least all is not lost.