View Full Version : GMS 47 Shims
I have a Gms 47 that seems low on compression. there is 3 shims under the head. my question is what happens when you take one or 2 of the shims out ? will it give me better performance? will it run hotter?
08-07-2003, 07:16 PM
Since we are talking about shims, first of all, are they those little copper circles? If so, I took apart my gms .47 to clean it up, and there were no shims in it. I noticed that I have a couple spares in the box, but I dont know what they are for.
08-07-2003, 08:18 PM
I am not sure if you know how to do it but measure your deck height. Top of the piston(tdc) to the top of the sleeve. Then measure the head at the groove for the sleeve and subtract the head from the piston height. I usually keep this close to .015 for sport. Add shims or take away and to get close. If you start blowing plugs add a shim. I hope I've got this right and if anybody wants to correct they can.
As far as it goes it will up your compression if it is a way out and add a little more power. It will also increase the heat a bit but you don't want to run lean. It will not increase the rpm or shouldn't. That has to do with the port timings. I have a rossi that (old Black head) when I set up for Q500 a few years ago would pull a 9x8 with real authority. Just was slow on winding up. In the air all I could get out of it was 18.2k and it did not matter which prop. The reason I know is I have an airborne tach and speed indicator.
If this helps let me know.
08-08-2003, 06:29 AM
In a number of engine threads just lately there have been many references to shims, adding or removing them. Most of the information in this regard is correct. However not all shims are the same thickness. If a manufacturer supplies spare head shims they are usually different thicknesses. To track results accurately one needs to know exactly how much the head is being raised or lowered.
Sorry folks, it is a fact of life that when you start playing with engines you have to be specific.
I found that I needed all the shims on my GMS before it would run smoothly. Because there are many external factors which affect the way an individual engine will run (including how tight it is, and its previous history..) the best thing is to experiment and see if the change (if any) is to your liking.....
08-09-2003, 07:52 PM
Also make sure it hasn't lost compression through a scored piston. Is this a new or used engine. Its a trial and error thing. Take shims out one at a time, running it each time till it starts to detonate, sounds like frying eggs. If it does, go to lower mitro and,or colder plug. If it doesn't stop detonating put one shim back in. Hope this helps.
Thanks for your comments and would appreciate hearing anyone elses experiences.
Here is what I tried and what I observed. first I tried removing one shim...it ran and flew but wasn't smooth and very hot. then I tried putting that one back in and added another one. this seemed to make it run much smoother and cooler... only had one flight on it like that so will know better after somemore flying time.
anyone elses comments are more than welcome
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