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Post  Iceberg Sun Jul 05, 2020 7:52 pm

I have talked with a few of the gurus with cylinder timing. I have heard that ports at about 45 degrees are optimum. However timing shims are hard to come by these days.

One of the gurus said to me. The little lip on top of the crankcase can be sanded down to allow the cylinder to be screwed into the crank another quarter turn to achieve the 45 degree port aligning.

However this would result in the piston to deck height being a bit higher. How to rectify this? Add another copper shim or two to get the piston. Does anyone have knowledge of this timing performance improvement and would it be worth it to sand the crank and screw the piston a quarter turn further into the block? If so on all cylinders? Or #1 #4 etc?

Just trying to get all the facts before doing anything too gutsy:) Huh... Huh... Smile

Thanks
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Post  pkrankow Sun Jul 05, 2020 9:42 pm

The threads land where they land. This means the exhaust ports are where they are. No, do not try to alter a case to change exhaust port location. When you shim it back the ports will be in the same place!

Yes alter a case so you can change piston port height by shimming to optimum locations... but only if necessary.

Mix and match several engines to get the exhaust port locations. In most situations it is cosmetic because these engines are not put on a plane that needs said performance.

There are blank cylinders to cut ports into for a specific case... but these are rare.

Phil
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Post  aspeed Mon Jul 06, 2020 10:22 am

Most mods to the port height would be shimmed up. Removing material from the case should be machined accurately to keep/make the cylinder square IMHO. Not sayin that I have not sanded any. If shimmed up, then there is a compression loss. This can be remedied by taking the same amount from the step of the cylinder where the plug seals. Lathe and boring bar. There may be some shims available somewhere. Maybe an .074 shim or from some other motor if there are none made specifically.
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Post  fredvon4 Mon Jul 06, 2020 11:40 am

between Paul Gibeault's excellent mouse race motor set up and very good info in your own thread , I would think you have all the physical and engineering info to be had

https://www.coxengineforum.com/t14169-cylinder-performance-figures
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Post  sosam117 Mon Jul 06, 2020 1:09 pm

@aspeed wrote:Most mods to the port height would be shimmed up.  Removing material from the case should be machined accurately to keep/make the cylinder square IMHO.  Not sayin that I have not sanded any. If shimmed up, then there is a compression loss.  This can be remedied by taking the same amount from the step of the cylinder where the plug seals.  Lathe and boring bar.  There may be some shims available somewhere.  Maybe an .074 shim or from some other motor if there are none made specifically.

I wrote a nice article (instructions) on how to make your own shims. You can use either brass or aluminum sheets to make the shims out of? Even had people ask "IF" I would make a few for them?
You can shim the cylinder up to have the exhaust ports at 45° angle from either front to back or side to side?
I have some where the exhaust ports are front to back, side to side, and 45° (1/2 way in between) that.

Here is the link:
https://www.coxengineforum.com/t13945-shimming-cox-cylinder-for-transfer-and-exhaust-timing#180685

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Post  Iceberg Tue Jul 28, 2020 11:11 pm

Hi HappyDad

Thanks very much for helping to locate and send the timing shims. Very much appreciated!! I will test over the next week or so. the three thicknesses .02 .03 & .06 I guess no idea which are the closest to perfect to get the 45degree timing as mentioned being optimal by some of the "know"?

Thanks again and talk soon
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