Claude,

The OS140RX header fits nicely, except the mounting holes need slight modification. They need to be opened for M5 mounting bolts and moved out about 1mm on each side.

<span style="font-size: larger">EDIT: The Macs header for the OS140RX ,not the 2 piece one OS makes..... Actually Macs makes 3 headers for 30-40 cc rear exhaust gas engines as shown on their website. It may be that one of them would be a drop in choice. I'll measure the bolt CL dimensions of the exhaust flange and see if one of them will work

</span>I'm planning to use a Mintor pipe on mine. I will start with an XOAR 19x10 laminated prop for break in and move up from there if the engine can tolerate it.... I'll post some photos when I have progressed enough with the pipe tunnel to warrant it