GMC 671 Diesel.

[Flyboy]

Hi,

I'm contemplating buying a 1964 GMC 4104.  Has a in-line 6, 671 GMC diesel (1978).  Spicer 4 speed trans.  It was used as a US army bus in Arizona and has been in Canada as a converted RV for 15 years.

The current owner did a general re and re 50,000 miles ago which included new sleeves etc. 

Does anyone have any advice regarding the 671?  Is it a reliable engine etc.?

Any help will be greatly appreciated.

Thanks in advance.

FB.

[GM0406]

Yup,  My 671 is at the DD hospital right now.  Normally these engines run as smooth as glass, and mine has a vibration and miss in it.  I think they have resolved the miss by replacing the injectors, but the vibration is a different story.  They found too much endplay in the crank and had to pull down the pan and go in and replace the mains.  She's been in the shop for almost two months now and we are hoping the surgery restores teh smooth 6 performance we need.  We also think efficiency should be restored giving us more power and better fuel mileage.  In these days of high fuel costs, these 671s make sense.  Good luck with yours and you will not be disappointed if it starts and runs properly.  Bill T.

[TomC]

Many, along with myself, believe that the 6-71N was the most reliable engine ever made.  It saw much action in the armed services since WWll. With proper care, it wasn't unheard of to have these engines do a million miles in trucks without overhaul-a feat that diesels have just been able to do in the last 15 years or so.  The first 6-71N came out in 1939 and was in production with the original 175hp @ 1,800rpm (considered to be the continuous hp setting) for 60 years until the 2 strokers stopped being in production in 1999.  No other engine has ever been in production for that many years.  That's not to say that there were other engines of the same horsepower that was more fuel efficient and produced less pollutants, it's just that GM had a perfect blend of balance on the inline 71 series.  When GM produced the V engines, they were getting essentially twice the horsepower out of the same crankshaft, so longevity was reduced.  Most V Detroits will only get about 500,000 miles before overhaul, with the exception of the non turbo'd engines with small injectors running at 1,800 rpm.  The 6-71N with the 4 valve head and N60 injectors make for a very reliable engine.  You can go bigger with the injectors and rebuild the engine into a turbo engine and still get close to 500,000 miles of engine life out of it.  To deminstrate how relatively low stressed that engine was at 210hp, Detroit made a 485hp version for boats at 2,500rpm with full warranty.  I doubt another engine like the 6-71N will ever be made again-it was too good.  Good Luck, TomC 

[RJ]

Flyboy -

What's the VIN on this coach you're contemplating?

4104s were not built in 1964 - production stopped on them in 1960.

If it's really a 1964, then it's a 4106, with the 8V71 powertrain.

Obtain the VIN, and I'll let you know who originally purchased it from GMC and when it was delivered new.

HTH. . .

[Flyboy]

Hi Tom,

The information is greatly appreciated.  Spoke with the current owner today.  The re and re consisted of new sleeves, pistons, bearings and head re-cond.  He called it a "wet" overhaul kit?

Russ,

Will get the VIN on Monday...curious what you'll come up with.

I think my wife and I are going to make an offer on Monday!  We've been wanting to get a bus for the last 18 months, this one came up and is in great condition at the right price.  We are stoked!!  We are a young couple with two kids, 7 and 5, have spent lots of time driving to Mexico and beyond.  Looking forward to doing it in style!

I am a first officer on the 767 at Air Canada so have a fair amount of time off to pursue our soon to be passion.

Thanks again for all the advice.

FB.

[TomC]

I don't know what a wet overhaul kit is, but he replaced the suspect items.  There can still be concern from not overhauling the governor, blower, water pump, oil pump, fuel pump.  What shape are the camshafts, crankshaft, drive gears?

I had my 8V-71N overhauled in 2001.  Had to bore the block .010 over since the block bores were out of round.  I then had .010 oversize liners installed with the same pistons, new rings, bearings, air compressor (rebuilt) and replaced one head since it cracked.  Since then, I've replaced the water pump, tested the thermostates, rebuilt alternator, new power steering pump.  Now am having the engine turboed which entails going from N65's to 7G75.  Had the oil pump opened up and discovered the drive keyway was about to give way (very close to failure)-probably never been replaced with past numerous in frames.  Also while the engine cradle was out, had the transmission (V730) gone throught and discovered the forward and third clutches were about gone.

My suggestion to everyone-when and if you overhaul the engine, have it torn down completely, thoroughly measured for wear, and inspected for possible future failures.  This also applies to the transmission.  Even though it sounds good, looks good, no smoke or leaks, things can go very wrong very quickly with hard working Diesels and transmissions.  I just makes for more relaxed travelling-rather than squeaking by on another trip.  Good Luck, TomC

[Paso One]

I don't know what a wet overhaul kit is, but he replaced the suspect items.  There can still be concern from not overhauling the governor, blower, water pump, oil pump, fuel pump.  What shape are the camshafts, crankshaft, drive gears?

I had my 8V-71N overhauled in 2001.  Had to bore the block .010 over since the block bores were out of round.  I then had .010 oversize liners installed with the same pistons, new rings, bearings, air compressor (rebuilt) and replaced one head since it cracked.  Since then, I've replaced the water pump, tested the thermostates, rebuilt alternator, new power steering pump.  Now am having the engine turboed which entails going from N65's to 7G75.  Had the oil pump opened up and discovered the drive keyway was about to give way (very close to failure)-probably never been replaced with past numerous in frames.  Also while the engine cradle was out, had the transmission (V730) gone throught and discovered the forward and third clutches were about gone.

My suggestion to everyone-when and if you overhaul the engine, have it torn down completely, thoroughly measured for wear, and inspected for possible future failures.  This also applies to the transmission.  Even though it sounds good, looks good, no smoke or leaks, things can go very wrong very quickly with hard working Diesels and transmissions.  I just makes for more relaxed travelling-rather than squeaking by on another trip.  Good Luck, TomC

I agree 100 percent about the complete teardown. I have my donor engine 8V71 on my car hauler getting ready to remove everything that the Detroit Diesel dealer doesn't need attached.  The only thing that scares me is the same dealer told me the V730 shouldn't be used in highway applications....  ( I hope it is just his opinion not everone at the DD )  I hate assumptions made with no background.

[RJ]

Paso -

I think the answer to your concern about the V-730 being used on the highway can be eliminated by the fact that GMC offered the transmission as an option in the late-production 4108s and 4905s. 

Do you think they would have done that if it wasn't designed for highway use too?

Go back to the guy who told you that the V-730 shouldn't be used on the highway and ask him to show you the Allison documentation that substantiates his claim.

FWIW & HTH. . .

[Paso One]

Hi Russ  I Just blew the comment off as a un-informed person  making a comment based on nothing.  Thanks for the models  #'s that came stock with the V730  I just said that I know of at least a dozen buses with the V730 operating on the highway with no problems.  Paul

[Dallas]

Since the V730 is a transverse transmission with a V drive, designed, as far as I know, especially for the GMC coach, what is it used for in an off-highway application?

Dallsa

[TomC]

V730 is an HT740 without 1st gear running through an angle drive.  What sometimes is referred to the V730 not being an highway transmission only has to do with the fact that you only have 3 speeds to play with.  So if you do gear the V730 like a HT740, the startability will be less than desireable with the V730.  As far as running the V730 at highway speeds, I haven't heard of any restrictions. I have the V730 with 4.625 rear gears with the 8V-71N.  In my opinion, you should to the 4.125 only if you have a turbo engine, then you won't loose startability.  Good Luck, TomC

[Dallas]

V730 is an HT740 without 1st gear running through an angle drive.  What sometimes is referred to the V730 not being an highway transmission only has to do with the fact that you only have 3 speeds to play with.  So if you do gear the V730 like a HT740, the startability will be less than desireable with the V730.  As far as running the V730 at highway speeds, I haven't heard of any restrictions. I have the V730 with 4.625 rear gears with the 8V-71N.  In my opinion, you should to the 4.125 only if you have a turbo engine, then you won't loose startability.  Good Luck, TomC

thanks, Tom! that sheds some light on the subject. But isn't the 730 a 3 speed and the 740 a 4 speed?

My question still stands.
What is the off road application for the V730?

Dallas

[RJ]

Dallas -

What I think the un-informed DD individual was referring to is city transit use vs highway coach operation.

I don't think he was referring to "off-road" use at all.

Obviously, this guy is a little confused, because I think we're all aware that city transit operations are far more demanding on a powertrain than running down the interstate.

If a V-730 can withstand the abuse given it in transit use, in RV service it's practically indestructable - when properly maintained!

And yes, a V-730 is a three-speed transmission with a lock-up torque converter that makes it feel, to the uninitiated, like a four-speed.  Just like an HT-740 feels like a five-speed, even though it's only four.


TomC -

TTBOMK, you could not get different gearing inside the V-730, except for the extremely rare 1.04:1 bevel gearset.  These don't even show up in the parts books, and may, in fact, be more of an urban legend at this point.

I know you can play with different rear axle ratios to affect performance, but not too many folk know that there were different torque converter stall ratios available, too.  These can have an effect on startability, something to consider when you're changing the rear axle from a 5:38:1 to a 4:10:1.

FWIW. . .

[Flyboy]

Hi Russ,

The VIN number for my newly purchased bus is PD4104931.

My wife and I are extremely excited about the start of the next chapter in our lives as newbie bus owners!!

I will post some pics of the bus when it is delivered tomorrow.

[Paso One]

Hi Dallas Yes Russ is correct he mentioned transit use of the V730 as to what it was designed for not any off road.

I Did in fact go from 5:38 :1 to 4:10:1  but I still have the VH9 2 speed tranny.  I just completed a 1400 mile hop and I'm really surprised how well that VH9 performed. The bus ran all day at 62 MPH 

When I install the V730 I should be able to cruise right along.

Although I am hoping to attach the 8V71 to the V730 at the same time.

But I would like to try the V730 before the swap just to see how it performs in the current application..... V730  4:10 on a 6V71

Sorry Flyboy to hijack your thread but it's a good topic if you ever go to a automatic.