Jeep Engines - Oljeep

4.OL POWER TECH IN-LINE 6 (BLOCK) 133Surfacing (“Decking”)Perhaps this topic would be more properly labeled“resurfacing” because the block already has a deck surface.Sometimes this operation is called “decking” the block. Ineither case, the top surface of the block, called the deck, ismilled. This operation decreases the block’s height.Why would you “deck” (surface) the block? One possibilityis to increase the compression ratio. Another is to takeweight out of the blocMengine assembly. Most enginebuilders DO NOT feel the production machining to beacceptable in the area of flatness. The engine builder maywant to remove the production tolerance of the decks.Usually a race machine shop will machine the stock deck ofa new block ,020” to ,040”. They will take very light cuts togain the desired specifications. If it is a used engine that hasseen a lot of heat, they may have to increase these numbersto get it flat enough. In decking the block, the machine shopmust be very careful that it does not make the resulting decksurface too smooth or the cylinder head gasket will not seal.Let’s look at an example. The 4.0L engine has a blockheight of 9.45”. A race engine builder may machine 0.020”off the deck surface to gain flatness, which gives a finalblock height of 9.43”. Never make calculations based onaverage numbers like the 9.45“. Or, if you have to, leaveenough stock so that you can get exactly where you want towithout scrapping parts (.040” is preferred). What if thecrank has a stroke that is ,010’ too long, or a connecting rodthat is .OOS”” too long, or a piston that is .0OS” too tall? Itwouldn’t be the block that is wrong, but the block can fixthem all with no problem if we left our engine builder witha .040” cushion.HoningBefore the honing operation is started, the cylinder wallsshould be inspected for cracks or pits and the bores checkedfor taper and roundness. A cylinder bore is most accuratelymeasured using a dial-bore gauge. For the actual honingoperation, approximately .004” (.002” per side of material)should have been left after boring. (The size of the pistonand the piston design are other factors determining the boresize and the piston-to-wall clearance.) The hone used togain the desired surface finish is stipulated by the choice ofpiston rings, but regardless of the actual finish (perfectlysmooth or slightly rough), it should have a 45” crosshatchpattern. (Moly rings, 10-15 micro inches; chrome orstainless steel, 15-25 micro inches. A 30 micro finish isfairly rough and is close to the hone finish on a productionengine. A 5 micro finish is very smooth.) After honing, thebores should be straight (no taper) and round within +.002”.Having a STRAIGHT bore that is ROUND with NOTAPER is THE single most important step in building arace engine that produces good horsepower, high rpm.and/or high specific output.1. Honing PlatesFor a serious all-out race engine, the use of a honingplate is strongly recommended. It should be made of a1 to 1-1/2” thick steel plate. The honing plate is held onby capscrews. The screws should be torqued to thesame specifications as the standard head bolts.Thickness of the honing plate and installation torqueare very important, but of equal importance (and anitem often overlooked) is the depth of threadengagement. Bolts should be turned all the way in andthen backed off one turn for proper thread depth.2. Why u Honing Plute?At the beginning of the race season when you’rerebuilding your tried-and-true race engine or building anew engine for the upcoming season, the block shouldbe honed using a honing plate. This is the only way tomake the bores straight and round with the throttlewide open. If the bores aren’t straight and round atW.O.T. (wide open throttle), the engine won’t makeany horsepower. If you’re building several engines, orplan to do so over the next few years, it will pay to getyour own honing plate. Most people/engine buildersand machine shops don’t have honing plates forChrysler-Plymouth-Dodge-Jeep engines. Honingplates don’t wear out. Cast iron plates are lessexpensive but can break if mishandled or dropped.Billet steel plates are much stronger arid more durable.3. Honing Plate Thread EngagementThe honing plate should be held on to the block withthe same thread length as the head. This sounds easyenough, but how do you get the two to be the same?Since cylinder head distortion is what you’re trying tomatch, the first step starts with the head aftermachining. The head bolts with washers, if they are tobe used with the head on final assembly, are insertedinto the head and the length of bolt protruding out thebottom of the head is measured. We’ll assume that thismeasurement is ,900”. The bolts that are going to beused with the honing plate are inserted into the plate.