JoeH

Is that the new stuff or pretty much all Allison's?

Looking into using an MR688 or 690 (mid '90s fully mechanical hopefully) to mount a concrete mixer (volumetric type, not barrel) on. Most are automatic, but the few that I glanced at look like the auto trans has very little ground clearance. Does anyone have any experience with MR's? We hop curbs once in a while, back into people's yards, the usual things Macks are supposed to do. How do the automatics perform in these conditions? Do they have a good torque converter lockup to be able to hop over curbs or soft ground without much fuss? Chances are an auto is a terrible fit for us. Ideally I'll find a gem e7-350 with an 8LL, but I'm curious about the automatics.

Short wheelbase trucks tend to "buck" like a bull rider more noticably, and with how far off the ground you are you feel it more than you would in different wheelbase cars/pickups.

Citing low back pain, get something with light duty springs and a longer wheelbase, plus an air ride cab is a must. Not sure when they started, but Mack offered an option on the R model where the rear cab mount is air bags. Night and day difference on operator back pain. You can convert any truck to single axle air ride, would likely give you a much better ride quality than traditional springs.

What kinda hobby truck? Hobby dump truck for picking up your own mulch? Hobby rollback to support an existing automotive hobby? 5th wheel for towing some old John Deere tractors? The possibilities are endless. A bit of mechanical know-how is important, and so are some big wrenches! As with all things metal, rust is the biggest enemy to watch out for. Solid bones are important.

The Go Pedal looks unused...

Unfamiliar with the cat cement. Proper procedure is to clean everything down, wipe it dry, position gasket, place Jake spacer on, position next gasket, make sure gaskets are properly positioned, set valve cover on, take the three bolts to the torque setting in the book.

No, you take them to the torque setting in the book, no more.

We probably have 100k hours on the one truck, 1979 R686ST, with the ENDT676 motor. Still working 5-10 hours a day.

We've been using the rubber ones through our mack dealer for a long time, they work fine. The key is don't use anything on them and do not overtorque the bolts. Do not use silicone, do not use high tack. They lubricate the gasket enough so it can shift and slide. Also, overtorquing the bolts distorts the valve cover, making it leak. We have had to straighten ours out a number of times over the years until we figured out the tricks to seal them up nice.

Reason to keep that lift axle: it's an extra set of brakes.

In PA those trucks are good for 73,280. Tandem axles are only good for about 58k I believe in PA. What you're able to get on the truck depends on the axle weight ratings. Example, steer is probably good for 18k, and the drive tandem is probably good for 44k. Combined you're looking at a legal weight of 62k loaded without that lift axle, depending on what your state regulations are. In PA the lift axle only needs to be down if you're over 58k or the weight rating of any particular axle.

Also, you should be seeing around 25 psi of boost under full load working hard. If you're trolling around empty on flat you'll never see more than 10 or 15. Need to hit a good hill at low rpm in a taller gear to make this engine work enough to build boost if you don't have any weight on.

Thinking about it more, I'm going to go out on a limb and say Fuel Filters are possible. My E7 did that, sounded almost like the Jake brake was coming on above 1500 rpm under load, and didn't quite have as much boost as it should as it neared the governor. Took 2 sets of fuel filters to clear it up, hasn't done it since.

When a cam lobe gets wiped it gets worn to nothing fast. Check for bent pushrod, driver could have over revved the engine and floated the valves a bit.

Before you pull expensive parts, pull the valve covers and see which rocker has too much slop. More often than not, you'll find either one of the engine brake valve plates popped out from a weak valve spring or a bent pushrod, or worn cam lobe. We have had all 3. If it's the engine brake valve plate missing on an exhaust valve then just delete the engine brake.

I'd be surprised if they are the same, but then again the oil pans use the same gasket.... Get VIN numbers from an E6 truck and an E7 truck, ask your dealer for the part number for each timing cover gasket. Same for the front main seal. The E7 can run the steering pump on the front of the timing cover, so I'm sure the covers are different. I'd just sift through a Mack junk yard and find a donor front engine mount. Don't know where you live, but Coopersburg PA has a good one at the Kenworth dealership.

Also make sure those 2 little pins in the side where it says "TOP" are mounted top, they're air bleeders so you don't get a pocket of air against the thermostat when you refill the system. Air won't conduct heat to open the thermostat properly on the first heat cycle. Automotive thermostats frequently don't have those so I drill a pin hole in them to serve the same purpose.

Should be running at 180, that's where all my Macks run, 1979-1995. Dont know what your truck setup is, if it's real light and you have no hills it'll take forever to warm up, it's meant to work hard. A new thermostat would have been a good idea, they're not expensive. Running cold slobbers up the valves, can make them sticky.

Not sure how to add photos at the moment, but we found the oil leaks! There are 10 holes in the oil pan flange presumably for spot welds to hold the reinforcing strip in place. Well, someone never spot welded it, so oil was able to seep past the gasket at the front and rear. $2,000 for a part 90% complete.

I've had fuel filters cause this; replaced them twice a few days apart and it cleared up. Almost sounded like Jake brake kicking on lightly. Fully mechanical '95 E7-350.

I'd never heard of 2 piece pistons until I looked up E7 rebuild kits. What is the advantage of 2 piece pistons vs 1 piece pistons?

Does anyone know if the oil pan threaded bosses in the timing cover and flywheel housing are open to the crankcase or if they're sealed?

It's looking like the oil pan gasket doesn't seal around the 2 bolts front and 2 bolts rear. Pressurizing it made oil pour out of the front around the oil pan reinforcement straps, where it wasn't leaking before. Didn't make it leak at the rear much, but we're going to pull the pan back off, smear some RTV silicone on it all around and see what happens!

Did you set the heads down on a table and smash the injector tips while you had the heads off?