Experts Wanted!

[thomastractorsvc]

I was surfing the inter-web and came across a 1989 Mack RD688S vin#2M2P141C1KC007994 for sale. It is a dump truck and I was surprised it was so light spec'd according to the right up.

Front Axle Weight 12,000 lb
Rear Axle Weight 38,000 lb

Most of the RDs I see posted are a lot heaver with double frames 44K or 46K rears , 14K to 20K fronts 8LL trans or 10 speeds.

So I suppose the bigger question would be what is the difference between this RD and a R? As this one seems to have similar specs to my old 1980 R road tractor.

[GOOG]

Every RD I've owned, had a double frame. The RD was a dump truck or a mixer similar to the DM. It was a heavier truck. Sounds like this truck has been molested.

[85snowdog]

Great question! With all the RD talk lately I was going to ask the same thing myself.

I have 2 RD's and an R . The only info I could contribute, and I'm not sure if it is limited to all R's an RD's is …..

The frame is approx. 10.5 inches on the RD . Approx 9 inches on the R. In my case they are both double frame.

The only other difference that I see is the way the fuel tanks are mounted. The RD has a different front mounting bracket. The R just has your normal tank straps.

My 85 RD has factory 38 000 # rears .

Hopefully there will be more input from the experts.

[Superdog]

The only thing I can figure is someone wanted a light tandem dump and wasn't planning on exceeding 50k with it. Gotta remember in '89 Mack would build a truck the way a customer wanted it. You could order a R model with double frame and heavy rears same as you could order an RD,but as mentioned above a regular R model rails weren't as tall,I'm sure the RD would have the taller rails even if it wasn't doubled.

[41chevy]

The Township of Oyster Bay had a dozen or so light spec RD's as Municipal units. When they put them up for sale in 1993 they had a bad time getting rid of them because of the light specs. Could be what that one is? Paul

[57 bcr]

Like 85snowdog & Superdog say,the big difference in the RD over the R is the frame rail height. I know of a '90 RD688 tractor running around here with a 5/16" single frame,12F 38R on Neway air ride. An RD came standard with a 12K front axle and was single framed. Heavy axles and frames were optional. Al

[Vladislav]

From what I've seen in the wiki section that's put there by our great Mack experts,

R-model frame rail heigth was 9" and RD was 10.62" or 10.75" as option.

But R-700S had 10.5" and whole 10.62" as option either.

On the other note what was the last year of an R-model in production? 1988?

Could you just buy an R in 1989 and later?

[tjc transport]

i knew a guy years ago that used to buy mack trucks out of canada that were speced very light so he could carry 23-24 ton in a tandem.

single frame, light rears and front axle. his reasoning was he only did on road work delivering stock from quarries to asphalt plants, never ever drove off road so he did not need heavy rears.

and he would replace the truck every 5 years before he had frame problems.

the last one he bought was a 89 superliner that only weighed 24,000 lbs full of fuel..

[mackey58]

Man wish we could carry 24 ton in Illinois with a tamdem dump

[kscarbel2]

I was surfing the inter-web and came across a 1989 Mack RD688S vin#2M2P141C1KC007994 for sale. It is a dump truck and I was surprised it was so light spec'd according to the right up.

Front Axle Weight 12,000 lb

Rear Axle Weight 38,000 lb

Most of the RDs I see posted are a lot heaver with double frames 44K or 46K rears , 14K to 20K fronts 8LL trans or 10 speeds.

So I suppose the bigger question would be what is the difference between this RD and a R? As this one seems to have similar specs to my old 1980 R road tractor.

The R had a 9" x 3-1/4" x 1/4" frame (with optional 1/4" inside frame reinforcement).

For tanker operations that wanted a stronger frame without the added weight of inside frame reinforcement, the base level RD frame was made available (for tractors only), that being 10-5/8" x 3-1/4" x 5/16".

The RD6 came standard with a 10-5/8" x 3-1/4" x 5/16" frame (with optional 1/4" inside frame reinforcement).

The RD6 could be optioned with a 10-3/4" x 3-1/4" x 3/8" frame (with optional 1/4" inside frame reinforcement).

[ekennedy21]

The difference in tanks has no bearing on wether the truck is an r or an rd, there are multiple options when spec'ing a truck

[Jeff M]

Possible original road tractor that later had a dump body installed on it....

[thomastractorsvc]

i knew a guy years ago that used to buy mack trucks out of canada that were speced very light so he could carry 23-24 ton in a tandem.

single frame, light rears and front axle. his reasoning was he only did on road work delivering stock from quarries to asphalt plants, never ever drove off road so he did not need heavy rears.

and he would replace the truck every 5 years before he had frame problems.

the last one he bought was a 89 superliner that only weighed 24,000 lbs full of fuel..

I looked up and called the original owner (power of the internet and sticker residue) he said they were ordered as dumps to haul asphalt on road and seen very little off road use. So as TJC said, apparently a lot of people had the same idea. In his case he said he kept the the two trucks until last fall when they were sold to a truck wholesaler who sold them to another guy that has a used truck lot.

They guy was really nice and said they never had any problem with them until last fall and one developed a miss, they thought it was an injector turned out to be the cam and that was when the decision to part with them was made.

[gearhead204]

sounds like left coast specs where we have to work with the bridge laws, we had several rd688s that were specked that way, the only thing that would happen is they would snap an axle shaft if the guys wheel hopped them in the sand, other wise no issues except for the failing cam shafts we lost 3 in less than 130,000 miles all in sister trucks.

[thomastractorsvc]

So the cams in the 300s were soft? First I ever heard of those engines having cam issues. Have not seen anything really posted on here about that.

Any experts on why and the fix?

[tjc transport]

the big problems we had with the late 80's 300 engines was with the rocker bosses snapping off the head when a jake brake was used.

the replacement heads had no problems.

[39 Baby Mack]

So the cams in the 300s were soft? First I ever heard of those engines having cam issues. Have not seen anything really posted on here about that.

Any experts on why and the fix?

We had a few cam problems. I thought the real problem wasn't so much the camshafts being soft but that the hardened wafer would come off the lifter. Once that happened that's when the lobe on the cam went.

Ron

Sent from my SM-G900V using Tapatalk

[tjc transport]

We had a few cam problems. I thought the real problem wasn't so much the camshafts being soft but that the hardened wafer would come off the lifter. Once that happened that's when the lobe on the cam went.

Ron

Sent from my SM-G900V using Tapatalk

saw that a few times also. but by far we had more bad heads. probably 10 bad heads compared to 2 or 3 bad wafers on the lifter.

[gearhead204]

the ones we had fail were on 350's and yes I think it was the followers that caused the cam failures (mack warranty issue) so I didn't do them my self.

[tjc transport]

same here, the few we had were still under warranty, but on 300's. the heads were under warranty also.

we only had 1 350 engine. the owner liked keeping everything the same so one part would fit all 20 trucks.

[Phase 1]

So from what I can gather, an RD would have been like the earlier X models. If there was an R like a B-61, an RD would have been like a B-61X model. The main feature was a heavier frame, but usually spec'd with other heavy components.