If you use these on a standard cox .049 reed valve engine without the muffler do they increase RPM's.
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT

The TD timing shim usually didnt do anything for increasing power. They reduce the compression and can give you a "friendlier" engine as far as starting and running. If there was any power increase I didnt notice it. You get more out of a TD glow head or one of the Norvel style glow heads as far as power increases... also letting the engine really rev up with a 5-3 prop gets the most out of it.
Bob

These timing shims go between the cylinder and the crank case and decrease the sub piston induction and adjust the gap. If the engine has more than .015 SPI, these shims can decrease the SPI. Thus increasing performance. Do not use a muffler, it will decrease performance greatly.

Edited to change backward info.

They DECREASE SPI - (raising the bottom of the exhaust port towards the bottom of the piston skirt) - they are intended to eliminate all SPI so that when using a muffler, you aren't sucking spent exhaust gasses back into the engine. There are other effects caused by raising the cylinder which are probably not intended, nor beneficial (timing changes, compression ratio changes). If you don't intend to use a muffler anyway, these won't be of any benefit.

OOps I had it backward, I used these in Open Class Mouse race.

As you can see from the notes results are mixed... they help with tuning the engine and any performance change (increase or decrease) depends on the individual engine and how well it was "fitted" by the factory. I would go ahead and buy a set if I were you ... but not expect a lot out of them. As with all model engines there is some variability in timing and they could be a help for you....
Bob

It is a juggling act, If you add shims and raise the cylinder, you can remove shims under the glow head to tailor the changes to maintain performance. One of my BW's had Two timing shims and one glow plug shim and it ran like a scared cat.