Sample Repair 3 - this is really a Top Overhaul of all 4 cylinders on a Lycoming O235 L2C.

(1) Received cylinder kits were inspected to verify that all required parts were present and part numbers were applicable to the engine.

(2) Piston rings ( 2 x Compression rings and 1 x Oil Control ring) had to be correctly gapped i.a.w.. Superior Service Letter L04-01A. This involves fitting each ring into the cylinder in which it will be used (using plenty of oil on the rings and cylinder wall) and then checking the ring gap at 1.2", 4" and 6" from the bottom of the cylinder (compression rings) and 1.2", 4" and 5,.5" from bottom of the cylinder for the oil control ring. The rings have to be square in the bore (the piston is used to push the ring into the bore). Feeler gauges are then used to measure the gap between the ends of the ring in the 3 axial locations. The O 235 cylinders have a choked bore so the internal diameter is a few thousandths of an inch smaller at the top of the piston stroke than at the bottom. The ring gap is critical to allow for thermal expansion of the piston rings when the engine reaches operating temperature. It's easy to calculate the dimensional change that could be expected in the piston rings:

 Change in length of circumference = 6.0 x10E-6/F x 441 x Pi x4.375 = 0.036" (Assuming middle of range of coefficient of thermal expansion for steel 5.5 to 9.9 x10E-6 per degree F, max cylinder head temperature for O235 of 500F according to operator manual and assuming rings are gapped at 15C/59F and cylinder bore is 4.375"). So if you don't have at least 0.036" gap, when the rings heat up to max cylinder head temperature, the ends will butt together and any further expansion can only be accommodated through increased rubbing on the cylinder walls and higher thermal stress in the ring. It's easy to understand then , why the cylinder kit manufacturer specifies a minimum gap of 0.045".

If the rings as supplied have too small a gap when fitted in the bore, then they have to be ground to achieve the required gap and the edges dressed to create a chamfer on the edges.

The oil control ring requires a different gap and is checked without the spring fitted. Note these cylinders were Nitrided which is a form of case hardening where Nitrogen is introduced into the surface of the steel at high temperature to create a hard carbide which is wear resistant.

(3) When all ring gaps were set up, the rings were fitted to the piston making sure they were the correct way up. Next, the piston can be fitted into the cylinder  so that the hole for the gudgeon pin is just outside the bottom of the cylinder or alternatively the piston can be fitted to the con-rod on the engine and then the cylinder is slid on over the piston. Either way, plenty of oil is used and a piston ring compressor is required to squeeze the rings so that they will slide into the bore. It's vital at this point to verify that the piston ring gaps are distributed around the piston circumference and are not in line with each other - ideally 120 degrees apart. Plenty of oil is needed on the gudgeon pin and the clearances are so small that just holding the pin in your hand for too long will cause it to expand and not fit into the bore in the piston.

(4) Before fitting the cylinders it was found to be wise to remove the blank screwed plugs for the fuel priming system from the cylinder head while the cylinder was on the bench. This particular engine had priming nozzles on all 4 cylinders and the plugs are extremely tight as received and can be very difficult to remove. It was necessary to alternately apply heat from a hot air gun to the area around the plug in the intake port and plenty of penetrating oil in order to free these plugs. This job would have been far more difficult after the cylinders were mounted. It's also easier to insert the fuel primer nozzle at this point.

(5) The torque plates fitted at cylinder removal were removed one at a time and the new cylinders slid on over the pistons. New nuts (2 different sizes STD2106  and 383-B) were used to secure the cylinder flanges to the crankcase and these were then torqued in accordance with the manual in a 2 stage process using a torque wrench and making sure to apply plenty of oil to the threads of the nuts and studs and also to the mating faces of the nuts and crankcase - where most of the friction is generated.  The original nuts had only only been used twice - once at last overhaul (assuming new nuts were used then - can't be sure of that) and then to clamp the torque plates, but even so there could be thread damage which would increase thread friction  and result in less of the applied torque being converted into required clamping force in the studs. The nuts were tightened in the sequence shown in the manual (starting with top right and working around in a clockwise direction). A smooth continuous movement of the torque wrench is required to have the best chance of achieving the required torque. Also this needs the correct base nut wrenches ( 2 sizes) as there is no access for a normal socket.

(6) With all 4 cylinders mounted, the next thing was fitting the original push rods  and rocker arms but with the new rocker shafts supplied with the cylinder kit. The O235 L2C has solid tappets/valve lifters and so valve/tappet clearances are set using adjustment screws and locknuts on the rocker arms and not by using different lengths of push rod as on those engines with hydraulic valve lifters. The tappet clearances needed to be set before fitting the rocker covers and gaskets. See  Lycoming Service Instruction 1068A and Superior Service Letter L07-01A for details. As the push-rods are now steel (15F22200) rather than aluminium, the target tappet clearance is 0.005" rather than 0.008" previously. LSI 1068A gives the procedure (i.e. first set zero clearance on Nos 2 and 4 cylinders (with each of those cylinders at TDC compression stroke) to side load the camshaft, and then set 0.005" using a feeler gauge on Numbers 1, 3, 2, and 4 cylinders while at TDC Compression for each. The jam-nuts/lock-nuts are torqued to 450 inch-lbs. Once that was done, the rocker covers and gaskets could be fitted.

(7) Now all the components which were stripped to allow cylinder removal had to be re-fitted - exhaust pipes and muffler (using new exhaust gaskets supplied with cylinder kit), induction pipes (with new gaskets), baffles, fuel priming pipes, oil cooler, crankcase breather pipe, cylinder head oil drain pipes, spark plugs, cowlings and finally the battery.

(8) Ground operational test i.a.w. Lycoming Service Instruction 1427C. Engine started first time.  After following the 1427C procedure, the engine was shut down and the oil filter removed and cut open for inspection - see photo - no metal particles or other contamination found. A new oil filter was fitted and wire-locked. A 2 -hour test flight  as per LSI 1427C was then carried out. Appropriate logbook entries and CRS issued etc. Engine now has to go through break-in procedure as per 1427C for next 50 hours or until oil consumption stabilises (formula for calculating acceptable oil consumption in LSI 1427C). 



 

 

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