 Techline-June09-H-D 110" CVO -Part VI: Ending thoughts and the wrap-up (cont.)
This article continues here and concludes in next thread.
Axtell Gaskets
Axtell supplies excellent Cometic gaskets with its cylinders, which are what I’m going to replace the stock CVO cylinders with. Twin Cams use an O-ring to seal the cylinder to the engine crankcase deck, which has proven to be very successful over the years. It combines with two OEM (#11273) Viton dowel O-rings to seal oil in the oil return passage in each cylinder on its journey back to the gearcase compartment. Axtell utilizes a Cometic coated-metal base gasket that replaces the main H-D O-ring on the Axtell 110" cylinder base. When using this kit and base gasket, still use the two O-rings for the base cylinder dowels. This base gasket has sealing ridges that mate with the O-rings around the two locating dowels, one of which also serves double duty as the oil return passage. Another sealing ridge is evident on the inner diameter of the base gasket. Do not use the OEM O-rings in conjunction with the CVO 110 or the Axtell multilayer-system (MLS) head gaskets.
The CVO 110 MLS second-generation gasket that H-D hoped would rectify the oil-leaking situation is a three-in-one gasket. It has a metal center with a rubber, Viton-coated, metal gasket on either side. There are two metal rivets, each on a gasket appendage opposite to each other on either side of the bore, to hold the multilayer gasket together. MLS gaskets will not tolerate a rough or warped gasket surface. There are three major problems with the CVO 110 rear MLS head gasket sealing correctly. First, the head gasket surfaces that I have thus far observed are not always smooth enough to allow an MLS gasket to work properly. Secondly, the CVO 110 rear head has a tendency to warp under the extreme heat it encounters due to an up to 16.0:1 air/fuel ratio in the rear cylinder.
Thirdly, the shifty CVO 110 cylinder liner at the top of the cylinders creates a bump that a MLS gasket cannot tolerate effectively.
Interestingly, the thick, fibrous TC 88 head gasket, complete with a fire ring to seal compression in the cylinder and dowel O-rings for oil-return sealing, will help answer the three concerns just listed. This style gasket can accept a rougher gasket surface and can cope with some head warping without leakage. The fire ring can also manage a shifty cylinder liner and will effectively seal oil within the oil return passage. The H-D MLS gasket looks so identical to the Cometic (#554-241) head gasket that I am going to say that, in my opinion, they are the same. Therefore, the irony becomes that I will use the same MLS gasket that fails on the H-D CVO 110 cylinder to help successfully deal with the problem using an Axtell 110 cylinder that has a smoother gasket surface and a cylinder liner that does not move. There is a similar but different head bolt tightening procedure needed for the Axtell cylinders.
Guess what? H-D’s next new attempt, after the failure of the MLS gasket, through no fault of its own, is similar to the TC 88 gasket! There was no part number for these gaskets at the time of writing, but markings on them include “IVAN-AC” and “This side up 4.00".” When using the Cometic MLS gasket with Axtell cylinders, ensure the rivets holding the three-piece gasket together are not pinched between the cylinder and head. If this happens, remove the rivets but keep the gaskets in the same configuration and relationship as when they were still riveted.
Checking Clearances
Mock up the piston and cylinder assemblies, so you can check the BDC piston clearance. The notched rear piston installs in the rear cylinder, notch forward, to provide relief for the front piston at or near BDC. The cylinder bottoms are stamped F and R for front and rear installation. On the mockup, with pistons and wrist pins fitted in the connecting rod bushings, raise the cylinders off the engine case deck far enough to look at the piston bottoms. Rotate the engine slowly and observe the piston skirt interaction as it approaches BDC, changes axis, and then direction. There must be a minimum of 0.060" clearance between piston skirts at their closest point. This is standard stroker and big bore building procedure. If there’s not a minimum of 0.060", remove the pistons and clearance accordingly.
Cam lift, cam duration, intake and exhaust cam lobe separation angle (LSA), valve margin, head design, and milling of the cylinders and/or heads determine piston-to-valve clearance. Piston dome (crown) design, dome height (compression), and bore diameter will reflect some of the above demands. I mention this because it’s crucial to use the pistons for the above applications. Generally, manufacturers save the mechanic and consumer much of this legwork with their designs. However, you must still check and read the instruction sheets! Different manufacturers will have different piston-to-valve clearance specifications. The Axtell piston, with the intake valve open at maximum lift, must have a minimum of 0.090" clearance between it and the piston at TDC. The exhaust valve must have a minimum 0.060" clearance between it and the piston at TDC when fully open. This procedure is done by using a putty such as Plastocine on the piston valve pockets or in the area of closest interaction between valves and piston at TDC when each valve is fully open.
To do this, check that the top end is mocked onto the engine. The engine is slowly rotated by hand. If you feel any resistance, stop immediately and disassemble the top end to determine where the valve contacted the piston dome and relieve the piston. After successful rotation, disassemble and measure the thickness of the putty where the valve and piston made contact. If the clearance meets the called-for tolerances, all is well, and final assembly may proceed after weighing the pistons to ensure equal weight between the two. If your components do not meet the minimum tolerances, relieve the valve pockets to attain minimum tolerance. Then reassemble and check them again until they do. Weigh the pistons and remove material underneath in the thickest area to equalize weight to prevent unwanted vibration.
After this check, slide the cylinders down onto the cases and seat them on the case deck. Rotate the engine to place the pistons at TDC. Then check the squish clearance, which is how far below, above or even with the head gasket deck the piston dome sits at TDC. This subject is complex and depends on piston dome shape and other factors that require a competent performance mechanic. The results of taking the time to engineer the proper squish clearance will pay off with, first and foremost, more power. A squish band is the area (or areas) where the piston comes in close proximity to the combustion chamber as it changes axis through TDC. Air and fuel caught in the squish band get squeezed out at high velocity, generating turbulence in the combustion chamber. Turbulence in the combustion chamber promotes better atomization and less separation of the fuel and air, encouraging it to burn. A proper squish clearance makes the engine both more efficient and less prone to detonation because the air and fuel are more thoroughly mixed. Moreover, fuel mileage will also increase, since you are using the available fuel more efficiently. If you’re confident in your mechanical ability, squish clearance should be set tight, to about 0.030". Closer than this risks piston-to-head contact. A looser squish clearance progressively begins reducing the chamber turbulence that is so necessary to a faster and more efficient engine.
Stock engines usually have 0.050" of squish or above. One Screamin’ Eagle head, a hemi version, has a squish band that is at an angle from the horizontal. If not looking closely, one might think it a simple (and thus inefficient) hemi head. However, the piston’s dome has a similarly angled squish surface to mate with the chamber’s squish band. More common in stock H-D engines, like the Evolution and Twin Cam, is a 0-degree or flat squish band, which is the same as the angled squish band except that the air and fuel are squeezed out horizontally. Axtell recommends a squish of 0.030" to 0.050". Obviously, 0.030" gives superior squish while 0.050" is for the conservative mechanic. In closing, too little squish between the piston top and the flat squish band surrounding the combustion chamber will result in less turbulence, a poorer mix of air and fuel, and, thus, wasted power potential. Too much squish may result in parts interference and disastrous results, apart from performance considerations. Gasket thickness can have a large influence on squish, after the piston relation to deck height, of course.
The last area to check is the lifter covers, which may need slight relieving if they contact the beefier Axtell cylinder bases.
This article concludes in the next thread. Check back issue for pix and extra information. |
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