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How to Degree a Cam: Step 2: Find True TDC

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Step 2: Find True TDC


00:00 - The next step of our process is to fit the degree wheel to the snout of the crank and find true top dead centre.
00:06 Ideally for improved accuracy we want to use as large a diameter of degree wheel as possible however this is a compromise with the engine in the car since we need to clear other components so in this case we're using a 240 mm diameter degree wheel from Kelford Cams rather than the larger Moroso wheel that I usually use with the engine on a stand.
00:28 There are various options available to attach the degree wheel to the crankshaft on the LS engine, including a special mandrill which makes it easy to align and rotate the degree wheel when finding true TDC.
00:41 Be mindful if you do use one of these that they do locate positively and that there's no free play in the mandrill.
00:47 In our case, we're just attaching the degree wheel directly to the crankshaft using the existing bolt.
00:54 To do this we're using an alloy washer that has the same inside diameter as the crank bolt and a step machined into it which locates on the inside of the degree wheel.
01:04 This makes sure that the degree wheel is concentric with the crankshaft when bolted down.
01:09 While we're fitting the degree wheel, we want to make sure that we locate TDC approximately in a location of a suitable bolt hole that you can use to attach your pointer.
01:19 We don't need to be too accurate here but this will help when we do install the pointer.
01:24 When we're happy with the location, we can use an impact gun to tighten the crank bolt.
01:30 This doesn't need to be super tight right now but we do want to make sure that it's tight enough not to come undone when we're turning the crankshaft backwards.
01:37 Now we can make up a pointer and attach this to the engine block.
01:42 Here I've simply used some TIG filler wire which is cheap and easy to bend.
01:47 We want to cut this and bend it so that it's pointing at TDC.
01:52 Again the accuracy right now doesn't matter yet but we do want to position the pointer as close to the degree wheel face as we can as this is going to help reduce the effect of parallax error.
02:03 This is a good time to mention the problems we'll have with parallax error in this example as it's not possible to view the pointer from straight on due to the engine bay confines of the car.
02:14 What I recommend in this situation is choosing a reference point in the engine bay that you always view the degree wheel from when reading the degree markings.
02:23 This will entail some degree of parallax error but provided you always use the same viewing reference, this won't affect your accuracy.
02:31 Now that we've got a suitable pointer, we can fit a positive stop into number 1 cylinder spark plug hole.
02:37 I'm using an adjustable brass stop for this purpose but you could also make one out of a modified spark plug.
02:44 We can then rotate the crankshaft clockwise by about 10 to 15° to move the piston away from TDC a little and then wind the stop in until it gently contacts the piston crown.
02:55 Next we want to rotate the crankshaft back and make positive contact with the stop.
03:00 You don't need to apply too much force to the crank but it's important to take up any free play that may exist.
03:07 In this case we can see that we're registering 13°.
03:11 It's worth mentioning here that due to the angle of our camera, we are being affected here by parallax error which is why it looks like the pointer is actually on 11°.
03:21 This really illustrates the importance of understanding parallax error and how it can affect our accuracy.
03:28 Now we can rotate the crankshaft clockwise until we come up against the piston stop again and we can note the degree reading which in this case is 8°.
03:37 If we not add 13° and 8° together and divide by 2, this equals 10.5.
03:44 This means that our pointer should actually be pointing to 10.5° each side of TDC when we're contacting our piston stop.
03:52 We can simply bend the pointer until it aligns with the 10.5° point before rotating the crank back to confirm that we're seeing the same reading at each side of TDC.
04:03 Once we're happy with this, we can then remove the positive stop from the spark plug hole and move onto the next step.

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