Cylinder shimming

I have read a publication that suggested to "Deck" the crankcase so the piston just touched the head but would still turn over, and then install a .005 shim under the cylinder. It mentioned this would set a known value for piston/head clearance  and combustion chamber scwish. The addition and subtraction of cyl. and head shims would adjust transfer timing and maintain commpression.

My question now would be... for high RPM or low RPM flying, would later or earlier timing be better suited for which type?

EGR isn't that bad with a muffler. The engine runs fine. Preignition is negligable although compression might need reduced due to the added retained heat.

The engine may run better without EGR and with a muffler but it would be better in most cases to select different parts.

With a throttle the story changes some but not much. Throttle range is improved without EGR. The difference is generally not very dramatic. I have little experience with throttles though.

My understanding of shimming is to bring an engine to design timing usually improves performance.

Phil

EGR is Exhaust Gas Recirculation

Would having the exhaust ports oriented to the sides (rather than front to back) have better performance than adjusting cylinder for optimal intake timing?
I realize that much of this is determined through testing but I would like to avoid reinventing the wheel as much as possible.

I found it a pure wast of time... BUT yes depending on airplane air flow... some will show 50 to 350 RPM increase depending on orientation...NOT KNOWN....did the new air flow change it or did the new timing change it

I can do more with prop selection than fussing with the Random Cox cylinder exhaust port orientation

I only now us the base shims to increase or decrease SPI... as well as judicious sanding of the crank case

Streak , the shims are for reducing SPI and that is about it IMO and i am sure COX did their research before they published the reasoning behind their use .. https://coxengines.ca/cox-.049-tee-dee-timing-shims.html As Fred said Good Pis/cly fit and porting, Boost ports, Hi comp. head , prop. size will get you top power / speed . I am not a fan of the slit cylinder need a clear flow for max exhaust for a fresh combustion , the slits reduce this ( people have cut these out to make TD cylinders ), so that should tell you something ? Air induction for fuel mix makes the difft. too as U have read from Paul tricks.

The shims are for adjusting timing

fit90 wrote:The shims are for adjusting timing

Yep.

You won’t be able to achieve the desired cylinder orientation with those tiny shims. You would be better off by doing careful parts selection for that.

Increasing SPI is better accomplished by correcting the timing with the shims and then grinding a little off of the piston skirt. Decreasing SPI goes back to part selection.

Honestly, if you are just looking for more performance you can save yourself the headache and just buy a new engine from Matt or Bernie. They assemble performers right out of the box.

Ron

A performance tech who worked at the Cox plant made a comment about cylinder orientation
in regards to exhaust ports. With the ports faced forward and rearward the flywheel orientation
to the boost ports would shoot up the fuel mix faster with more volume while the gate was open
providing an increase in power regardless if SPI or not.

SkyStreak wrote:Can some one explain "How To" and the Effects of "Cylinder Shimming" on the "Bee" type engines.
I enjoy flying Oldtimer 1/2A Texaco with my current engines but I am wanting to build a Bee engine and plane a little racier. Not like a GLH but but more like a 1/2A pattern plane. I don't necessarily want a Mouse motor but I do want it to be fast.

Here's an article I wrote about the matter some time ago. Hope you find it useful.   Cheers,  Paul G.

-Hi Guys,

I do get a few inquiries asking how to use cylinder shims. I have to assume you either have your own or want to order some direct from Darren Albert.
Due to random cutting from the automatic screw machine, not all Cox crankcases are machined exactly same. The difference can mean that your (factory spec.) Top Dead Center (TDC) may be off a bit. Strangely enough, a few thousandths of an inch difference can cause some serious RPM changes. The factory spec. (w/ a tight ball-socket joint) is measured when the piston top is flush with the cylinder land (cylinder top shelf). I find an easy way to check this is by drilling out the center of an old glow plug. When the old plug is screwed into the cylinder (w/ no head shims), it will come to rest right on the land. Now, with the piston on the c/shaft, screw the cylinder onto the c/case ass'y. Now gently rotate the c/shaft & see if the piston 'hits' at TDC. If it locks up, you will need shimming, if it doesn't hit at all, you'll want to gently sand down the top of the c/case using 400 wet/dry paper & oil. If the piston 'just' bumps the glowhead fixture but goes over, then the TDC is bang on.
The standard K/K shims come in .002/.003/.006" sizes. The 'shim holder' itself is .005" -.008" thick. So one has to play around a bit to find out what's needed for a particular engine. The Davis Diesel shim measures thick at .015" (which most often eliminates any SPI your engine has). When using these shims, it's most useful to have a micrometer that reads in the ten-thousanth of an inch (.0001"). Such an instrument can tell when a given shim has been compressed over time & is no longer what you thought it was.

Once the piston height is set right, then most often the spec SPI (Sub Piston Induction) will be right on as well (between .008" - .012").
Set this way, your head clearance (or "deck height") will be exactly what the thickness of your head shim(s)are.  i.e. If you have two standard .005" shims, then your head clearance is now set at .010" EXACTLY. For top performance, this is something you'll need to know. How important you ask? If you get it wrong your engine can lose 1,000 RPM or more. Get it 'spot on' & you can see 1,000 RPM gain or more.

BUT, for some reason cylinder shimming isn't necessarily the FINAL WORD on set up. The cylinder exhaust ports orientation is also a part of the performance equation. My best performing engines seem to run with the exhaust ports facing between 45 to 90 degrees from the front. A straight forward facing exhaust port seems not as good.
Also, moving the cylinder up too much can cause the "pinch fit" of the piston to be less, because the piston does not go up as high. Sometimes this matters.

SO, moving the cylinder up or down affects:

1. Compression ratio
2. Glow plug height (from top of piston)
2. Piston to cylinder fit (pinch)
3. Exhaust port orientation (external) relative to the slip stream
4. Boost port & exhaust port orientation (internal relative to the c/case)
5. Cylinder port timing (relative to the piston)
6. Amount of SPI

I am simply AMAZED how this one little item can affect so many different parameters! There is a ray of hope though. If you need to raise your cylinder .005", then it's best to LOWER the head by the same amount (1 shim) in order to at least keep the compression ratio & plug height the same.

The bottom line is whatever you do, the result must be measured with a tach & noted on paper for reference. This is the MOST IMPORTANT TUNING TRICK I know of. On occasion, minor shimming doesn't seem to do very much so you need to know that as well. Some engines respond to shimming a lot while others, not so much. You can now tell from the above that there are numerous COMBINATIONS that can be arrived at by shimming. About the ONLY thing I'm sure of, is that you won't go wrong referencing your TDC at flush & it's a GREAT place to start off from. I can't say with authority that it's the ONLY way to set up an engine, but I've had good success personally.


FINAL NOTE: A standard Cox .049 can run OK with the spec. dimension being out of tolerance. It's mostly the discerning modeler or competition engine man that will want to pay attention to this. Also for the "tinkerer" looking for that last bit of high performance, he can spend a lot of time on the test bench pursuing the ultimate set-up'
for his own particular engine. It's certainly tedious work, sometimes even boring... BUT, once in awhile a super combination shows up & makes it all worthwhile.
Then the next task is to duplicate the powerhouse motor (by the numbers) & wonder why it doesn't run as fast...

AHhh Coxes, ya gotta lov 'em!

Paul Gibeault ( a.k.a. Mr. Open Mouse)

Thanks for clearing up several grey areas, Paul.
Bob

Paulgibeault wrote:
Here's an article I wrote about the matter some time ago. Hope you find it useful.   Cheers,  Paul G.

This is the article that generated my question. I tried some of the techniques mentioned in this article with widely varying results which just caused more confusion.
I did get out a Baby Bee that had some favorable traits base on your article and ran it for the first time in probably twenty years. It started on the first flip and finally peaked at 13-14 thou. with 5.5x4 and 10% fuel. I'm not sure but I think that is pretty good. I guess I could just buy a built engine but I am the kind of guy that needs to know why, and how come, and then do it myself.