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Adapt a roller tappet from another engineby Jeffrey Diamond |
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The missing factor is that roller tappets must be locked in alignment. Any increase in lash allows the roller to turn slightly sideways, and after a certain point it will cock at an angle and refuse to rotate, and instead scuff across the tappet. Result: instant junk. Flat tappets, by comparison, should rotate to prevent wear. Alignment by attached linkage Usually, some form of floating anti-rotation linkage connects each pair of adjacent lifters. The length of the link or bar varies with the inter-tappet distance. This is generally the measurement on centers between a pair of adjacent tappets; this may be 2 intakes, 2 exhausts, or one of each depending on the engine design. The SBC distance is fairly small, and the links are too short to permit direct installation in an engine with (generally) a substanbtially larger bore pitch or centerline distance. The link length must be sufficient to permit the tappets to assume any position (zero to full lobe height) without binding. The correct link length will always have some slack in it, regardless of the position of either tappet. The greatest distance required is when one tappet is at full cam lobe height (probably less than .500” in every case), and the other is not. As long as the link can be “jiggled”, you’re safe. You can easily make a dummy out of stiff cardboard with an Exacto for a trial. Turn the engine over two full rotations; if the card rips, it’s too short. As a link alternative, the upper tappet body could have two flats ground in them and some form of flat “figure 8” bar, similar to that used in modern engines, could be fabricated to loosely connect the tappets in pairs to prevent rotation. Google cam manufacturers for ideas on link bars – there are many designs, you may need clearance grinding, &c. All the manufacturers make links for a small range of inter-tappet spans to make their roller tappets semi-universal (not one for each motor), since there has to be slack in them to allow for “1 tappet up, 1 tappet down”. Several cam manufacturers sell replacement link bars (specific to their own tappets) at very reasonable cost in different lengths. Your cam lift may be smaller than their designed-in maximum, allowing for a somwhat shorter link to work. There’s no reason why the links you have with the wrong (donor) tappets (or another set that’s not the right length) can’t just be widening by welding a strip of flat stock in-between the ends. Saw or grind them in the center to insure that the locating buttons don’t pass across the cut. Use a jig to make sure they’re parallel. The weld can overlay the cut ends, but should present to the cover side to prevent interference with the block. Keep the heat away from the finished ends of the slots to retain the hardness and fit to the buttons. Here are some potentially useful tappet-to-tappet center measurements for common engines. Note that the link dimensions roughly track bore pitch, except whjere the pushrods are not parallel, such as BBC. I have only listed reliable data, but some other engines are included so that their pitch distances can be weighed for possible interchange.
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It’s possible to machine a groove or slot in the tappet bore ID, or use a grooved sleeve, to accept a key or lug located into a recess machined into the tappet body. Alignment by fixture Instead of a moving link, a fixed plate (rather than an attached link) could be used to save reciprocating weight. The original tappet button or lug could be used, with a plate slotted for vertical travel, but it must be very strong, absolutely straight & parallel, and hardened against wear. The plate can be attached in several places. The height is obviously the lobe height plus a safety factor for strength, perhaps ¼” on either end, so a .600” lobe needs a 1.100” plate minimum, etc. As thick as the lug is probably safe, but stiffer if it’s solid behind the slot. However: I’m not sure where to attach it. There is are no flat vertical surfaces inside the pushrod compartment, no available threaded bosses, nothing substantial to tap. A single plate could run the whole length and tie all the lifters together, but it’s got to bolt somewhere. If you had the nerve, you could just bolt it into the inside of the pushrod cover in a stovebolt L6. “Hang” it in place on the lugs, its several studs protrude out beyond the gasket edge and through the wall of the modified cover. Slip machined spacers on the studs, put the cover on, put nuts and Stat-O-Seals on the studs.
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