How To Set Timing On A Twin Cam

HOW TO SPOT Bank check CAM TIMING

Quick Links: Introduction Positioning the Motor Measuring Lifter Position While on the Base of operations Circle Measuring Lifter Position at Overlap Doing the Math

Introduction If you've ever installed a gear up of cams, did your best to line up the marks, then stood back and scratched your caput wondering if they're right, this article is written for y'all. Sometimes cam marks leave room for estimation. Many times I've establish myself going back and forth, moving this cam or that cam a tooth each mode, trying to figure out if I've got them correct or not, without going to all the trouble of attaching a degree wheel and punch indicators and making the measurements. So one day, many years ago, I mentioned this to a friend and mentor. Ron said something to the issue of "You @#$%! Aaron, why don't you lot just spot check your cam timing?", to which I responded "Huh?". "Only mensurate how much lift you have at overlap and compare information technology to the spec!" Too freakin simple. So since then, whenever a set of cam marks leaves me wondering if it'south right, I make some very quick and unproblematic measurements to check my work. It could be done with a lot of precision if you wanted to find true TDC and utilise more precise measuring methods, just you can brand this measurement quick and muddy using crude tools and become precision that'due south much tighter that 1 cam gear tooth. Then it'southward a great method for verifying a cam installation. Actually, all you need to do this is a 6" machinists ruler and a pocket calculator. So here'due south the details on how I go about information technology. In a nutshell, we're going to measure the lifter position with the cams sitting on their base circle, and then we'll do the same with them sitting at overlap, calculate the valve lift from these numbers, and and so compare the answer to the cam specs. It matters not whether y'all do the base circumvolve or the overlap measurement first.

Positioning the Motor We're going to brand two pairs of measurements, 1 pair at Compression TDC and another pair at Overlap TDC. So it'south important to empathise the divergence between those positions and be able to place them. While that may sound basic to some of y'all, it's too important to ignore in this article. Peculiarly with procedures similar pushrod adjustments, I tin can't count how many times I've heard someone requite advice to "position the motor at tiptop expressionless center" without specifying which TDC they're talking about. Well, it's enormously important! Report the above advisedly and you'll see the obvious problem with the aforementioned advice. Information technology takes two total revolutions of the motor for all 4 strokes to happen. That ways the piston passes through TDC twice. And yes, on one of those TDC's, both valves are closed (which is what you lot want for things like pushrod adjustments or removing/installing rocker boxes), merely on the other TDC, both valves are slightly open. So slightly, in fact, that information technology's very piece of cake to mistake them for existence closed. So unless you go through a process of turning the engine while watching the valvetrain, you lot have a very high chance of doing it wrong. Sure, many HD engines accept a timing window and a mark on the flywheel to tell you when you're at TDC, but it'south up to yous to figure out which TDC it is. To identify Compression TDC, watch your valvetrain every bit you rotate the motor in the forrad management. When you encounter the intake valve closing, you know you're on the pinch stroke, as shown in the cartoon higher up. Proceed until the intake valve is fully airtight and and then you accomplish TDC. For the purposes of measuring the lifter position while on the cam base of operations circle, it's not critical that you're precisely at TDC. Technically, anywhere between where the intake valve closes (on the way up the compression stroke) and where the exhaust valve opens (on the mode down the power stroke) will piece of work. Compression TDC is most halfway betwixt those two events, so it'due south a dainty safe place. The valves move very slowly when they're most the seat, and so to exist absolutely sure both are in the closed position, it's safest to be at or nearly Compression TDC. Even so, for the purposes of measuring the lifter position during overlap, how precisely we're positioned at Overlap TDC does thing. It takes very little move abroad from Overlap TDC to change the measurements pretty significantly. So pay close attention to positioning prior to that measurement. If you don't have a timing window, or if you lot're working on the rear cylinder (most HD flywheels don't have a rear cylinder TDC mark), y'all need to determine TDC through the spark plug hole. Use something that won't risk impairment, for instance a plastic straw works actually well. Movement the motor back and forth beyond TDC equally you feel the straw for movement. The piston moves very slowly through TDC and then this is a piddling bit slow, but it tin be done. When you've convinced yourself the motor is halfway betwixt where you can experience the straw move on each side of TDC, stop.

Measuring Lifter Position While on the Base of operations Circle OK, and so you've got your motor somewhere near Compression TDC as described above. We're going to now make a unproblematic, crude measurement of the valvetrain position. There are diverse places and ways to make this measurement, depending on the accurateness needed and the access you have available. Shown here is 1 of the best places to practise it, if y'all take access, which is between the case deck and the top of the lifter. The measurement can exist fabricated with a simple machinist'southward ruler equally shown, or assuming the heads aren't in the style, it can too be washed with a dial or vernier caliper positioned as a depth judge. The machinist's ruler has graduations to x one-thousandths of an inch (.010"), and this is adequate for what we're doing. A caliper can typically read to one i-thousandth of an inch (.001") though, so it'south the preferred way to make the measurement, assuming you lot have both the caliper and the access. I've even done this with the pushrods and valvetrain completely installed, when the motor was equipped with collapsible pushrod covers. Sometimes I'll line up the example deck by sight and put a reference marking on the pushrod itself, and then later on when I do the Overlap TDC measurement, I measure how far that mark has moved up away from the example deck. But fifty-fifty with pushrod covers in the mode, it's possible to practise a similar kind of measurement up at the rocker box. Just precisely make a marker on the pushrod or rocker arm, referenced to something that's not going to move. When you brand the Overlap TDC measurement, merely measure how far it has moved. Of course, in the ideal situation, we'd make this measurement at the valve itself, and that's certainly possible. But it's typically in a difficult to access place on an Hd motor, particularly ane that's installed in a frame. Withal you exercise the measurement, write down the numbers, and repeat the measurement for the exhaust side besides.

Measuring Lifter Position While on Overlap Turn the motor one full rotation from Pinch TDC to put it at overlap TDC. Remember, for this measurement, the position of the motor is critical. It's not like the Compression TDC measurement, where the motor only has to be somewhere near TDC. Even a very fiddling scrap of motor rotation will change your measurement pretty significantly. This can peculiarly be a problem when yous're dealing with an assembled valvetrain, every bit the valve leap force per unit area may very well desire to rotate the engine abroad from where you want it. You can always put information technology in gear and lock the back brake to hold everything however. In that location's some other issue, though, virtually making this measurement with an assembled valvetrain. If you have stock hydraulic lifters, the valve spring pressure is applied to the lifter plunger, which is slowly making it go downwards every bit the oil "bleeds" out of it. Well, if you're taking your measurement at the pushrod, or upstairs somewhere in the cam box, that drain downwards effect is irresolute your measurement. Either y'all've got to exist very quick positioning the motor and then making the measurement, or you should make the measurement as I've done here, which is non at the plunger itself, but rather at the perimeter of the lifter. But if yous're doing this measurement equally a preventive measure out later installing cams, so at that place shouldn't be any valve jump pressure applied however. Of course, if yous're doing this measurement because you call back you might have done it wrong and the motor didn't run correctly, the impairment is probably already done and the rocker boxes volition have to come off anyway. Again, write down your measurement, and repeat the process for the frazzle side.

Doing the Math OK, and so we've taken our lifter position measurements, now we simply run the numbers. It's fairly obvious that lobe elevator is only the deviation between the 2 numbers for a given lifter. In the example pictured, I measured .620" on the base of operations circle and .490" on overlap for the intake lifter. .620" minus .490" tells me I have .130" of lobe lift on the intake side. Not pictured, just trust me, my exhaust lifter position besides measured .620" on the base circle, simply it came up more like .500" on overlap. .620" minus .500" tells me I have .120" of lobe lift on the exhaust side. Before I go grab the spec sheet on the cams and compare, though, I have to retrieve that these are lobe lifts, not valve lifts. The TDC lift specifications for the cams are given in valve lift though. And then I take to catechumen. That'southward pretty unproblematic, though, merely multiply by the rocker arm ratio, which is 1.625 for HD all Evo and later HD air cooled motors: .130" intake lobe elevator times one.625 equals .211" of intake valve elevator .120" exhaust lobe lift times 1.625 equals .195" of exhaust valve lift And so my quick and dirty measurements come up up with TDC lifts of .211/.195". This motor is equipped with Screamin Eagle commodities-in cams, every bit came stock on many Buell models. Then a quick visit to the Cams and Valvetrain page on the Hammer Performance web site reveals the following: Our cams are the kickoff entry in this table. Every bit shown, the TDC lift specs for these cams are .211" intake and .191" exhaust, both of which are very close to the .211" &.195" we measured. And then it's safe to say our cams are installed correctly. For the measurements to come out this close is a flake unusual, although they truly did come up out this fashion for me as I walked through the process to take pictures for this article. More often than not, your measurements will exist slightly higher than spec on one side and slightly lower than spec on the other side, which can be explained mostly by lack of precise Overlap TDC positioning. For example, if the intake in this case was to come up out at say .220", and the exhaust at say .180", more than likely it would mean that the Overlap TDC measurements were taken with the motor turned just a little as well far, because at overlap TDC the intake is opening and the frazzle is closing. Likewise, a lower than spec intake TDC lift coupled with a higher than spec frazzle TDC lift would mean that more likely, the Overlap TDC measurements were taken with the motor not quite turned enough. The important matter, though, is that both measurements are reasonably close to spec, and that they make sense in terms of one existence low while the other is loftier. Even 1 molar off on a cam is enough to put a measurement out in the weeds, and it volition be very obvious. So this method, while quick and dirty and not terribly precise, is however very useful to verify the cams are installed correctly.

Source: http://www.hammerperf.com/ttspotcheckcamtiming.shtml

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