In an ideal world, the combustion chamber in an engine would be completely airtight during the compression stroke. However, due to basic mechanical inefficiencies, there will always be blow-by somewhere. But by keeping this to a minimum, your engine is able to produce good power.
Possible ways for the air/fuel mix to escape could be via the piston and rings, through a failed head gasket or the valves in the cylinder head. Because we're dealing with valve clearances today, we'll stick with those.
In modern engines, the poppet valves in a four-stroke engine are either machined perfectly to form a seal against the 'seats' in the head, or lapped in with grinding paste to match them together (older engines). Springs then hold them tight against the seat and, voila, no air leaks.
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| Suzuki GS550E. In this pic you can see the two camshaft lobes above their respective bucket and shim. The cam cap is removed because some clown (previous owner) stripped a thread — in fact they mullered a few of them and decided to replace them with some coarse-threaded bolts! Marvellous. |
Because seats and valves wear after prolonged use, the tiny clearance between the camshaft and valve can decrease until the valve is actually held open slightly. It then leaks, which lowers compression and the engine will no longer start or run properly.
Increased friction and wear will also take place between camshaft and follower because they're always in contact with each other.
Generally, clearances are checked with a feeler gauge when the engine is cold because, when the engine warms up, everything changes — gaps can close or open up as the different metals in the engine expand at different rates. And it varies with engine design.
This is why we need a clearance. A factory-tested method to ensure that the valves have the best of both worlds, hot or cold. You need a slight gap to prevent blow-by, excessive wear and loss of power, but not too large a gap that the engine becomes noisy or is subjected to valve-train damage when the camshaft comes round and hammers the valve open every revolution.
So how much?
Stick with what the factory manual says. For the Suzuki GS550 I'm playing with today, it's 0.001" - 0.003" or 0.03 - 0.08 mm. Double check yourself when looking at the reading on the feeler gauge. It's easy to get confused and mix up thousandths of an inch with mm. If both readings are displayed on the gauge it's much easier, just remember what you're working in.
If you're just checking clearances, anything within that range is good but, if you're going to be adjusting them, go with the wider tolerance. It'll save you doing the job again anytime soon.
Types of adjuster.
There are different methods for adjusting clearances in an engine. Some use a screw and locknut, the benefits being ease of adjustment and the fact that you don't have to change parts. The downside is the they tend to wear (on the loose side) and not really suited to very high revs due to the extra moving parts.
In this GSX750 engine, Suzuki have been clever and used rockers with screw and locknuts enabling them to use a camshaft that was probably designed for an eight-valve motor. Very easy to adjust, and no parts to swap over.
Then we have buckets with a shim for adjustment. It makes for a very compact, neat arrangement with the camshaft usually sat right above the valve. Different thickness shims are used to give the required gap.
The downside for this arrangement is that you actually need to change a part when adjusting. And for some engines, with shim under bucket, this requires camshaft removal. Big job, and you'll need to measure your clearances very accurately beforehand to work out what size shims you'll need.
On the humble GS it's relatively easy. Push bucket down with special tool, use a magnet to pull shim out. Work out what size is needed and refit. Happy days.
You'll need to turn the engine over by the bolt in the end of the crankshaft so you can check your gaps for each valve. According to Suzuki, you can set the lobes in two positions to check clearances — either lobe straight up, or horizontally facing away from the valve. Get a piece of paper and write down your results for each cylinder, leaving enough space to put down the thickness of the shim too. You'll need it later.
The Motion Pro tool slips under the camshaft and allows you to push the bucket down and remove the shim. Easy, years ago I would've struggled with a screwdriver.
The orange-handled pick is to release the shim from the bucket (it will be held by the oil). Once the grip is broken, the magnet will pick it up easily. There is a slot in the bucket to enable access. Just turn it round to the correct position before pushing it down.
The shim itself comes with the thickness marked on it — this one is 2.35mm thick. The writing always faces down into the bucket so the camshaft lobe doesn't rub it away. You may find that some shims have been ground down to another size, so a digital vernier is needed to make the job really easy.
Results for my engine:
Clearance Shim
No.1 inlet — 2.60
No.1 exhaust 0.002" 2.60
No.2 inlet — 2.45
No.2 exhaust 0.0015" 2.70
No.3 inlet — 2.65
No.3 exhaust 0.002" 2.65
No.4 inlet — 2.60
No.4 exhaust 0.002" 2.65
While the exhaust valves are all within spec, the inlets have no clearance at all. The buckets were also hard to turn because of camshaft pressure. This engine should sound and run a lot better once these are adjusted. No matter how well your ignition or carbs are set up, the engine will never run properly until the valve clearances are put right.
Also note I'm working in imperial, whereas the shims are measured in metric. It's going to be easier to work out accurate shim sizes if everything is in millimetres, but I'll sort that out when I get some more shims.
A quick look on good old eBay should have a few secondhand shims on their way to me and we'll swap some round. Might even put a new camchain on while we're in there because it sounds a wee bit rattly.
Cheers folks, more soon.
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