|The MGA With An Attitude
ENGINE OVERHAUL - BE-201D -- Pg 4 of 5
Pistons and Connecting Rods
Section A.21, Removing and Replacing Pistons and Connecting Rods
- - Re-ringing Pistons
If you have in mind to replace piston rings while re-using the old pistons, it is important not to damage the ring grooves when removing the pistons. Use a ridge reamer to remove any detectable wear ridge from top of cylinder bore before lifting out the piston. Do not mix up con-rods, as they are offset on the bottom end away from the main bearings. #1 & #3 are the same. #2 & #4 are offset in opposite direction. If you are going to re-ring using the same pistons, do not mix up the pistons, keep them matched to their original cylinders.
Use a cylinder hone to break the glaze in the cylinders and remove all signs of buff marks and/or scratches. If you can't remove all marks with the hone, it needs to be re-bored. When in doubt, measure the bore in several places with a bore gauge. If cylinder is 0.005" (or more) oversize in any one place, or 0.003" (or more) out of round, it should be re-bored.
Clean carbon off top of piston. Look for numbers stamped on top of piston. If standard size it may be blank or may be market "STD". If oversize it should be marked with the overbore amount, like "010" "020", etc. Do not depend on this, as some oversize pistons may not be marked. The pistons might also be odd size if they came from a different type of engine, transplanted by some prior mechanic with a "better idea". Use a dial caliper to measure the cylinder bore at top end as a quick check.
If you don't have a bore gauge, you can use a new piston ring, same size as the piston. Get a new piston ring (not a used one). Insert ring into cylinder and position it about 1" down the bore. Measure the end gap with thickness gauges. Brand new spec (or re-bore with new ring) should have .008" to .014" gap (preferably closer to .008"). Add .003124" to gap for every .001 diametrical wear in the cylinder. A .005" oversize bore will have .016" larger ring gap. If the ring gap is .030" or more, it needs to be re-bored. Beware that some piston rings may be intentionally oversize, so you may find a gap smaller than .008", in which case the method doesn't work. The idea for oversize rings is that you can selectively grind or file the ring ends to achieve the desired 0.008-0.014 inch ring gap even in a somewhat worn engine.
If you hone the cylinder bores with the engine in place, be sure to clean everything thoroughly afterward. You do not want any of the grinding swarf left inside the engine. This is especially important if you are leaving the crankshaft in place while you hone the cylinders. Clean everything clean, clean, clean afterward.
When in doubt about any of this, take the block to a machine shop for evaluation (and expect them to tell you it needs to be re-bored (because that's how they make a living). If the engine block goes to a machine shop for re-boring they will most likely hot tank the block for cleaning. That process ruins the cam bearings, and new cam bearings will have to be installed using some special tools.
Section A.22, Dismantling and Reassembling Piston and Con-Rod
If installing new pistons, remove the pinch bolt at top end of the con-rod. Push out the wrist pin to separate piston from con-rod. Do not re-use the old wrist pin. Every new piston should have a new wrist pin hand selected for perfect fit. These are hand push fit when new, but might require a light tap with punch for removal of the old one. New pistons will almost always be supplied with new rings included (not installed), but check with the supplier to be sure.
MG pistons should be symmetrical, but some may have an offset wrist pin or selective oil slots in the skirt. If it says "FRONT" on top of the piston, please oblige. Otherwise, if there are numbers on top, it's just nice to put them in all the same way round. Pay attention to orientation of the offset con-rods.
Section A.23, Fitting Gudgeon Pins
Follow the Book. Important point here is that the wrist pins are selectively fit to the piston journals with very close fit. Wrist pins are not interchange with different pistons except with some extremely accurate measurement. New pistons always come with matching wrist pins, so don't mix them up.
Section A.24, Removing and Replacing Piston Rings
If old pistons are to be re-used, remove the rings. A ring expander is a very handy tool (but you could do without in a pinch). Clean dirt and carbon out of the ring grooves. A groove cleaner is a handy tool, not very expensive. In a pinch you can use a broken piston ring held in pliers to scrape the groove clean, but be careful not to remove aluminum material from the flanks of the grooves. New rings must slide into the grooves with no interference, no stick or drag.
Install new rings on the pistons. The ring expander is even more handy for installing rings than for removal. Be very careful not to break a piston ring, because new sets will never contain any spare rings. Follow instructions supplied with the rings. Some may have a small mark on the top side, small dots for instance. Some may have a small chamfer on the outer top edge. Some may have a very small angle on the OD, which makes it important to get it right side up, but those will always be marked. Some may have a one piece cast iron oil wiper ring (bottom ring), others may have a three piece oil ring consisting of two thin rings with an expander piece in between. Follow instructions to the letter. The three piece ring has to have the ends of the inner spacer butted together, not overlapping. In rare cases the oil ring may have an additional wave spring expander to be installed first in the groove. Position the rings with the open ends rotated to various positions, intentionally misaligned about 180 degrees (but none exactly in line), to make the compression leakage path as long as possible.
Section A.25, Piston Sizes and Cylinder Bores
While the Book notes replacement pistons available in .010" increments up to .040" oversize, there are also .060" oversize pistons available as aftermarket parts. 1600 engine pistons might also be fitted to a 1500 block with matching re-boring. 1622 engine pistons (usually flat top pistons) might likewise be installed in 1500 or 1600 blocks with appropriate re-bore. Before re-boring and installing any pistons more than .060" larger than standard you may want to consult "those who know" about cylinder wall thickness, sand core offset, and electronically checking cylinder wall thickness.
- - Re-fitting Pistons
With new rings on the pistons, oil the cylinder wall, soak the piston and rings with oil, and use a ring compressor (generously oiled) to squeeze the rings prior to inserting the pistons into the cylinders. Install a bearing half shell in the big end of the con rod and oil generously before installation into the block. Also oil the crank journal. A series of light taps with a wooden hammer handle (while holding the ring compressor secure against the engine block) moves the piston nicely from the ring compressor into the bore. Rotate the crankshaft to position the rod journal at the bottom center of stroke. Tap the piston down the bore as you guide the big end of the con-rod onto the crank journal. When fully seated, install the big-end bearing and bearing cap.
Install the locktab and nuts, and tighten gently. Rotate the crankshaft to be sure the bearing does not bind. Be aware that the locktab will most likely hit the camshaft so you will not get full rotation of the crankshaft. Tighten the bearing a little and test rotate again. Torque the bearing cap to spec and test rotate again. If at any time the crankshaft goes tight, disassemble and start over. Do only one piston and bearing at a time, testing for free rotation at every step. When finished with full torque and free rotation, then bend up the lock-tabs. In recent years it is common for people to not trust lock-tabs, figuring they may have an adverse affect on determining torque of the securing nut or bolt. If the lock-tab is omitted, then the thread may be secured with a touch of thread-lock adhesive.
Repeat the process for each piston and con-rod bearing. As you install each piston the piston ring drag on the cylinder walls will increase, and the torque required to turn the crankshaft will increase accordingly. When the pistons are at top dead center and bottom dead center you should still be able to turn the crankshaft a little with your fingers, even with all bearing caps torqued up to spec.