1986 GMC

[Randy1974]

Hi all! New Guy here!! I bought a 1986 GMC C7000 single axle dump last fall from a guy I work with. I drove the truck for him a few years ago so I have some knowledge of the truck. It has been sitting for a few years so I plan on doing some work to it. The Vin says it has a 427 BBC engine in it. Its back by a 5 speed transmission and a 2 speed rear axle and air brakes. The odometer says 81,096. My question is what oil should I run in the engine? I was thinking about 15w40 but not sure. I am gonna change the rear axle oil and transmission oil and probably the hydraulic oil. It also has a rear axle seal leaking. What size is the locking nut for the rear bearing? Thanks for any help!

Randy

 

[Welder Dave]

15W40 is typically a diesel oil. I have a newer similar truck with a 366 and run a good 10W30 gas oil. 10W40 would be fine too.

 

[Junkyard]

I wouldn't run a diesel oil either. Too much detergent action for an older high mileage truck. Who knows what kind of crud it might knock loose. I would say a multi-viscosity as mentioned above would be fine. I don't recall the part number but I used to run a larger oil filter, twice the size of a standard filter for a big block gas motor. A little extra media and volume won't hurt a thing.

As far as nut size, it should be like a trailer hub so odds are a big pair of channel locks or a hammer and chisel will get it loose. I'd say in the 3 to 3-1/2" range. Any heavy truck parts place should be able to sell you a socket for it. Doesn't need to be impact it can be the lighter duty setup.

Junkyard

 

[Tenwheeler]

Ryder Truck Rental had thousands of big block Chevy's and ran 15W 40 in them. I run it in most everything but my chainsaw. To my understanding it meets and exceeds all the requirements of the gas rated oils.

 

[Randy1974]

Thanks for the help. I keep 15w40 around for the skid steer that's why I thought about using it but I also use 10w30 in my pickup so I have that as well.The truck was a former township truck and a guy that drove it then said he thought they used 10w30 in it.So I think I will use 10w30. Should I use 80w90 in transmission and rear axle?

 

[56wrench]

If it's always had 10w-30 se, sf, or sg type oil, do not use diesel eng oil( ch, cj-4 etc. as it may unseat the piston rings and cause it to burn excessive oil which can only be cured by a ring job. I had it happen to me many years ago when I mistakenly put 15w-40 cd oil in my pick-up. It had not been using oil previously but it did after that right up until I re-ringed the eng. If a gas eng has used 15-40 right from new, it likely would be ok, but not in an eng with high miles. just my 2c

 

[Old Doug]

I use 10w30 diesel oil in my old flat tapet cam engines from what i have read new oils dont have what it takes for older engines,

 

[Truck Shop]

I use 10w30 diesel oil in my old flat tapet cam engines from what i have read new oils dont have what it takes for older engines,

Your right O-D the oil today has the zinc removed or ZDDP which is what kept the flat tappet cams alive in the older engines. Every oil change on an older flat tappet cam engine
should have a bottle of Lucas engine break-in lube added to up the level of ZDDP. The reason ZDDP was removed was it will plug a catalytic converter over time. SBC and BBC
engines run a narrow lobe unlike say a dodge 318/360 or AMC plus GM didn't use a cam thrust plate so the cam can move slightly more fore and aft with a worn chain. Depending
on the amount of crown machined into the lifter face can determine just how long the cam will live. The norm is .002, any lower and lifter/cam life is cut by 25%. There are two
later designs of lifters that oil better. One has three narrow flat areas machined the length of the lifter to allow more oil at the cam lobe the other is the EDM lifter which has a .015
hole burned through the lifter face before machining, which puts more oil directly to the lobe.

One of the many hi-perf engines I've built over the years

Truck Shop

 

[Metalman 55]

I don't know anything about high perf motors, but that looks like a real beauty.

 

[Randy1974]

What brand of oil do y'all recommend? I think I will run 10w30.

Randy

 

[Old Doug]

I use Rotela because its easy to get and alot of guys use the 15w40 in thelr diesels.I dont think their is a bad oil if you use the grade and weight the builder recammends but you will all ways hear the story of how brand x oil did bad things to my engine.