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I'm not sure I've seen the e-mail with the original qustions, or all the replies, so I may be missing something or repeating what has already been covered. That said, here goes:
The STC for an O200 crank in a C85 requires O200 rods and pistons. No other changes are required nor are any authorized. No timing changes, cam changes or carb changes. I am not sure if the STC covers the C75 or not, but that's easy to get around as Continental allows converting from a C75 to a C85 and the Ercoupe TCDS allows use of the C85. Just more paperwork, no approval pitfalls. The engine remains a C85 and no airframe modification has occured, so no paperwork or STC is required.
I know of no legal way to install an O200 cam and/or carb in a C75 or C85 short of a one time field approval. (good luck...) Same goes for building an O200 using a C75/85 case. That case was never listed on the O200 parts list, and therefore is not authorized. However, I know that Don's Dream Machines does hold an STC to make modifications to the engine mounts, but I'm guessing it's to modify the C75/85 case to the -14 configuration, so it can use the O200 style mounts. Probably not what you were looking for...
(Don's also owns an STC to allow use of the O200 crank, similar to the one Aircraft Specialties owns).
There is an STC to install a modified O200 carb on a C85. This is not a performance enhancement, just a way around the fact that the correct M-S carb for a C85 is more rare than hen's teeth. (Why anyone would want to spend all that money to get rid of a simple, functional carb like a Stromberg is beyond me...)
If your engine is not a C75 or 85 as defined in the appropriate TCDS., or properly altered by STC or field approval, then you are going to find it hard to get anyone to sign the thing off at annual. If you hide the fact that the engine is not properly documented you are putting the inspector in jeopardy and should be drawn and quartered. It's hard enough to find good folks to inspect these old birds without tricking them into putting their heads in a noose.
Skyport does not sell an STC to convert a C or CD to a D. No STC is necessary. An STC is approved data to support a modification. The manufacturer's TCDS is also approved data and describes the modification, so the approval is already there. Skyport does sell the parts, placards, etc. required to accomplish the modification.
The reason the STC for an O200 engine does not extend to the C and CD models is due to the additional down force on the elevator caused by the propwash.
Did I miss anything?
John
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