How to Degree a Cam: Considerations for Pushrod Engines
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Considerations for Pushrod Engines
03.24
| 00:00 | - When you're dealing with a push rod engine, we have a few more options and considerations when it comes to degreeing the cam. |
| 00:06 | Firstly with a push rod engine the intake and exhaust cam lobes are both ground onto a single camshaft. |
| 00:13 | This means that we can't alter the lobe separation angle as this is part of the cam's mechanical grind. |
| 00:20 | What this means is that if we have accurately measured and degreed the cam based on the timing events for the inlet cam lobe for example, we don't necessarily need to go through the process for the exhaust cam lobe too. |
| 00:32 | Essentially if our inlet timing events are correct, then the exhaust timing events by definition must also be correct. |
| 00:40 | Of course you're welcome to confirm these too as a further sanity check and it will allow you the opportunity to check and confirm that the cam grind is as per the manufacturer's specification. |
| 00:53 | One of the biggest considerations with a push rod engine is that we may have the option to degree the cam prior to fitting the cylinder heads. |
| 01:01 | This is because the cam is located in the block and we can use our dial gauge along with a lifter and push rod to directly measure the cam lift and degree the cam based on this. |
| 01:12 | This is ideal since the cam specification card is often going to represent the timing events in terms of cam lift or lift at the cam lobe rather than lift at the valve. |
| 01:24 | I'll point out that this isn't always the case and some cam cards will express valve lift details or even both. |
| 01:32 | The reason this is so important is that between the lift at the cam lobe and the lift at the valve, we need to account for the rocker ratio and if we're dealing with a solid lifter cam then there's also valve lash to consider. |
| 01:44 | What I mean here is that if we're looking at cam timing events, measured at 50 thou cam lift, and we have a 1.7 rocker ratio, then the equivalent lift at the valve would be 0.050 or 50 thou, multiplied by 1.7 which equals 0.085 or 85 thousandths of an inch. |
| 02:06 | If we also have 10 thou of valve lash then this will also affect the net lift that we will see and in this case 10 thou of valve lash would reduce the net valve lift from 85 thou to 75 thou. |
| 02:19 | You can hopefully see that there's a big difference between the cam lift and the valve lift and failing to account for this will lead to a lot of confusion and wasted time when it comes to degreeing the cam. |
| 02:31 | If the heads are not fitted to the block then it also makes it a little easier to confirm TDC as we can either use a positive stop that bolts across the deck surface of the block with a stopper that contacts the piston, or alternatively you can use a dial gauge on the piston crown directly. |
| 02:49 | For more information on these techniques you can refer to the practical skills section to learn more. |
| 02:55 | If you're performing a cam swap and the heads are not being removed then you can still measure the cam lift directly by temporarily removing the rocker gear and locating the dial gauge on the end of the push rod. |
| 03:07 | Alternatively if you're going to measure the valve timing events then you can locate the dial gauge on the edge of the retainer as you'd normally do. |
| 03:15 | But the key point is that you need to understand that you'll then need to account for rocker ratio and lash if applicable. |
