What is the compression supposed to be for a good Cummins 5.9 6BT Diesel? I've found the compression RATIO (17:1) but I can't find the target pressure I should read on the compression gauge. I'm getting 470-480 pounds in 5 cylinders and 425 in #3, dry. Wet compression came up to 510 in all 6 jugs.

I'm getting ready to change the lower end bearings to get my oil pressure up. Should I do a ring job while I'm at it with 470-480 pounds compression?

 

Mine are around 380psi at cranking rpm dry. I'd say your top end is fine. I would not fool with the rings, #3 is about 10% low from the high, but I think 470psi is on the high side of things.

I have to ask why your changing the bottom end? Whats your oil pressure at? Where did you messure it? How many miles/hours on it?

 

Wet compression testing is a bad idea in a diesel. They can start on the oil.

 

That would do some bad things to the gauge if it fired on that cylinder.

 

Uh huh. Might be pretty rough on your hand if you're holding it, too.

 

I'm replacing the lower end because the oil pressure is quite low, I haven't put a gauge on it yet but the dash meter reads minimum and the light comes on at idle. Also there are some serious knocks coming from the crankcase. The engine has 170000 plus miles, at least the odometer stopped at 170000 and I don't know how long ago that was. I've only owned the truck since January.

 

Before you put the time and effort into rolling in new bearings, slap a mechanical gauge on there and see what the pressure really is. The oil pressure senders on these things are notorious for going bad and reading low.

 

Thanks, I'll do that today or tomorrow.

 

Low oil pressure cure? Today I replaced the main bearings in-frame on my 1990 5.9 and the old ones look like new!! The crank journals look excellent and the new bearings turned in quite easily. (I did squash a .001-.004 PlastiGauge in the old ones and it measured .001 but was smashed to oblivian.) The new bearings felt about the same going-in as the old ones did coming-out so I did't plastigauge them. I'm starting to get the feeling that a previous owner already replaced the main bearings on a slightly worn crank.

Tomorrow I'll be replacing the rod bearings in-frame as well. I have two questions now.
1) When I get the old rod bearings out, will I be able to get a clue of piston pin play by feeling-for slop? If I twist the lower rod-ends slightly just to feel the play above at the pistons, should I feel anything? I hope I'll only feel the normal side-slop.

2) After replacing the main and rod bearings, the oil rifles will be empty. Before I'm ready to fire-up, is there a method to re-fill the oil rifles prior to starting the engine?