HK Trucking

A 74 DM 685 would have originally been equipped with a TRXL 107 or 1071, which are both direct (1:1) in high gear, so the automatic should allow you to achieve the same top speed as you have now. That said, I doubt if the automatic transmissions in those other trucks are causing them to "pull away from you like nothing". What horsepower do those other trucks have? Probably a lot more than the 237 horsepower that your ENDT 675 has. It's almost a certainty that the other trucks are pulling away from you due to more horsepower and not due to the fact that they have automatics in them. A manual transmission is far superior to any automatic in dump truck work, you just need more horsepower.

As long as you're changing to the 22.5 hubs, I would suggest using 445/65 R 22.5 floatation tires. Wider footprint, larger diameter for more ground clearance. The diameter would more closely match the 12.00 24's on your drives.

So what you're saying is that if I don't like those custom butcher jobs on "Trick My Truck", then that means I'm not "legit" and I have no "Trucking Cred"? Oh well, I guess my career for the past 33 years was all a waste of my time then.

You're absolutely right. Therefore stop b*tching about our opinions.

What Glenn said. R & DM radiators are way too big to fit in there. With a recore, you'll have a nice new radiator.

Everything they do on that show is all f---ed up. It may be awesome to the general public, but to guys like us who understand trucks, it is just a load of bullsh*t. I don't watch it anymore To me, a nice completely stock restoration is much more appealing.

Check for a blown gasket where the exhaust pipe attaches to the manifold, or any other loose connections in the exhaust system upstream from the muffler. I remember a long time ago I had a Ford F 750 with a big gas engine in it, and it would backfire something awful on deceleration. Noticed that it had a broken stud where the crossover pipe connected to the left exhaust manifold, which allowed the pipe to come loose. Fixed the stud and got that pipe to manifold connection tightened back up, and the backfiring went away. I've heard others talk of this happening too, caused by a loose connection in the exhaust system. Evidently on deceleration, air is pulled into the exhaust system thru the loose connection, and mixes with unburned fuel in the exhaust system, and then is ignited by the heat in the muffler, then you get a big "bang".

Are you sure it came from the factory with a Fuller 9 speed? A 690 with an EM7 300 and a 9 speed doesn't sound to me like a factory set up.

While you've got the torch & welder out just fabricate the necessary hardware to make it a combination tailgate. Then you can either use the tailgate normally (hinged at top), or by changing a couple pins around you can use it as a swing tailgate (hinged at side) for dumping large material without jam ups.

That is because with the Mack power divider, if there is a big difference between the diameters of the tires on the front drive axle compared to the diameters of the tires on the rear drive axle, the result will be that the pins in the power divider will constantly be jumping over the cam & cage and this can sometimes be felt as a "clunk" in the driveline. I've had it happen before, and rearranging the tires so the largest 4 were all on one side and the smallest 4 were all on the other side cured the problem. I found it best to put the 4 larger tires on the curb side because it helps to counteract the "crown" of the road.

When you say that "they both have the same motor", do you mean they externally appear to be the same, or does the ID info on top of the valve covers indicate that they are the same engine model? If the engines are indeed the same model in both the RD690 and the RD688, then someone has done a transplant, because as I said, the 690 would have a Maxidyne such as "EM7300", or "EM7300L", while the 688 would have an Econodyne such as "E7350", or "E7400" etc. The Maxidyne engines were made to work with the T2050, T2060, T2070, and T2080 Mack transmissions which were 5, 6, 7, and 8 speeds respectively. The Econodyne engines were made to be used with T2090, TRTXL1070B, T2130, and T2180 Mack transmissions which were 9, 12, 13, and 18 speeds respectively, and some of the Econodyne equipped trucks were set up with Eaton Fuller 8LL, 9, 10, 13, and 18 speed transmissions.

The 690 would have a Maxidyne engine and the 688 would have the Econodyne engine.

What is the model number? For example RB688?, RD 690? DM690? The three digits after the letters will give us a hint of what engine it may have.

My Mack manual from that era shows 150 to 175 ft lbs torque on the flywheel nuts. Start at the low limit (150) then tighten til the cotter pin hole in the nut aligns with the hole in the stud. Do not exceed the upper limit (175). Cotter pins are OK, I've seen them used before on that application. I would also suggest using Loctite.

But you will have one hell of a mess! Not only that, but you will lose most of the gear oil that's in the wheel bearings as soon as you pull the axles. (if the truck has the old type seals with grease in the wheel bearings then this does not apply). And if it has one piece axles, you'll have to make some sort of cover plates to keep dirt out of the wheel bearings during a several hundred mile trip. If the truck has 2 piece axles you can remove the shafts and then replace the end caps to keep dirt out during the trip.

There's a little plunger type oil pump on the input shaft in the TR 72, 720, 7220 series transmissions, Rob.

That appears to be a classic. ENDT673 @205 HP. It has to be a VERY early one (mid 50's) because there weren't many turbo motors made without an oil cooler and without a full flow oil filter. That water cooled turbo is also a rarity, I ran across a motor with one of those in a junkyard back in the 70's. Didn't need a motor at the time or realize then what I had found. BTW, that was at that big truck junkyard in Elmsford NY. (don't know if it's there anymore). While it is a rare engine, it also looks like it's been sitting outside unprotected for years and that appears to be the air compressor intake line thats pointing upward and has probably been swallowing rainwater for some time. Chances are the engine is stuck, but for $250.00 what the hell, I'm sure you could use something from it, or from the trans.

Yes.

If you can't find a number on the block in front of the inj. pump on that curved area where the timing case cover bolts on then look at the tops of the valve covers. There's a square pad on top of each cover. At least one of them should have an embossed label on it that says "Maxidyne" or "Thermodyne". If it says Maxidyne, then the engine is an ENDT 675 because that was the only 6 cyl. Maxidyne in 1969. If it says Thermodyne then it is an ENDT673C because that would be the only turbocharged 6 cyl Thermodyne in 1969. Of course this info is assuming that someone hasn't put different valve covers on the engine at some point in the past 38 years. As I said in an earlier post, that truck should have an ENDT675 Maxidyne in it.

Here's what you get when you look up Mack powertrain facilities at macktrucks.com: http://www.macktrucks.com/default.aspx?pageid=198 Note that it says"Volvo" Powertrain North America. Kinda removes all doubt as to where this is going, doesn't it?

Based on the fact that it is a 1968 R685T, I'm assuming you have the 14" pull type clutch with the "pot" type flywheel. The center plate has notches which engage with the drive pins in the flywheel, enabling the center plate to also transmit torque to the clutch discs, therefore you have twice the friction surface as you would with a single disc clutch of the same diameter. Incidentally, when you have the flywheel machined the drive pins have to be removed. When you reinstall the drive pins, use a small machinist's square and be sure the drive pins are installed perfectly straight, or the center plate will bind on the pins and your clutch may not release properly.

The 711's with the large studs at the corners were notorious for having head gasket problems, so I'm betting that's where the trouble is. These engines did not have the "fire rings" to seal the top of the liners to the head, therefore the head gaskets had to hold compression and combustion pressure as well as seal the water and oil passages between the heads and block. Chances are that the heads haven't been retorqued in years (maybe decades), and the head gaskets themselves can deteriorate over time also. Add to that the chance that the deck (top of the block) and the bottom of the heads may no longer be perfectly flat or smooth and theres a good possibility of seepage. Yank the heads, check for cracks in the block & heads while it's apart, and put a new set of head gaskets in it Follow the proper head bolt torquing procedure, and re torque after you run it for a day or two.

Here's how I always did it with my dump trucks, and the company I work for now does it this way too. Complete grease job every 5 working days.(and while under the truck take a good look at everything) Oil & filter change & check all lube & fluid levels every 300 engine hours. Mileage is a poor way to determine service intervals for a dump truck, due to the fact that the truck may be on short hauls / jobsite work which puts few miles on the odometer, but the truck is working just as hard or harder than if it was on the highway, and there is more dust & dirt than in a highway application. Sample your engine oil and have it analyzed by a reputable oil lab. Trans & rear end oil should be changed every year or so, it's cheap insurance against lube related problems. Air filter? Change when the indicator shows that maximum allowable restriction has been reached, or once a year, whichever comes first. Do not!!!!! under any circumstances remove your air filter element, blow it out with compressed air and reuse it!!! This is a sure way to cause a premature engine overhaul. A new air filter element is much cheaper than a new or rebuilt engine.

An R685T would have originally had an ENDT675 Maxidyne.

It sounds to me as if one of your differentials has failed. Probably lost a tooth or 2 off the ring and pinion, or maybe the bull gears. I can't count the times someone has said "my transmission is bad, it's making a banging noise", and it turned out to be a rear end problem. The noises from a failed rear end get carried up the driveline and you feel it in the shifters and it sounds like a trans. problem. There's always the chance it could be the transmission, but have someone stand next to the truck while you slowly move it, and see if they hear noises coming from the rears. If so, pull the side covers and have a look at the gears in the rear ends, before you automatically assume it's the transmission.