The 100 hour Radial Engine Annual Inspection Series – Valve Adjustments

This is the second topic in the series about the 100 hour / annual inspection. This series focuses on maintenance performed during an effective 100 hr. Inspection on R1340 and R985 engines:

  1. Oil Change with filter/screen & sump checks.
  2. Valve adjustment – Positive or compression.
  3. Ignition timing check – Spark plug servicing.
  4. Compression check – differential.
  5. Air filter and carb – heat system check.
  6. Fuel System Screens.
  7. Cylinder Head Checks.

Valve adjustments provide you with an excellent opportunity to check the hottest side of your engine for unusual wear.

General Info on Valve Adjustment:

Grumman Mallard R1340 Beautiful PropellerThe most potential for required adjustment of the valves usually occurs at the first 100 hr. valve check. Always question; “Why” when more than 1 turn on an adjusting screw (either way) is required to obtain a good clearance.  Problems such as bent/broken pushrod, recessed valve seats, stretched valve stems, worn or failing rocker bearings are often first detected during a valve adjustment!

R-1340 Exhaust pushrods are roughly 1/8 inch shorter than the intake pushrods. R-985 exhaust and intake pushrods are the same part number and can be interchanged. The manual recommends marking the tips “Ex” for exhaust and “In” for intake, a practice that has been dropped over time. However, those markings still provide some excitement when a novice finds pushrod tips marked intake in an exhaust location?

It’s important to know whether your R-1340 engine is equipped with steel exhaust pushrods (unusual!) or aluminum. Engines with steel exhaust pushrods require adjustment utilizing the positive method. R-985 engines do not have a steel exhaust pushrod option. We currently install aluminum pushrods in our overhauled 1340s.

Be Patient! Remember that valve adjustment on an R-1340 or R-985 engine is not an exact science and a feel for the process will develop with practice. I prefer the “Compression method” of adjustment to the “positive” method. If you set them one way and double-checked it the other, you would be hard pressed to find any differences in the final clearance.

That is some serious #radialengine torque going on!

A video posted by Covington Aircraft (@covingtonaircraft) on

The Compression Method of valve adjustment on the R-1340 engines is described in the Pratt & Whitney PN: 123440 Overhaul Manual as: Valve Clearance Adjustment (Optional Method). It reads as follows:

The valve clearances are adjusted in a sequence which conforms to the firing order of the cylinders: 1,3,5,7,9,2,4,6,8.

            Back off all the valve clearance adjusting screws until at least six threads are visible above the rocker. Turn the crankshaft in the normal direction of rotation until No. 1 piston is at T.D.C. (top dead center) of the compression stroke. Insert a 0.010 inch leaf of the clearance gage between the valve adjusting screw and valve stems on No. 1 cylinder, and tighten the adjusting screw until 0.010 inch clearance is obtained. Tighten the valve adjusting screw locknut to a torque of 300 to 350 pound-inches.

            After the valves in No. 1 cylinder have been adjusted, rotate the crankshaft until No. 3 piston is at T.D.C. of the compression stroke. Adjust the valves in the same manner as described for No. 1 cylinder.

            Adjust the clearances of the valves in the remaining cylinder(s), always turning the piston to exact T.D.C. before making the adjustment.

            After the valves in all nine cylinders have been adjusted, rotate the crankshaft two complete revolutions and recheck the clearances. Reset any valve clearance found below 0.010 inch. Do not disturb a greater clearance unless it is in excess of 0.025 inch.

I normally set each valve at .010″ clearance (slight drag on .010″) then check for at least .010 and no more than .017 (book says .025″). Each valve should be checked at least 4 times, as the cam has 4 sets of lobes. Valve Adjuster lock nuts should be torqued 300 to 350 in/lbs.

I find that it’s generally achievable to establish clearances between .010 inch and 0.017 inch. Don’t be upset if you have one or two that are slightly looser than that when finished. If you are installing a replacement cylinder and don’t have time to adjust all the valves, the valves on the replacement cylinder should be adjusted slightly loose  using 0.015 inch as minimum clearance. Full engine valve adjustment should be done as soon as is practical following a cylinder replacement. Slightly loose is better than slightly tight!

The engine Maintenance Manual valve adjusting instructions are posted on the web-site for your review! Call us for tips on making this involved maintenance less painful!

Fly Safe!

Ron Hollis

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