Jeep Engines - Oljeep

4.0L POWER TECH IN-LINE 6 (PISTONS AND RINGS) 155The best way to show the aspects of this number is byexample. Let's assume that you have a 4.0L In-Line 6cylinder engine which shows a 98.85 cc volume for its 1/2"down number. If you have a flat piston at zero deck, youwould measure 98.85 cc for your 1/2" down fill volume. Ifyou have a 15 cc dish in the piston, you would measure11 3.85 cc. On the 4.0L, the typical (stock) deck clearance is.022" below, which calculates to 4.35 cc. Therefore, if yourdished piston was actually .022" down, then the measuredvolume would be I 18.2 cc.If you have valve notches in addition to these other featuresthat are large enough to hold 3 cc for the intake and 2 cc forthe exhaust, then the total volume would be 123.2 cc. If youswitch pistons to a IO cc dome design and keep your 3 and2 cc valve notches and the ,022" below deck (deckclearance is measured to the tlat of the piston, not the top ofthe dome), what is your resulting volume'? First, the 98.85stays the same. The IS cc dish disappears and is replaced bya negative IO cc (-10) resulting in 88.85. The deck is thesame so that adds 4.35, and the notches are the same at 5 cctotal. Therefore, the end result would be 98.2 cc.Now run through a compression ratio calculation. (Use the4.0L engine and the IS cc dished piston example justdiscussed.) You measured 123.2 cc for your 1/2" down fillvolume (ordinarily you don't know how big the dish is orhow large the notches are. so the total volume is all youknow). List your measurements (known data): Head -64.5cc, Cylinder volumc - 666.52 cc, Gasket thickness - .065",1/2" down fill volume- 123.2 cc.First you need to convert the gasket thickness into a ccvolume. This is easily done by multiplying the gasketthickness by the 1/2" down fill volume, and then multiplythat answer by 2. (In this example, .065 x 98.85 x 2 = 4.35cc.) The piston volume is found by subtracting the two 1/2"down fill volumes (123.2 - 98.85 = 24.35). This is a'positive' volume since it is larger than the calculatedvolume from Chart 2. The total combustion chambervolume is 64.5 + 4.35 + 24.35 = 93.20 cc. This is thevolume above the piston at TDC. The volume above thepiston at BDC is equal to 666.52 + 93.20 = 759.72 cc.Therefore, the actual compression ratio for this example is759.72 + 93.20 = 8.15.Measuring 1/2" Down Fill VolumeWith an ordinary flat top piston, the block is not cc'd. Theneed for cc'ing arises only if the piston is domed or if valveclearance notches or reliefs on the piston top are quite large.The same basic technique is followed in either case.Assuming that part of the piston protrudes above the block,choose some common measurement, say 1/2" (SOO"), androtate the crankshaft until the piston (with piston ringsinstalled) has lowered itself from top dead center (TDC) bythis amount. (If not 1/2", a measurement must be chosenthat will ensure that the entire piston is below the block'sdeck.) This amount should be measured very accurately bya dial indicator. The piston should now be sealed to thecylinder wall with a thin film of light grease, being carefulnot to have any grease extend past the top of the piston. Aflat piece of Plexiglas with a hole drilled through it shouldnow be placed over the cylinder bore and also sealed witha thin film of light grease. With a burette (graduated in cc's)containing colored fluid, fill the cylinder. By knowing thecc's that were added to the cylinder, and by knowing theamount that the piston was lowered from TDC, the volumeof piston's dome (or "dish") can be calculated.PISTON RINGSNote: For piston ring availability and recommended usage,refer to the Cust Pistons und Rings chart earlier in this section.The subject of piston rings seems to come and go as a hotspot for performance activity. Piston rings obviously haveto do their job if the engine is going to do its job-makepower. By the same token, piston rings cause very fewproblems by themselves. If there's a problem with a ring,it's usually caused by something else such as the enginebuilder. We've all broken rings trying to get them on to thepiston or when putting the whole assembly into the engine.We'll be more careful next time, but we still broke the ring.This isn't the ring's fault. With the ring causing so fewproblems once installed, why try to fix it? New inventionsburst upon the scene and everyone tries them. They thenfall back into their position and everything quiets downagain. Rings today are in this position.Basically there are three styles of "trick" race rings: Dykes,Headland and gas ports. A Dykes ring is an L-shaped ringthat we've used in Hemi engines since the 1960s. TheHeadland is a bigger section Dykes that's located right at thetop of the piston. They're usually used with flat-top pistons.Both require custom cut ring grooves, so they typicallyrequire special pistons. Gas ports are typically used withvery thin rings -043" or .03 1 ". Gas ports do work, but theyload the cylinder walls very heavily and tend to wear outquickly. The Headland ring works best at lower speeds. TheDykes ring works best at high speeds. The moly-coatingworks best on any of the rings, but you cannot butt the ringends under load or the ring seal will be gone. You can't takechances with too tight a ring gap with a moly-coated ring. Inspite of this, moly-coated rings are still preferred.The main action today in piston rings by volume is in the5/64" and 1/16" designs. They are NOT interchangeable. The5/64" is the production ring and fits on the production piston.The 1/16" ring is a race/performance ring and only fits onaftermarket race/performance pistons that are cut for thatstyle ring. Typically, all domed pistons use this ring style.One of the characteristics of the ring that is often overlookedis the clearance of the ring in the piston's ring groove. It isimportant to keep this to a minimum. Also, check the sideview of the groove to be sure that it hasn't been worn into aV-shape where the clearance is at a minimum.