Hi 120 guys,
I am happy to finally find this thread!

I bought two new RCV 120's about 8-9 years ago, for a VQ P-61 ARF. At the time I had read glowing reviews concerning scale performance and reliability, and the fact that the engines fit neatly inside a cowl. I broke them in on the bench over two hours each. After completing the model, it sat in my basement until recently. I decided to revisit this old project, and thought it would be a good idea to try one engine out on a single engine arf. It's in a Seagull Yak 54 90 size. I have a 4 bladed 15.5/12 prop, intended for the P-61.

During break-in run I lost several muffler nipples, and the mufflers reliably came unscrewed. I assumed the vibration was from the axial movement of the piston, but after reading this thread I realize there is a lot going on inside that engine.

During a ground run with the plane secured, the engine overheated. The aluminum spinner was nearly as hot as the muffler. The muffler was on the grass. Also, the tabs securing the canopy to the model both broke due to vibration. Got those tabs fiber glassed and epoxied down now.

I have four flights now on the Yak. The first flight was exciting and noisy, ended in a dead stick with the muffler nipple missing. And the pilot figure lost his head, it rattled right off his shoulders.

That evening I put 5 layers of fiberglass securing the firewall to the box, and one layer inside the box. Yesterday I removed the cowl, ran the engine and re-checked the low needle. Mine seems to run best at 2 revolutions open. The high needle is about right at one revolution. I re-tightened the muffler while it was hot, as per the manufacturers recommendations. I am using

I had to make a cowl baffle using a Quaker Oats cardboard container, and also opened up the grill vents and epoxied window screen painted yellow to camouflage the hack job.

The high speed needle needed a long extension, and it vibrates like crazy when the engine runs. I don't think this will hold up long.
Flew her again, and the muffler was once again lose but was retained by the vent hose.

I will try the Great Planes vibration isolation product you recommend, and I want to get that muffler too!
These engines were very pricey, and I have serious concerns about flying a twin powered by jackhammers. But as I am this far into it, call me stubborn. I want to make this work. Any other advice from you with experience will be appreciated.

Don