I don’t see the point of not boring an old engine, it is highly unlikely that the bores are suitable for new pistons. Assuming just normal wear from miles accrued the bores become tapered and out of round.
Tapered is ring wear, while at the very tippy-top above the top rings highest travel point the bore is often of original diameter and decent quality, right below by 1/4 to 3/8th of an inch is a ridge. Below that ridge the bore has become larger than the original bore diameter while from there it tapers back toward the original diameter toward its bottom. This wear is caused by ring pressure and lack of lubrication where the combustion pressure is highest at the top driving the ring outward from the piston against the cylinder wall, this pressure dissipates as the piston travels down the wall so wear reduces; this combines with the least amount of lubrication is found at the upper bore due to the previous forces, distance from crank splash, and very high heat at this point. No engine escapes this it is normal wear the more the miles, the harder the engine is used for power output the more this wear occurs.
Out of round is thrust wear this is a combination of the forces of rotation of the crank assembly on the piston, offset from center of the piston ring, and combustion pressure on the piston. These cause the piston to rock about its pin resulting in different amounts of skirt force being applied to the cylinder walls at various points as the piston cycles through loads imposed upon it. This results in egging the bore diameter in the width direction of the block, what engineers would call ‘normal to the pin axis‘. As with taper this is normal wear and as with taper is related to miles accumulated and how much power was demanded from the engine in those miles.
Needless to say the harder an engine is worked the faster these normal wears occur and the the higher the mileage the more they accrue.
Overfueling has a big effect on this as rich mixtures whether from improper jetting or operating so the power valve is turned on a lot supplies extra fuel which washes the marginal top end cylinder lube off the walls and rings greatly accelerating and increasing bore wear. Oil quality and change frequency and how often the engine is operated either too cold or too hot come into this equation as well for just what we call normal wear.
The hidden surprises occur when you take it apart to find a ring broke and gouged the cylinder wall, the casting had a slag inclusion that wear exposed and is now a depression in the wall, or the casting has cracked in a wall. At this point bore job is mandatory perhaps the affected cylinder(s) need to be sleeved, or the block scrapped. These are all surprises that old engines hold in store and often more.
The wisest approach to engine building is to get it apart to have it firstly cleaned so it can be inspected for damage and dimensions before ordering parts. Guys that skip this and start accumulating parts often months or years before using them are unpleasantly surprised when the castings need corrective work that changes dimensions that cannot use the parts you bought and then you find their return window to the vendor purchased from has long ago closed so you eat the cost of those parts and new ones that will work.
In real terms most blocks clean really nice at .030 over, this leaves plenty of wall strength from a thickness standpoint, actually most mass rebuilders bore the SBC .040 as their standard, your’ll find most 383 bare blocks sold by stores like Jegs, Speedway, or Summit are point oh-forty over and already relieved for a 3.75 inch stroker.
You will find details count a lot, to that end 6 inch cap screw rods and matching pistons allow for less relieving for clearances and have a better shot at internal balancing which if you pull high amounts of power or like to wind it out is a lot easier on the bearings and their caps (mainly the center 3) than external balance.
I’m not a big kit fan as this really isn’t hard to figure out and most kits use to my thinking odd combinations like a forged crank with hyper-cast pistons or a cast crank with forged pistons. So I like to do my own parts picking. If you stay with us here the membership is more than happy to take you through this one step at a time.
Bogie
