Adjusting Valve Clearances on Cummins engine

The Cummins Owners Manual lists all routine servicing for the engine – except valve clearance adjustment. Odd, because its not really that difficult to do…..must be like a 401(k) plan for their mechanics. I did this for the 330HP 6BTA5.9 engine, but the same procedure applies to the 370HP.

The newer ‘Electronic’ QSB engine is slightly harder, because it has twice as many valves (4 per cylinder or 24 altogether).

Why is this necessary??

Just some basics: the whole point of setting a specified valve clearance (also called ‘valve lash’) is to allow for  expansion of the valve opening mechanism. As we all know, metals expand when hot, so Cummins has carefully calculated the amount of clearance to be set on a COLD engine, so that when everything heats –up there will still be minimal clearance in the valve mechanism. Too large a clearance and the engine will be noisy and lose power: too small a clearance and the valves won’t close properly when hot…and that’s a very bad thing.

When to do this:

The manual calls for checking this clearance at 600 hrs, but according to Tony Athens (the resident Cummins guru on www.boatdiesel.com  ) its best done at around 500 hrs, and then every 1000/2000 hrs after that.

Incidentally, his website at www.sbmar.com   has many useful Cummins articles.

 Tools needed:

The good news is that no special tools are needed. You’ll need a good torque wrench (always a good idea to have one), plus 15/16” 10mm 14mm 15mm sockets, hex wrench and a really good set of feeler gauges.

 Parts needed:

Even better news – none. Unless you find that the reusable rubber gaskets on the valve covers are worn, but that’s not likely.

Procedure:

·        The engine MUST be COLD. i.e. left overnight (or longer).

·        Remove the Belt Guard (3 10mm bolts). Now would also be a good time to check the Accessory Belt, like you’ve been meaning to do for a while …

·        Remove the 6 Valve Covers (15mm bolt), Clean and check the gray gasket and ‘O’ ring on each cover. They can be reused if OK.

·        If you feel flush, you can buy a special ‘Barring Tool’ from Cummins to rotate the engine. However, a 15/16 socket on the alternator pulley bolt works fine, and provides a 3X mechanical advantage. You will be rotating the engine Clockwise.

·        Take a good look at the Data Plate, on the Injector Pump on the port side. Note the Valve Clearance (Mine are .010” Intake, .020” Exhaust) and the Firing Order. That’s 10 Thousands of an Inch for the Intake, 20 Thousands of an inch for the Exhaust. Yours may be different.

·        You need to find TDC – Top Dead Center – for each Cylinder. That’s the point where the piston is at the top of the power stroke and both valves are closed. There’s several ways to do this:

o       The Official Cummins way is to find a tiny ‘pin’ behind the water pump on the lower port side of the engine.

o       Then you push this pin in, while turning the engine slowly. At TDC for #1 cylinder, the in slides into a hole in the back of the timing gear.

o       However, this is a lousy method ! because:

o       Unless you are a contortionist, it’s a 2-person operation

o       You MUST remember to remove the pin before rotating the engine, otherwise it can break and ‘cause engine damage’ as they say

o       They helpfully covered mine in white paint. It’s stuck…

o          OK, a much better way is to rotate the engine (via the alternator pulley) a few times, and watch the valves move. You start to see the pattern of: intake valve opens..then closes…then things get still as the engine goes over compression (that is TDC)…then exhaust opens….then closes just as intake opens.

o          When you think you have TDC for a cylinder, try to move the valves. There should be a little side to side clearance, and a little up-and-down clearance. The exhaust valves (with green springs) have more clearance than the intake.

o          Test the clearances with the feeler gauge.

o       For the intake, a .010” gauge should just slide-in, but an .011 or .012 should not. And a .008 gauge should be loose.

o       Similarly, for the exhaust, a .020 should be a snug fit, but a .022 should not fit.

o          I recommend rotating the engine, and checking and recording ALL clearances before adjusting anything. You should find a few are OK, and the rest are no more than .002 or .003 out of specification.

o          To adjust a clearance, loosen the locknut (14mm) a little, then turn the adjuster until the feeler gauge just slides in. Tighten locknut, then check clearance … it is probably now wrong, as tightening the locknut inevitably turns the adjustment screw. Try again… you’ll soon learn to compensate for this. Then tighten the locknut to 18 ft-lbs (which is 24 newton-metres for the metric-inclined). Then check again.

o          Then do 1 complete rotation of the engine, checking all clearances one last time. Just being paranoid…

o          The valve cover torque is also 18 ft-lb.

That’s it for another 1000 hrs. Fairly straightforward if you work carefully. Took me about 3 hrs the first time, on a rainy day when I had nothing better to do.

Jeremy Bell. AT34 - #48  “Tardis”