JoeH

There should be a sideways torque arm from the right frame rail to the rear differential. The eyes on these wear out, but if the truck is acting this way from new I'd wonder if someone forgot to install it... Reread, you stated it's missing the transverse torque arm. There's your culprit. I assume all your other trucks have it. For shits, go to your dealer parts dept, and run the trucks VIN and ask for that torque arm. If it comes up as a part on the truck then I'd tell the dealer to file a warranty claim and install it on Macks dime, citing that it never had it from new so they must have forgotten it at the factory.

You'd blow up the motor at 14,000 rpm! Change your fuel filters (2 I expect) when they clog they become a restriction point and act like a governor.

I think you're overthinking it. Its thin wall metal, and gets softer the more you heat it up. I'd think the pipe would give before anything else. My '79 endt676 is rigid pipe off turbo, down along transmission, and below frame. It then has flex pipe before heading to the muffler. Pyro says downshift at 1125 degrees, and it's factory rigid.

Its not an expansion joint, it's a flex pipe. You're fine on rigid pipe until you make the transition to the frame mounted muffler stand. That transition will need to be flex pipe. Never used the bellows type. The engine and transmission can rock sideways in the frame a good bit under load. Especially if your transmission bolts work loose.... 🙄

Heck, I'd be fertilizing my pants on those inclines! My truck runs a V box type body that is about 11'6" overall height at 73,280 lbs so I get nervous at any kind of angle.

Drove a Peterbilt once that had an air piston to raise/lower the passenger window via a switch on the dash. I thought that was slick. Driver side was hand crank. On those steep inclines I'd be a bit nervous on light springs. I'd want heavy springs to a) create less lean angle between frame and wheel alignment, and b) more safety factor to offset overload fatigue on the downhill side.

I'd love to see the rig you're hauling around at 30 degrees! That's a great gauge to have in the cab!

That's good work for one of these trucks. Camelbacks have phenomenal articulation over rough terrain as I'm sure you already know.

Reading into your post a little more thoroughly, what are you using the trucks for? How many miles are on them? The older camelback suspension lacked a sideways torque arm, so the axles tend to wander. You'll want your steering and suspension components nice and tight to help stay in a straight line going down the road. King pins, drag link, tie rod ends, spring u bolts all tight up front, and trunnion bushings, elephant pads and torque arms in the rear. You'll also want to keep an eye out for vertical frame cracks under the cab where the frame starts splaying out to go around the engine and transmission. This is a single frame issue, haven't heard of a double frame cracking there. We've cracked/replaced both frame rails at different times on our '79 R686ST.

Best thing to do is keep everything in the same engine family. That way one spare engine can supply the fleet, or if a truck goes down it still has Parts value.

The real driving force behind whether or not you'll see them in another 10 years is a) they aren't building them any more b) they're so old that it's unrealistic to expect Mack to keep making parts for them and c) the repair industry is so much further down the road simply because of a) and b). There's no real reason you shouldn't expect one of these to perform admirably for you. Nothing's bulletproof, new or old. On these, if you're prepared to dig for spare parts and fix it in house then I'd say go for it. Just remember, Mack engineered their whole engine/drive line to work together "as engineered." Dont go ramping up the HP, Mack already built their engines to their limits.

If it happens again I've got 11 more good lifters in the spare engine, and I'll send the currently bum spare cam out to have a new lobe welded on while the good engine gets ready for cam swap.

*shrug* last time we lost a lifter the truck got the cam and a lifter out of our spare engine.

These motors will run until you break the carbide face on a valve lifter and wipe that lobe off the camshaft. Then you're borrowing the cam and a lifter from a spare engine. Could happen tomorrow, could happen in 20 years.

Yea, modern diesel is "bio" and grows stuff pretty quick. They make an algicide fuel treatment that will kill stuff, but won't break it down. Our front end loader sat for 6 months once while we pulled the motor to deal with oil leaks. When we got it back together it kept clogging the strainer in the electric primer pump.

What's with all that pitting around the machined part of the cast iron? Combined with the chipped teeth I'd wonder if something small went through there...

The truck my dad adjusted is a 1979 R686ST...

Glad someone chimed in with the "how to". I had no idea how. I only know that my dad adjusted the link in one of our trucks not too long ago so I threw it out to you as a possibility! Reasonable that a 20k axle is not adjustable. The one my dad adjusted is rated for something in the 10.5-11k range.

Humor me, change the fuel filters. If you already did, then do it again.

I've only heard one once here in PA, he pulled in next to me at the fuel island. There's no mistaking it's sound for an inline 6. He has it turned up to 600hp IIRC.

If you take the power to displacement ratio of the e7-460 at 12 liters and apply it to the Big Six at 14.5 liters then you wind up around 555hp. Decent, but unknown upward potential.

Fact is, Mack missed out on the Big Power market when they didn't move forward on their "Big Six" prototype sitting at the museum. Their E9 V8 is the only real Big Power option, but all I've heard is parts are hard to get and they suffer in the longevity department. Whether that's just scarcity and age or design flaws, I couldn't say. I have heard that V engines' main flaw is that they have to run 2 connecting rods on each crankshaft offset.

Pyro on my E7 says downshift at 1025 degrees. Pyro on my ENDT676 (1979 2 valve precursor to the E6, pretty much the same engine except for fueling/powerband) says downshift at 1125 degrees. You'll have 13 gears so that'll help you keep in the powerband. The E7 is only 12 liters, so at 11 on the E6 youre not far off.

11 liter, not 10. Don't be fooled by its size, Mack rated their HP at the wheels, not at the flywheel. So a 350 Mack should make more power than another brand 350. I haven't driven an E6-350, but it should pull well. Only turn it up if you're the only one that's going to drive it, and don't do much. They're incredible engines for their size, and Mack maxed them out at the perfect power vs durability ratio. If you tip more towards the power then you'll lose durability.

Our 2003 MR688 is an automatic. I love it. There's no going too fast in that truck, being a heavy spec cabover every bump in the road hurts.