Tuning option

Great eyes! That is cool that you found a perfect fit around. Can you mic the thickness too? I'd like to know how much compression is lost by lifting the glow plug/cylinder.

Wow yes Nice Find

Yes, it’s good however, when you eliminate SPI on an SPI cylinder you are also altering deck height which can be detrimental to performance.

Optimal performance/timing/compression is achieved when the the piston is allowed to travel to exactly TDC. You may be able to play with heads and shimming to restore the loss of compression however, the timing can’t be corrected.

Depending on how you plan on using the engine it may not even matter.

Ron

Yes the shims are about .005", or about .14 mm.
I'm learning a lot from members of this forum.  My previous experience with control line was never about speed. Now i'm eager to learn about getting more speed from these little gems from Roy Cox.  Never knew about SPI and how it affects running with a muffler.  I want to try running a muffler on a Tee Dee .049 using the number 4 cylinder.  These shims will help me do that.  Since shimming the cylinder up lowers compression, maybe 60% nitro could be safely used.  

Using high nitro was never a problem in any Cox engine. My standard Cox fuel is 45% nitro. When I run TD's my standard fuel for them is about 67% nitro. My son makes the fuel for me. There's nothing crazy about using it. They start easier, needle terrific and work better. While most use 25%-35% nitro fuels, you really start seeing a difference at 50% nitro. The plug takes a beating if your not shimmed properly. The only problems I've had is the cranks on TD's cracking at the cutout in the shaft. Funny thing is the two I had that broke never separated, and the engine was still running with the broken shaft.

I have some cylinder-shims somewhere. I'd thought about using some for the option of "indexing" the exhaust-port(s) position up to 90 degrees from its non-shimmed position, to possibly eliminate a hot-spot. Running a cylinder "open-face" with an exhaust that's facing the needle-adjust gets hot on the fingers.

I just now did some investigating of my own (on a Cox Bee engine) and found that when using a snug-fit ball-socket jointed piston/rod; that 1/4 turn (90 radial degrees) loosened-from tight.. (no shims) of my #2 cylinder w/SPI....... closes the SPI gap by approx 25%. One "half-turn" back from tight appears to close the SPI gap by 50%. One full-turn appears to close the gap completely.

What I "don't know" (yet..) is how much a given-thickness/cylinder-shim will affect the above discovery? I'll need to find my shims to measure them.

The "brain-damaged" Roddie...

The thread is 40TPI, .025" per rev. Shimming the cylinder will change the clocking accordingly. Generally, you would remove as much head shim as you add cylinder shim. If using low compression head, this means you may have to change to high compression head to get the compression where you want it.

Greg typed faster than me, so as he says. it is a simple calculation.  There are 40 threads per inch on the case/cylinder thread.  That is .025".  A half turn would be .0125" shim.  I think that would pretty much get rid of the SPI on an average motor.  To line up a cylinder 90 degrees, would need an .0063" shim.  Now if someone was to shim up, say, .015", then there would not be much compression even with no shim/gasket.  We used to face off or sand off a bit of the face of the head to get the compression back.  Often the cylinder ring would interfere and need to be removed as well.  I suppose a permanent fix would be to remove an amount of material from the bottom step of the cylinder where the head screws on. Then a similar amount should be removed from the top ring on the cylinder so the plug can bottom out properly. Of course a lathe would be needed as well as a good magnifying glass.  Overall the bypass timing would be higher which may give more power at a higher rpm.  Of course the ball socket may not take the abuse.  It might be a simple way to test out for pipe timing.  That is usually about 180 degrees total exhaust opening.