Why would you use a head shim?????

I did not want to hi-jack the other thread.

So why would I want to put a head shim on my LA .40 or .46?


Thanks
Paul Taylor

~I wish I knew everything you have forgotten about C/L.

Check out my Blog@
http://rootbeard.rcuniverse.com
A link to a slide show of a fun day: http://www.youtube.com/watch?v=V-_sR0ZydSQ

To lower the compression which in effect changes the ignition timing of the engine which allows you to run the engine with different percentages of nitro and/or different prop sizes. Think of the shims as a stepped compression adjustor like diesels have. All in an effort to get the perfect engine run and lap times. At least that's what I think it's for.
Andyh

>I did not want to hi-jack the other thread.
>
>So why would I want to put a head shim on my LA .40 or .46?
>
>
>Thanks
>Paul Taylor
>
>~I wish I knew everything you have forgotten about C/L.
>
>Check out my Blog@
>http://rootbeard.rcuniverse.com
>A link to a slide show of a fun day:
>http://www.youtube.com/watch?v=V-_sR0ZydSQ


Paul,

It appears as if the primary purpose of adding head shims to an otherwise fine engine is best summed up by what one can say at the flying field and during contests:

"Yeah, she runs good now. But I went through blah-blah-blah (this part of the conversation can take many forms, many minutes, many inconsistencies, many total dumps on logic--but I will let you write your own material) before putting everything back to stock and then adding a head shim.

Ya see, what that head shim does and that is so magical...is more blah-blah-blah..."

All of that aside, once you slip in a head shim or two or three y'all become a customizer of engines.

Dan

>All of that aside, once you slip in a head shim or two or
>three y'all become a customizer of engines.
>
>Dan
>
>

Actually it is more like tuning. Changing head shims is a common occurance.

Basically you are adjusting the engine for how hard it hits. You want it to pick up rpm when you point the nose up, but not too much.

When Matt got his first PA 75, he was quite happy with it. But when I first saw him fly, I noticed right away that he was accelerating going up hill in the wingover. What we are seeking is constant maneuver speed whether the plane is flying level, going up or going down. A good test that I learned is to time the plane for two level laps. Then time the wingover. If you fly the wingover correctly, and your engine is set up properly, then the time for two level laps and the time for doing the complete reverse wingover should be approximately equal.

In Matt's case with the PA 75, we added a head shim. This reduced the amount of kick and enabled us to equalize the two times.

Now with the PA series of engines, it is quite easy to do. We have head shims of 2 thousandths, and 3 thousandths and 5 thousandths, etc. Mix and match. With the Magnum engines the head shims are 4 thousandths which isn't too bad. The OS engines come with head shims that are 16 thousandths and that is too much. Going from one to two to three--you are killing the power. It is way too much of a jump. We have some at 10 thousandths for the 35/40, and with these you can use one OS (16 thousandths), two of ours (20 thousandths), one of each (26 thousandths), etc. A range of 2 thousandths at a time would be better, but this isn't bad.

Some people take this to extremes in trying to "tame" their engines. I got one from a modeler once that he wanted me to rework that had 5 OS shims in it (that is 80 thousandths). He had to put new, longer, head bolts on it just to tighten the head down.

You can adjust the number of head shims for tuning, but make sure that the engine is running properly and that a slight addition is all that it needs.

My reason for adding head shims is to keep the engine from running away. If an engine breaks, from a 4-cycle run to a 2-cycle, as I go into a maneuver, but doesn't go back to a 4-cycle when flying level, it has too much compression.

I have a fine-running K&B Series '61 .35. All it needed was a K&B Series '75 venturi restrictor and one Fox Stunt 35 head gasket.

Years ago, I had one of the early, cast-iron FP 40's. It needed 3, super-thick OS head gaskets to tame the beast.

>My reason for adding head shims is to keep the engine from
>running away. If an engine breaks, from a 4-cycle run to a
>2-cycle, as I go into a maneuver, but doesn't go back to a
>4-cycle when flying level, it has too much compression.
>
>I have a fine-running K&B Series '61 .35. All it needed
>was a K&B Series '75 venturi restrictor and one Fox Stunt
>35 head gasket.
>
>Years ago, I had one of the early, cast-iron FP 40's. It
>needed 3, super-thick OS head gaskets to tame the beast.

What you are doing here is trying to solve a problem in the wrong way. Small venturi, super low compression, kill the power. You are probably also running too small of a prop (necessary because of what you have done above). How does the engine behave other than how it sounds. Hold the plane level and tach it. Then point the nose up about 45 degrees to where it breaks into a 2-cycle and tach it again. You may find little or no increase in rpms, and sometimes even a decrease. I might also ask how much nitro you are using. We never used over 5%. You might be killing the power just to try to restore it with nitro.

The amount of head shims needed will depend a lot on prop size, venturi size, nitro used, altutide (affected also by heat), and even the plane you are towing along with where you set your engine.

Some people running in constant 4-stroke will actually increase compression. It is just a matter of how you set your engine.

And "If it runs like a Fox and sounds like a Fox it probably has the power of a Fox."