OS FS91 Surpass Four Stroke
This OS engine was badly neglected. It didn't have a lot of run-time but it never got AFTER-RUN oil and then sat for a very long time. This is what happens!
The engine is all be destroyed from corrosion as depicted in the photo. The valves are stuck, the piston ring is stuck, and the rod is seized to the wristpin! The bearings are totally destroyed and the crankshaft is rusted as well as the camshaft. The only good news here is, no one attempted to run the engine in this condition. If it happened to start (un-likely) it would have ruined ALL the moving parts. Like it is, the engine is very salvagable with some work.
I'll begin with restoring the crankshaft.
There is some pitting on the counterbalance but it won't be a problem. The main thing is, the rod journal and bearing surfaces are OK. The worm-gear (that drives the cam) is still in good shape and just needs a little extra cleanup.
The crankcase will have to go back to the cleaning vat for several more hours.
These parts (except the piston ring) are in good condition. The piston ring "suggests" the engine did not have much run-time on it! It wasn't broken-in and there wasn't any fuel residue on the top of the piston. I bet this engine didn't have 20 flights on it.
The new front beraing is seated by heating up the crankcase enough for the bearing to drop in without force. A hot-air gun is perfect for this job.
The same method is used to install the rear main bearing. Using the crankshaft to seat the bearings insures perfect alignment. While the engine is still hot, put several drops of oil on the bearings. The heat provides great penetration for the oil. After it cools off rotate the crankshaft to be sure it's smooth.
Lubricate and insert the piston & rod assembly in the jug and connect it to the rod journal on the crankshaft. This may take you a few tries, but it will go on. Do not force it!