Fitting Pistons To Bores

Fitting Pistons To Bore

Care starting the rings in the bore

Tapping the top of the piston with a soft hammer handle

The use of rod bolt protectors


Fitting pistons in the bores…… check that the rods fall under their own weight.( don’t let the rod contact the piston skirt ) If press fit rods don’t fall under their own weight, don’t fit them as they have been assembled incorrectly. Repair before fitting as tight pin bosses can seize up on start up. Before fitting the pistons and rods in the bores mount all the big-end shells.

The big end shells and con rod tunnels should be dry and clean. The most common mistake made at this point is to damage the big-end journal with the thread of the big-end bolts as the pistons are fitted in the bores. To guard against this care has to be taken to ensure the thread of the big-end bolt does not contact the shaft on the way-down. One method is to hold the big-end bolts with one hand while the piston is pushed down the bore with the other. With some engines this is hard to do so the use of rod bolt protectors is the preferred method. These protectors are simply a couple of pieces of plastic tubing around 50mm long that fit snugly over the threads.

Ring compressors come in a few different designs. The most versatile are the ring clamps using the ratchet compressor pliers. Select a ring compressor that is of good quality. The more trouble you have starting the rings in the bore the higher chance you have of damaging the rings.

Oil all the cylinder bores and the big end bearing shells. Oil all the piston pin bosses. Oil all the rings, moving them back and forth in the ring lands to ensure complete lubrication. Make sure the big end is on B.D.C. Clamp the ring compressor firmly and squarely onto the piston. Have the compressor positioned so as to cover about 12mm below the oil ring. Position the piston in the bore with the front mark aligned. Square up the compressor against the block face. Tap the top of the piston with a suitable tool, a soft hammer handle is best as the weight of the hammer head allows you to control the pressure placed on the rings as they enter the bore. When the top compression ring enters the bore stop tapping with the hammer handle.

Remove the compressor and take hold of the big end bolts so as to guide the rod into place on the big- end journal. DON’T KNOCK THE PISTON THE REST OF THE WAY DOWN THE BORE WITH THE HAMMER HANDLE…… Use a smooth pushing motion until the rod is positioned on the journal. The reasons for doing this instead of roughly knocking the piston to the bottom is quite simple. If you encountered a problem as the rings entered the bore(such as lapped over expander or the like) bashing the piston home with a hammer is not going to alert you to any problem. By the time you realise there is a problem the bore will be damaged and the job ruined. Once you smoothly push the piston down the bore you will realise how rough it is doing it the other way. You will also be very confident of correctly fitted rings, correct ring pressure, free running pistons and never be caught out damaging a bore. Any problem you may be able to think of will be noticeable once you experience the correct feel of a piston traveling down the bore. As an example imagine the machinist damaged the bore leaving a small high spot on the lower edge of the bore. If you are pushing the piston down the bore, opposed to knocking it down you will be made aware of the tight spot as soon as the piston skirt contacts the damaged area. Knocking it down would eliminate any hope of you finding and repairing that fault and the piston could pick up in service causing an expensive warranty.

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