C85 to o-200 STC

[Robert Lees]

Has anyone got a copy of either of the two STC's, that they would scan & email to me? I just want to peruse what is required before I decide whether it's a worthwhile excercise.

I know about the crank & rods, but are the low- or high-compression O-200 pistons used?
What case machining is required?

etc

etc

Thanks,
Rob

[Ragwing nut]

Re: C85 to o-200 STC

You can only legally use the 530488 O-200 piston per STC. I know a few that have kept the 85 piston for higher compression, great for experimental, but made sure you have heli-coil sparkplug cylinders or the pistons are notched to clear the brass plug bosses.

Mike

[dmcneil]

Re: C85 to o-200 STC

Rob,

Sorry but I don't have a copy of either STC. And my apologies if the following is stuff you already know...

If you are changing the guts of a C-85 using the C-85/O-200 STC, it is simply a parts swap. No machining is required as long as you are going from a -8 to a -8 or if you are going from a -12 to a -12. The STC uses standard O-200 parts. The standard O-200 crankshaft has a slightly longer stroke than the C-85 crankshaft. Combined with the O-200 pistons, this gives a small increase in both displacement and compression ratio. But, the STC calls for all standard O-200 parts.

The conversion from a -12 to a -8 is a completely separate STC not related to the C-85/O-200 STC. The -12 to -8 STC, available from Don Swords only, does require some machining of the crankcase. I think your best source for information on that STC would be an email directly to Don Swords.

Dan

[VictorBravo]

Re: C85 to o-200 STC

Someone just did this conversion at our airport, and had a problem with the big end of the rod hitting the lower edge of the cylinder skirt if I recall correctly. I think he had to take it all apart again and relieve the skirt, or grind some excess off the end of the rod cap or something. It was just a tiny little bit of interference but you could feel it and hear it. He called the STC holder and reported the problem from what I heard, as such the STC holder can advise you whether this problem only occurs with a certain choice of cylinders or rods, etc.

Bill

[Ragwing nut]

Re: C85 to o-200 STC

Someone just did this conversion at our airport, and had a problem with the big end of the rod hitting the lower edge of the cylinder skirt if I recall correctly. I think he had to take it all apart again and relieve the skirt, or grind some excess off the end of the rod cap or something. It was just a tiny little bit of interference but you could feel it and hear it. He called the STC holder and reported the problem from what I heard, as such the STC holder can advise you whether this problem only occurs with a certain choice of cylinders or rods, etc.

Bill

I will bet a dollar he did not use C-90/O-200 rods with the machined cap. Those are also required when using the O-200 crank to maintain clearances.
There is no other way to have that interference problem.

[NY86]

Re: C85 to o-200 STC

There are two places where interference can be an issue. The O200 crank has a thicker oil slinger behind the nose seal and it can interfere with the case. Later cases have a larger hole in the fillet between the slinger and the seal. the solution is to mill the case bigger. I believe there is a service bulletin on this as it also applies to early C90 engines with newer cranks.

The C85 cam shaft is much thicker between the lobes than the C90 and O200 cams. The rod bolts can interfere here due to the increased stroke.

There should be no issue with the piston skirts as all the associated parts (crank and rods) are changed to match. Maybe if you use C85 pistons? (I'd think not as the pin is set lower, not higher up.) Or rods?