Question for Paul McBride, the General Aviation News engines expert: I have a Lycoming O-290-D2 engine with about 80 hours since major overhaul (SMOH).
The engine sat for several years without being run.
The #1 cylinder had zero compression, indicating a stuck valve. The engine was then started and the run-up was normal, indicating that the valve un-stuck itself.
After several run-ups with no problems the aircraft was test flown. After about 20 minutes the engine started to backfire.
The cylinders were then removed for inspection. Everything looked good. It was noted that four of the rocker arms had the oil hole plugged and that these rockers were on the exhaust valves.
I could not find any information in the manual that I have indicating which valves should have the rockers with the plugged oil holes.
Other mechanics that I have talked with seemed to think that the plugged rockers should go on the intake valves and the un-plugged rockers on the exhaust valves. Could you verify this and give me the reason for which cylinders get the plugged and un-plugged rockers?
Dan, Arlington, Washington
Paul’s Answer: Dan, I’ve got to admit that you ask a question that I’m afraid I don’t have an answer for — or at least it seems that way to me.
In doing my research on the use of rocker arms, I, too, was unable to find any information regarding the description of the O-290-D2 you mentioned. I found nothing with regard to the O-290-D2 rocker arms having oil holes. I find this somewhat strange because there is no mention in the O-290-D2 Parts Catalog other than the engine uses eight each of the same part number of rocker arms, which is P/N 68519. The only reference to rocker arms that incorporate oil holes is Lycoming Service Instruction 1454A, which excludes the O-235 and O-290 series Lycoming engines.
The only thing that comes to mind is the fact that the engine hadn’t been run and possibly the #1 cylinder, which had zero compression as you mentioned, did, in fact, have a stuck valve.
With zero compression, if you had done a differential compression check, it would have been interesting to learn just exactly where the leak was coming from.
If an exhaust valve is stuck, doing the differential compression check would clearly show the leak by putting your ear near the exhaust stack where you would hear the air leaking. Isn’t hindsight a marvelous thing?
I am at a loss but have included here a section from the Lycoming Direct Drive Engine Overhaul Manual P/N 60294-7 just to refresh the proper way to set the dry tappet clearance on the O-290-D series engines. There may be a chance that, for some unknown reason, these clearances were not quite to specifications when the engine was last overhauled or operated.
From the Overhaul Manual: 6-137. (O-235-C, O-290-D.) Set tappet clearance on these models in the following manner: Check dry or unloaded valve tappet clearance by pushing in on push rod end of valve rocker and checking clearance between end of valve rocker and valve stem tip, using:
- Set tappets on numbers 2 and 4 cylinders at zero clearance. This will load the camshaft on one side.
- Rotate the crankshaft until No. 1 piston is at TDC of the compression stroke. Both valves will now be closed. Adjust each for a 0.007/0.009 clearance.
- Follow the procedure set forth in step B for numbers 3, 2, and 4 in that order.
- Tappet clearance must be checked after engine run-in. At that time the clearance desired is 0.010 inch. However, 0.006/0.012 is acceptable.
Do you know why this engine was removed from service after only 80 hours on a major overhaul? Learning the true history of the engine may provide some clues regarding its past and offer some specific areas to focus your troubleshooting efforts.
Dan, I’ll apologize now for coming up short of a positive answer, but I’ve got to admit, you’ve got a strange situation here.
Paul
The way I read it is that the engine has 80 hours SMOH and then it sat for several years. I think you are confused about the history based on your comment
“Strange ” is an odd word to put in the title of this story. Why would anyone think it a “strange” thing that an engine that had not been started for several years could go wrong.
Probably because they usually don’t have anything go wrong…at least in my 60 years of restarting long setting engines.
Long term inactivity of unknown history can complicate matters but it’s entirely possible the backfiring is caused by corroded magneto points and/or failing condensers, among other possibilities.
The valve train situation could also be at fault but some overhaul specifics are needed. The O-290-D series engine was assembled with solid tappets requiring a dry clearance adjustment procedure and a special rocker arm, P/N 60831. This is the rocker with the adjusting screw assembly at the pushrod end. The O-290-D2 uses P/N 68519 rocker arm without the adjusting screw and hydraulic tappets, not solid, P/N 72877 and P/N 71105 respectively. Your removal of the cylinders would have revealed which tappets are installed.
The drilling and plugging of rocker arm holes was done by overhaul facilities to boost oil flow to the top end of the engine. Some of these changes are sanctioned by Service Instruction and some are industry practice without proper authority. It can get very confusing. I suggest calling Aircraft Specialties Services 918-836-6872 for more detail around this aspect.
Older engines can be a mix of technologies none of which are to your advantage without documentation. Tread carefully here.
See the Lycoming service instruction ; Service Instruction No. 1454A.
The rocker arm with the valve end oil lube hole is used on the exhaust valve, as on the small TCM engines. [ valve stem cooling ].