Bill Scott wrote: This is a rather large subject, but I'll give it a try. Compression checks are dangerous, so take care. Check should be done hot, so operating clearances of parts are near what they are in flight. Do mag saftey check at end of run by turning mags off for a second to see if P leads are working and then quickly back to both. Chock all three wheels. Remove all spark plug leads. Remove lower spark plugs. Remove oil dip stick. Install compression tester probe in cylinder you are testing. Find top dead center for the cylinder you are testing by rotationg prop in normal direction while you cover the probes opening with your finger. When you feel compression remove your finger and continue to rotate prop till impulse snaps, this is top dead center. Its best to have a helper to hold gage and connect air line while you firmly hold prop a top dead center. 80 psi is usually applied into cylinder. Wiggle prop around TDC. and note highest reading. Move prop tip beyond TDC at least a foot and the wiggle prop again and note reading. Dont let prop get away from you, and have helper ready to disconnect air if prop starts to get away from you. Place prop back at TDC and have helper listen for air leakage at dip stick,exhaust outlet, and air filter intake. Air noise at the dip stick tube is from rings and alittle is expected. Air noise at the exhaust outlet is not good news and is the sign of a leaking exhaust valve, this is usually caused by worn out exhaust guides that do not force the valve into proper contact with the seat. Air noise at the engine air intake is cause by a leaking intake valve and is also not good news, and allows hot gase to enter the intake system every time the engine fires an causes lean mixture, lower power and high temps. Parting thoughts: Rings that seal good at TDC and then read low when prop is moved from TDC usually have a wear steps in the barrelfrom ring reversal at TDC. The ring snaps into the wear step and provides near 80/80 readings Leaking exhaust valves can not be fixed by staking the valve. Staking the valve is the striking of the valve stem with a hammer to allow any lead or anything stuck on the valve or seat face to be disloged. Remember 200lbs of spring force close the valves all the time and slam it shut hundreds of times a minute. In my 25 years as an overhauler I have never seen any thing stuck to a valve or seat face. Staking the valve may allow the valve to rock in a worn out guide and make a better seal on its seat and allow a higher compression reading, but this does not fix the worn out guide. Engines with worn out pistons, rings, and cylinder walls may have 80 over 80 compression readings because all the excess oil in the cylinders top end makes an air seal during the test. The amount of oil coming out the engine breather and on the aircrafts belly is a pretty good indication of ring problems if it is excessive. Cerichrome cylinders and current contential cylinders have one thing in common, they are not worth a darn. Bill Scott/AA1B Owner, mechanic with F double A letters Precision Engine 2200 Airport Road Owensboro, KY. 42301 502-684-1083 ============================= From: Bob Steward Subject: Re: SB 388 Wobble Test (long post) One peace-of-mind item I have been meaning to get done on the Tiger is the SB388 "wobble test." (1800 hrs SMOH, and I can find no record of the test in the engine log.) I've looked over the SB, but, as a non-A&P, I'm not sure from the description how much labor and expense is really likely to be involved. Any educated guesses or opinions? If it is done during a regular inspection cycle (cowl off, plugs out, etc.) Then it isn't TOO labor intensive. I'd say ~3 hours for all 4 cylinders *IF* it is just an inspection, and no problems are found or corrected. I should qualify that by saying it would take *me* 2.5-3 hours, your mileage may vary. The basic procedure is quite simple, and for those willing to invest $115 for a nice tool to check their own (and perhaps all the locals?) it is pretty painless. Take the valve cover off of all 4 cylinders. On all hydraulic tappet parallel valve engines (which all but the original engined AA-1x are) you will need the lock plate which holds the shroud tubes for the push rods in place, and for all models you will need a set of valve cover gaskets unless you have the silicone rubber ones already. The purists would say that one needs the seals at both ends of the pushrod housings, too. I find that they can be reused with success until the get stiff. With the plugs removed so that the engine will turn freely (and can't possibly fire!!) Rotate the engine to TDC on #1 and slide the rocker shaft out. "Catch" each rocker and keep them straight as to which location they came from. For most Lycoming engines the rockers are different intake and exhaust. All the new rockers are the same (all "exhaust" style) but older engines that have not had them replaced will have 2 styles and they are NOT to be interchanged! Next slide the pushrods out of the shroud tubes so that they are out of the way. Remove the "rotator cap" from the exhaust valve stem (O-320/360 engines only). If you have purchased the tool from Aircraft Spruce for $115 go ahead and get it WITH the dial indicator... unless you are a machinist had have your own. Bolt it to the cylinder head valve cover flange (where the gasket seals the VCG) and install the valve "extension" on the end of the exhaust valve. This requires a 3/16" allen wrench. Wiggle the valve up and down holding the end of the extension. You should see a movement on the dial indicator. Now hold the extension UP and zero the indicator. Press down on the extension and read the total thousandths of an inch deflection. Record it. Repeat this procedure on all of the remaining 3 cylinders. Now you have your "baseline" info to go by. Note that you didn't measure the actual clearance between the valve stem and the guide, but like a compression test, we now have something to compare to NEXT time you do this. To complete the process, O-235 engines can just be re-assembled. All the rest (hydraulic tappet engines ... O-290 D and O-320/360) all need each shroud tube taken out and then carefully remove the hydraulic plunger from each tappet body. They are then dis-assembled (I suggest 1 at a time, so there is NO chance of mixing components!!) and cleaned and dried. Then they are installed in the original holes with the original pushrods and rocker arms when each cylinder is at TDC. The valve clearance between the rocker and valve stem is checked with a feeler gauge. It must be within the range of 0.028"-0.080" to allow the hydraulic unit to function correctly. Why bother to check this, you ask? It was running "fine" when you took it apart you say. Well its possible, some say LIKELY that the last person to have it apart got the pushrods or hydraulic units mixed up (or even the rocker arms), and in MOST cases the guy who swapped the cylinder for you 2 annuals ago has NEVER even HEARD of SB388B, so he didn't bother to check it after swapping the cylinder (think manufacturing tolerances). If any are out of spec. the pushrod is swapped for a differrent number (longer or shorter as needed) and they are available in ~0.050" steps, so there isn't really much "fine tuning", just try and get in in that 28-80 thousandths range. Be sure to replace the lock tabs which hold the shroud tube keepers, and be sure you don't have any "spare parts". If everything is torqued, then its time to crank it up. The above assumed that none of the readings were outside the factory limits. If they are too loose, then the jugs come off and the guides are replaced. Be sure you ask for the new "hi-chrome" exhaust guides, as many are still not using them. Lycoming says this gives you an increase in interval for SB388B compliance from 400 hours to 1000, but I'd still check it after 4-5 hundred to be sure. If the valves were too tight (How can this be you say, don't they wear loose?) then the "rope trick" is used to release the keepers on 1 exhaust valve at a time, and the valve is pushed back out of the guide into the exhaust port. The guide is then honed to remove the coked oil (carbon) that has built up, and the guide is carefully cleaned of all abrasive and the exhaust system has to be dropped to that the valve can be guided from the exhaust port back into the guide, followed by the "rope trick" to re-install the keepers. Note that this post is meant to give you an overview of the process in layman's terms. It is not intended to be a replacement for the correct factory information, so I have avoided giving specs or enough detail to lead one through it without the factory info. I'd have, at a minimum, sitting right there at the plane, the SB388B and the factory OH manual with current table of limits. If you are "by guess and by gosh"-ing it, then you are a fool of the highest order, and certainly should not be working on an aircraft engine, yours or anyones. This means that if you PAY someone to do this and you "assist" or just stand by and watch with a favorite beverage in hand, you should be able to detect right away if this person meets the criteria of the above mentioned fool, so that you can take the necessary steps to protect yourself. >Can pretty much any competent mechanic do this test, or do I need to find a real engine surgeon specialist? I was surprized that the owner of a very reputable local major repair shop was not familar with the SB when I mentioned it to him. It isn't difficult, a little oily, so wear appropriate clothes, but anyone who can follow instructions and knows the difference in which way one turns the rachet or screwdiver to loosen or tighten can successfully perform this test. However, I would have great respect for the components being serviced, and would suggest that someone who has actually performed this test would be a good side kick for the afternoon. Note that I have actually seen all of the screw ups that are mentioned above. Far from being "theoretical", this is real life examples of what you may find. Bob Steward, A&P IA AA-1B N8978L AA-5A N1976L