Jeep Engines - Oljeep

4.0L POWER TECH IN-LINE 6 (PISTONS AND RINGS) 153Piston ShapeNot all pistons are the same “shape.” Each engine has apiston shape and resulting combustion chamber shape thatwill provide the best engine output at a given compressionratio. The simplest way to solve this problem in wedgeengines is to use a flat piston. Typically, an engine with11 .O: 1 compression ratio will make more power with a flatpiston than it will with a domed piston.For higher compression ratios, a domed piston must be usedand, therefore, the shape becomes important. The sparkplug relief needs to be enlarged, the top of the dome shouldbe “radiused” with as large a radius as possible, and allsharp edges usually located around the valve notchesshould be removed and smoothed as much as possible,especially in the piston notch and spark plug relief areas.Also, don’t use any more dome height than is required.Stated another way-keep the dome as low as possible. Insummary, a low, round and smooth shape is best formaximum power.The vast majority of production engines use flat pistons.Many high performance racing engines use a domed piston.The problem is that the domed piston isn’t always best forperformance. The flat piston engine typically makes asmuch power as the domed piston engine, if not more. Why?First, a typical domed piston is heavier than a typical flatpiston. The higher compression ratio of the domed pistonengine means that you have to use racing gasoline ratherthan “pump” gas or “blends.” Without racing gas, thehigher compression ratio is wasted and actually results inlower output. Domed pistons can be made to work, but theactual shape of the dome is very important. Each enginefamily uses a different shape. If reshaping is not done, thenthe flat piston will provide more power.In summary, for most racers and most racing engines, flatpistons are the best performance choice. However, if goodracing gas is used and the dome is properly shaped, thendomed pistons can be beneficial.Piston-to-Head ClearanceWhen putting an engine back together, the most oftenoverlooked clearance is the distance between the piston andthe cylinder head at top dead center. The minimum pistonto-headclearance should be .040” (low) and .060“ (high).Piston-to-head clearance becomes very important in highcompression ratio engines, especially with domed pistons.This isn’t a good place to gamble. If the piston hits thecylinder head, it can cause the piston skirts to fail and thepin locks to beat-out or fail. In some cases, it can cause thehead gasket to blow. Blowing the head gasket or failing apin lock isn’t a big failure by itself, but what happens nextcan be!The easiest and most reliable way to check piston-to-headclearance is to put the piston at TDC with the head installedand then remove or loosen the connecting rod cap. With adial indicator installed parallel to the connecting rodcenterline, the connecting rod and piston assembly can nowbe pushed up until the piston touches the cylinder head. Theindicator will read precisely the amount of clearance. If theclearance is measured without a head gasket, then add inthe gasket thickness for the total installed clearance. Flatpistons are easier to measure than domed pistons. Modelingclay can also be used to measure the clearance, but youshould leave an extra .025” clearance (i.e., .080” total) justto be safe, especially if you like to cut it close.Valve-to-Piston ClearanceValve-to-piston clearance is equally important. Engines thatmust race in conjunction with 4 or 5-speed manualtransmissions should have the following valve-to-pistonclearances: Intake 0.100” minimum; Exhaust 0.100”minimum. The valve-to-piston clearance is usuallymeasured by placing modeling clay on the piston, manuallyturning the engine over with the head bolted on, the cam,lifters, pushrods, etc., adjusted to zero lash, and the camproperly centerlined. The thickness of the clay left on thepiston is then measured by sticking a steel scale in at thethin spots. A dial indicator can also be used by using a light,‘checking’ spring and measuring the actual clearance ateach point in one degree increments from 10 degrees beforeTDC to 10 degrees after. This second method is moreaccurate but takes a little longer. Also keep in mind that ifyou change the camshaft centerline, you’ll change valve-topistonclearance. Generally, if the centerline is advancedtoward TDC (actual centerline moved from 110” to 106”,for example), you’ll end up with less clearance. Thisclearance should always be measured with a new camshaftat the proper centerline.Piston Race PreparationThe performance secrets for pistons are that they be light inweight and also have a smooth, round top (if it’s a domedpiston). The secret to performance in rings is to get them toseal to the piston and to the cylinder bore at W.O.T. (WideOpen Throttle).For obvious reasons, pistons are not interchangeablebetween various engines. Factory pistons have the pinoffset to reduce piston slap. By reversing the offset(reversing the piston), you can reduce engine friction.High compression pistons have a large dome that sticks intothe combustion chamber. In some cases this dome willstrike the spark plug and close it up. Therefore, the end ofthe plug should be given 1/4” clearance in the shape of aspherical cup. This will also aid in flame propagationbecause the plug will be less shrouded.