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How to Degree a Cam: Measuring Piston To Valve Clearance

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Measuring Piston To Valve Clearance


00:00 - Any time we're fitting an aftermarket set of cams to an engine that are going to provide additional lift and duration for the valves, we need to be very certain that we've got sufficient clearance between the valves and the pocket cut out in the top of the piston.
00:14 Now of course if we're dealing with popular cams for a given engine that are designed with the stock piston pockets in mind, then these cams may provide sufficient clearance.
00:24 It becomes more of a problem when we're looking at developing custom cams with really large duration and lift values.
00:33 Of course as with any engine building aspect though, it never pays to make assumptions so in this module, we'll have a look at the technique for correctly checking your piston to valve clearance.
00:45 With this module, we're going to be using our Honda B18C engine, and because the Honda engine uses a VTEC system for its valve actuation, where it has both a high and a low load for the intake and exhaust cams, it's very important before we go about measuring our valve to piston clearance, that the VTEC system is locked to the high cam position.
01:06 This will give maximum lift and duration to the valves, and this is where we need to check our clearance.
01:12 There are a couple of ways that we can go about locking the head into the high VTEC operating mode.
01:19 One if by supplying high pressure compressed air into the oil gallery that actuates the VTEC mechanism, however in this case what we've done is simply disassemble the rocker assembly for number one cylinder and we've mechanically locked the system into the high VTEC operating mode.
01:36 The method of checking our piston to valve clearance that we're going to use today is to lay some playdough across the top of the piston and we're going to then assemble the engine, we're going to place the head back on the engine, time the engine up and then we're going to turn the engine through one full engine cycle before removing the head again.
01:54 Now we're going to be using playdough for this demonstration but you can also use plasticine, it really doesn't matter, it's just whatever you've got easy access to.
02:03 What that's going to do is leave an imprint of the valve opening in the playdough, we're then going to remove the playdough from the piston, and by slicing the playdough with a sharp knife or razor blade through the centre of the valve pockets, we'll be able to get a really good visualisation of exactly how much clearance there is between the piston and the valve.
02:23 So let's go ahead and do that now.
02:28 What we're going to do is start by cleaning down the top of the piston, this is just to make sure that there's no oil there and the playdough or plasticine that we're using is going to be able to stick to the top of the piston.
02:39 I'm simply using a clean rag here and some brake clean.
02:50 Once we've got our piston cleaned down, we can take our playdough, and what we want to do is just apply a thin layer of our playdough to the top of our piston and it's really important to make sure that we get this playdough to sit properly into those valve pocket cut outs.
03:07 So we're going to do that on all four of the valve pockets on the piston.
03:43 Now that we've got our playdough applied to the crown of our piston, there's one more step we need to complete before we can fit the cylinder head, and that's to apply a thin coat of oil to the playdough.
03:54 And this just helps stop the playdough from sticking to the valves as we turn the engine through a complete engine cycle.
04:01 In order to do this, all I'm using is just some clean engine oil, and we'll just apply a little coat of that oil on top of the playdough here and then I'm just going to make sure that that's coating the entire top of the piston using my finger.
04:15 OK so we're at a point now where we can refit the cylinder head onto the engine, so we'll go through that process.
04:58 So at this point, we've refitted our cylinder head, we've torqued that to the engine block and then we've refitted our camshafts and our cam belt.
05:05 At the moment what I've done is timed the engine up on the factory Honda timing marks and now we can go ahead and turn the engine through one full engine cycle.
05:15 It's important for me to just mention here though that when we are checking our piston to valve clearance, we want to do our final checks after our cams are dialled in.
05:25 Obviously if we are checking our piston to valve clearance and our cams aren't dialled in to our specifications, then when we move the cams, this can influence or affect our ultimate piston to valve clearance.
05:37 OK so we're going to turn the engine now through 720 degrees of rotation, one full engine cycle and all I'm going to be doing is looking to come back up on the timing marks that we're on at the moment.
06:05 With our engine now turned through one complete cycle, we simply reverse the process, we're going to remove the cam belt, the cams and the cylinder head again so that we can inspect our playdough on the top of the piston.
06:38 With the cylinder head removed, the first thing we want to do is inspect the indents that you'll see from the valves in the playdough.
06:43 Now these straight away should give us some indication of whether we're likely to be getting close to vavle to piston contact.
06:50 In the case of our Honda B18C here, we can see that there are only light indents in that playdough so we can be pretty confident at this point that valve to piston contact is not going to be an issue.
07:03 However we can go one step further and we can remove that playdough and cut through it with a sharp razor blade or knife and that's going to give us better indication of exactly how close our valve is getting.
07:14 In this instance we're also able to see how close the valve is getting to the crown of the piston both vertically as well as how much clearance there is around the outside diameter of the valve, compared to the outside diameter of the valve pocket.
07:30 So we'll go ahead now and I'm just going to remove a section of that playdough and we'll cut through the centre of it so we can have a look at that.
07:47 So with our playdough cut through the centre, what we can now see is exactly how close our valve is getting to the piston.
07:53 In this case again, we've got absolutely no problem with these cams in our Honda B18C.
07:59 What you can also do once you've cut through the playdough like this, you can use a pair of vernier calipers to measure exactly how much clearance you've got and of course if you're getting a little bit too close to the piston, then it may be that you'll need to have those pistons modified and have those valve pockets cut a little bit deeper.