Drive Sprocket (Part 3)...

Now that we have mentioned the different types of sprocket (rim & spline, and spur), and also seen how to remove or replace the sprockets, it's time to take a closer look at how they wear. Just how do you decide when to replace them? More after the jump.

Drive sprockets have a hard life - they're constantly pushing the chain around the bar, they have the centrifugal clutch weights working on them and they have the chain brake band clamping the outside edge of the sprocket. All in all, they get it tough and it's no surprise that they don't last forever - in fact, you should be looking to replace your drive sprocket every 2 to 3 chains.

There are other ways of telling when the sprocket should be replaced. The teeth will wear down as they transfer drive to the chain by pushing the drive links along, the result is a groove that runs down the face of the tooth and wear on the top edge of the tooth caused by the tie-straps - when this wear reaches 0.5mm it's time for the sprocket to go (as shown in the image to the right).

The action of the clutch weights spinning out and contacting the inside of the sprocket will wear a grove in to metal, and it's not long before a slight lip appears on the inside. This is not normally a problem, but I have found that on Stihl saws, if you don't take the sprocket off for cleaning every now and then, the build of gunk and grime caused by sawdust and chain oil, can actually make removal of the sprocket very difficult.

Whilst mentioning Stihl, the other thing to watch out for are hairline cracks. I've noticed this on just a couple of sprockets in the past and they're caused by that little notch on the edge of the sprocket that drives the oil pump. I guess it's a weak point with the constant heating up and cooling down causing a stress fracture... eventually (if you don't catch it in time) it will break, as you can see from these photographs. Click on the images to see them full-size).

The chain brake operates on the outer edge of the sprocket and when used correctly created very little wear. However, many chainsaw users get in to the habit of revving the saw up, making their cut and then slamming the chain brake on - without waiting for the chain to come to a halt first. As well as greatly increasing the wear on the sprocket, the stresses involved in stopping a saw running at 12,000rpm to a dead halt in a matter of milliseconds does the chainsaw no good at all. Please wait for the chain to stop before putting the chain brake on! The wear can be seen as a series of grooves around the outer edge.

In fact, we'll take a closer look at the chain brake mechanism in a later post. But for now, you know what to look for on your drive sprocket.

Drive Sprocket (Part 2)...

In the previous post we looked at removal of the drive sprocket from a Stihl chainsaw, which uses an inboard clutch (which makes removal very simple).

In this post we'll take a look at the slightly more awkward procedure when dealing with machines fitted with an outboard clutch - in this example we'll be using my Husqvarna 350 chainsaw. Find out more, and watch the video, after the jump...

Those chainsaws that use an outboard clutch need to be tackled differently from those fitted with an inboard clutch. Husqvarna, Partner and the Stihl climbing saws all utilise outboard clutches.

The issue with these machines is that the clutch actually prevents you from removing the drive sprocket, and attempting to undo the clutch weights just turns the engine over (let alone the fact that it's threaded the 'wrong' way).

So, remove the drive sprocket from these machines is a bit more involved; here are the steps:

1. Remove the side plate, bar and chain.


2. Remove the engine cover to gain access to the spark plug.


3. Remove the spark plug and fit in a piston stop tool.


4. Knock the clutch weights off (remembering the thread is reversed).


5. With the weights off, the drive sprocket can be removed along with the roller bearing.

The following video, although based on the Husqvarna 350 chainsaw, should be relevant to all chainsaws fitted with an outboard clutch.

Drive Sprocket (Part 1)...

The drive sprocket is all-important in transferring the drive from the engine to your chain, and it's another consumable item that's under a lot of stress from various elements.

In this post we'll take a look at removing one common type of drive sprocket that you'd find in a Stihl chainsaw. Find out more after the jump...

Removing the sprocket on a machine such a Stihl ground saw, is a very simple process and certainly much easier than a Husqvarna (or a Stihl climbing saw for that matter) - as you may have already seen from the video in the last post.

The reason that it's so simple is due to the method of clutch drive chosen - in these saws the clutch is fitted in the main engine housing and is known as an inboard clutch. With this type of clutch, the drive sprocket can be removed just by taking off a circlip.

It's really that simple - but let's just go through the steps one-by-one:

1. Remove the side plate, bar and chain (make sure you wear gloves when handling the chain).


2. Make sure the chain brake is off (handle pulled back). If the chain brake is left on, you will not be able to remove the sprocket as the brake operates directly on the drive sprocket.


3. Remove the circlip holding the drive sprocket in place.


4. Withdraw the sprocket, it should come out easily, but if you haven't cleaned your saw for a considerable period then it may be nigh on impossible more difficult.


5. You should be able to see a [needle-roller] bearing. This will either be left on the end of the crankshaft or will have come off with the sprocket... either way, don't loose it!

The image below shows this in pictorial form, click on it to get the larger image (it's around 1Mb).

With the drive sprocket removed, you can now assess the wear on it - if you've been using it for a while then you'll probably notice scoring and general wear and tear pretty easily.

In our next post, we'll take a look at how to remove the drive sprocket from a chainsaw such as a Husqvarna, as they use a different method. After that, we'll take a closer look at that wear and how to decide when you need a new sprocket.

Replacing Stihl's Drive Sprocket

I thought I should even up the scores a bit - after doing a short video on taking out a Husqvarna drive sprocket, I thought I should create one for Stihl saws (although due to my own ineptitude you'll see the Husqvarna video in the next post). Watch the video after the jump...

As I mentioned in an earlier post, removal of the drive sprocket from a Stihl ground saw (or indeed most ground saws with an inboard clutch), is particularly easy; and that makes for easier maintenance. So the following video, based on a Stihl MS260, should be relevant for most saws having the same clutch setup. It's about 3mins 30sec long and represents my most advanced video to date! (Although the YouTube quality leaves a little to be desired - I just wish I could show the full MPEG-2 video CD.