Shims for OS 25FX?

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gsjames
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Shims for OS 25FX?

Post by gsjames »

I "miked" the head shim on my OS 25FX's and found it to be 0.007" thick. It is often possible to adjust the performance of the engine and/or increase/reduce the incidence of blown plugs my changing out head shim. Does anyone know of shims of other thickness that will fit the OS 25FX or any machinist that is making them? Thanks!
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lightning
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Post by lightning »

I just take 'em out.
gsjames
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Post by gsjames »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by lightning</i>
<br />I just take 'em out.
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

That's what I did too, BUT I'm thinkin' that it might be a good idea to have a couple of differing sizes just in case I start blowing plugs.
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lightning
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Post by lightning »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by gsjames</i>

That's what I did too, BUT I'm thinkin' that it might be a good idea to have a couple of differing sizes just in case I start blowing plugs.
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">I run OS A3s and I can't remember the last time I blew one.
Typically run 5% fuel for Lim-B and 2548 and 15% for Open.
M. Gillette
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Post by M. Gillette »

You can cut your own using a drafting compass with a blade installed. All you need to find is shim stock the thickness you want.
sgilkey
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Post by sgilkey »

This is a question I answered for myself a long time back but can't remember the answer- I checked a bunch of other engines' shims to find which, if any, fit the FX. I will have to check my shim stash and look at my notes. Turns out I never played with FX shims after all, I run all mine with the shims installed. The stock squish band on the FX with the factory shim is very conservative, so there is probably more power there if the head is lowered. Which helps to explain why the FX seems to like even up to 30% nitro so much.

You could certainly try removing the shim but that might drop it more than you're looking for. Using a set of dividers, you can "cut" shims from a coke can, similar to what Mike describes (but I think a bit easier than trying to fit a blade into a compass). I just scribe the ID till it falls out, and then cut the OD with scissors, eyeballing the width of the shim.

Also, there is a small projection in the squish band of the FX head that is the same ID as the shim, you can lap that down on a piece of wet or dry on a plate of glass. Not easily reversible, though..... I lapped one head and put it in my box for those days when I need just a few more r's and it's been sitting there for five years, never used.
sgilkey
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Post by sgilkey »

i checked my shim stash. it looks like Mag 28Xl shims might work, they have the same ID but smaller OD. nice thing about them is they come in .002 and .004 (in the same pack). but the way the FX shim is located by its OD rather than the ID, it might be tricky to keep it centered when installing.

the GMS25 shim looks like it will work, same ID but bigger OD. its .004" copper and could easily be trimmed on the OD to proper size. they're cheap and readily available from Tower.
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