What do shims do?

Can someone explain to me in layman's terms what role "shims" play in valve clearance and why having the thinnest shims fitted is a bad thing?
Thanks. (KZ1000P-9)

yorkieUK wrote: what role "shims" play in valve timing

In laymens term's nothing.

Shims are for clearance not timing.

Yes, shims are for clearancing. When the engine is first built the shims are some nominal thickness, but as wear occurs the clearance reduces as the valve sinks into the head. As time goes on you need to replace the original shim with one that's thinner to get back some clearance. This cycle repeats again and again. Some people (the idiots) don't properly adjust their valves, and when that happens the clearance goes away and the valve hangs open (where both the valve and seat get cooked and damaged). You can sometimes salvage these engines but you may have to go down 2 or even 3 shim sizes. There are only so many sizes of shims, and when you reach the thinnest possible you have to pull the engine apart and service it with a valve job and reducing stem length. This will restore the shims back up near the original nominal level. By this time your engine should have about 200,000 miles on it.

Shims don’t really affect valve timing very much, although they do have some minor affect.

Shims are used primarily to adjust the clearance (gap) between the end of the valve stem and the cam shaft - the thicker the shim the less space between the two. If the shim is too thick there will be zero gap between the two even when the cam lobe is pointing away from the valve. This may result in the valve never closing completely. This is bad as it will reduce compression and also can ruin the valve seats, etc. from excessive heat. If the shim is too thin the space between the two will be too great which means the valve will never open all the way – not good for performance since proper intake of fuel and removal of exhaust gases depend on the valves being open a specific distance.

Kawasaki has published gap specifications for each engine. Their specifications take into account expansion of valve train components caused by the engine reaching full operating temperature. If you ignore the specifications and simply install the thinnest shims available, the resulting clearances probably will not fall within the specified range which can then cause problems with the engine.

Nessism wrote: Yes, shims are for clearancing. When the engine is first built the shims are some nominal thickness, but as wear occurs the clearance reduces as the valve sinks into the head. As time goes on you need to replace the original shim with one that's thinner to get back some clearance. This cycle repeats again and again. Some people (the idiots) don't properly adjust their valves, and when that happens the clearance goes away and the valve hangs open (where both the valve and seat get cooked and damaged). You can sometimes salvage these engines but you may have to go down 2 or even 3 shim sizes. There are only so many sizes of shims, and when you reach the thinnest possible you have to pull the engine apart and service it with a valve job and reducing stem length. This will restore the shims back up near the original nominal level. By this time your engine should have about 200,000 miles on it.

Thanks Nessism. My KZP-9 has 104,000 miles on the clock and is in the garage with low compression. While the mechanic was checking it out, he mentioned that he was concerned about the clearance of the valves which had the thinnest shims available.
I'm just trying to understand what is wrong and the implications.
Cheers Mate.

650ed wrote: Shims don’t really affect valve timing very much, although they do have some minor affect.

Shims are used primarily to adjust the clearance (gap) between the end of the valve stem and the cam shaft - the thicker the shim the less space between the two. If the shim is too thick there will be zero gap between the two even when the cam lobe is pointing away from the valve. This may result in the valve never closing completely. This is bad as it will reduce compression and also can ruin the valve seats, etc. from excessive heat. If the shim is too thin the space between the two will be too great which means the valve will never open all the way – not good for performance since proper intake of fuel and removal of exhaust gases depend on the valves being open a specific distance.

Kawasaki has published gap specifications for each engine. Their specifications take into account expansion of valve train components caused by the engine reaching full operating temperature. If you ignore the specifications and simply install the thinnest shims available, the resulting clearances probably will not fall within the specified range which can then cause problems with the engine.

Thanks 650ed.
Do you think it would be a good idea for me to start looking for a good top end and get it swapped before things get worse?

I first would have a pro take a careful look at the cylinder head to see why the thinnest shims are in use (and make sure that is actually true). For example, it could be that the valve seats are eroded or that a previous mechanic recut the valve seats a bit too deep. On the other hand, it could just be that the valves have a problem and just need to be replaced. Perhaps a previous mechanic tipped the valves (shaved a bit off the end of the valve stems) for some reason.

In any case, someone who is a pro should be able to identify if there actually is a problem, and if so, what can be done to correct the problem. Ed

Just don't take it to a regular auto shop. Too many heads have been ruined by hogging out too much seat area. The valves can be "tipped" to give more adjustment room.
Steve

Was it used as a cops bike for most of those miles? I remember reading on here that they do a valve job every 18,000 miles so it may have had 2 or three maybe 4 valve jobs and does need valves, seats or at least tipping ? I would first find out if it really does have tight valves and if the shims are truly the smallest you can get......... like Ed said take it too someone you can trust to check it out.

yorkieUK wrote: Thanks Nessism. My KZP-9 has 104,000 miles on the clock and is in the garage with low compression. While the mechanic was checking it out, he mentioned that he was concerned about the clearance of the valves which had the thinnest shims available.
I'm just trying to understand what is wrong and the implications.
Cheers Mate.

Sounds like it's time to pull the head. Hard to say what's going on without seeing it, but could be as simple as normal wear and tear.

When refurbishing an old head like that the valves and seats will have to be cut. Doing this causes the valve to sink even deeper into the head. In these instances the valve stem needs to be shortened otherwise all the available shims will be too thick. Instead of cutting the valve face I'd just get new valves. They will still have to be shortened a little, but less so than would be the case if the face was cut. And speaking about shortening the valves, it could be that some inexperienced mechanic performed a valve job without shortening the stems. That would put you into valve shim never never land right there.

One other thing, you can only shorten the valve so much before the keepers stick up above the top of the stem. Some machine shops will shorten the keepers in these instances but at that point I'd definitely want new valves to get as much valve face thickness as possible.

The valves wear faster than the seats, so chances are new valves (even if un-tipped) and recut seats will get you back into the middle of the shim range. Might as well tip them while you've got it in bits though.

Are the intake valves out of adjustment too, or just the exhaust? The exhaust valves take much more of a beating than the intake do...

I put in new valves in my 40 year old head. I knew I should have tipped them. In a year or two, I will have to do it all over again.
Steve