FL350 Rebuild Tips

The Honda FL350R Odyssey has had a bad reputation for piston scuffing and seizure problems. However, utilizing the information here, a 350 can be rebuilt to last for years on one piston.

The FL350R has a 68mm stroke and was first released from Honda with an 80mm bore. The fins on the original cylinder heads were straight front to back. Do to many engine seizures, a recall was issued. The entire top end of the motor was replaced. If the motor had lower end damage caused by a severe top end melt down, the entire motor was replaced. The replacement cylinder had a reduced bore of 78.5mm. The new head had larger cooling fins that are angled out at the front to bring more air in from the sides around the seat. Another improvement was to coat the pistons with a dry lubricant. I am not sure when this change was made.

Since Honda replaced all the 80mm bore cylinders with 78.5mm ones, in theory there should not be any of the old cylinder or heads left. However, many of the old top ends and entire motors made, came in the front door and went out the back, along with brand new replacements.

The source of the problems is the exhaust port bridge. The exhaust port is very wide. Without the bridge, the rings may slide out into the exhaust port then get snagged on the other side. The bridge is a strip of metal from the top of the exhaust port to the bottom. It holds the rings in the piston. The bridge is in the exhaust port, the hottest part of the cylinder. It is only connected to the rest of the cylinder by a small area. The heat does not transfer off the bridge fast enough causing it to expand and protrude into the cylinder, scuffing or melting the front of the piston. The high heat causes the oil to burn off the bridge, also causing piston scuffing.

The 350 is not the only machine with an exhaust port bridge and the solution to this problem has been around for years. Two small holes need to be drilled in the front of the piston where the piston makes contact with the bridge. These holes should be .080" in diameter at 3/8" and 7/8" below the lower ring groove in the center of the front skirt. The holes bring fuel/oil mix from the crankcase to the bridge which cools and lubricates the bridge.

The bridge will still run hotter than the rest of the cylinder causing it to expand some. Material needs to be removed from the bridge in a crescent shape from top to bottom. Remove .004" in the center and taper out to the top and bottom of the port. A good tool for doing this is a flapper wheel. It has many squares of sand paper attached to a drum. I use an air grinder, but a drill may work also. Start in the center of the bridge then move up and down from the top of the port to the bottom while the wheel is spinning. Do this from both ends of the cylinder so that the wheel is spun across the bridge in both directions. I also tilt the wheel at an angle first to one side then the other. This will produce a cross hatching on the bridge similar to the hone finish. The flapper wheel is not quick, but does a very good job. I had a cycle shop relieve a bridge for me once. They used a ball grind stone. Very quick and very messy. I ended up redoing it myself anyway. I start by using the corners of the wheel in the center of the bridge. The wheel will round off. After that I keep it flat to the cylinder wall and slide it up and down. To check the depth, I flex a .004" feeler gauge into the cylinder over the bridge and place a straight edge over the gauge. If the straight edge rocks, not enough material has been removed.

Wiseco piston with lubrication holes drilled then ceramic coated on top and dry lube coated on the skirts.

The bridge will get the hottest in the center so when swelled, it should be level with the rest of the cylinder. When cold, the ring will slide out, but be pushed back in by the taper. Never run a 350 hard without proper warm up.

The discussion between Honda pistons vs. Wiseco vs. anything else is irrelevant, you don't have any choice for an FL350R. No one else makes them and for the other two, it depends upon how much you need to bore it to clean up the damage. Honda pistons are available in 78.5mm, 78.75 and 79. Wiseco is available in 80, 80.5, 81, 81.5 and 82. Wiseco lists the 80mm as a standard bore. Know exactly what you want and ask for exactly that in mm. If you are on a Honda standard bore and order a 1 over Wiseco, you will be throwing away three possible bores. If you happen to have a pre-recall motor with an 80mm Honda Piston, you can replace the rings with a set ordered for a Honda Pilot. Once that piston and/or bore wears out, your only choice will be a Wiseco piston.

The yellow sheet that came with Wiseco pistons say to use .003" clearance. I would not suggest running only .003" clearance. Those with green sheets reference the box which also says .004". The Honda service manual lists .004".I normally run .005" clearance with Wiseco pistons. If you have modified your motor and it is still air cooled, you may need to run more.When you get it bored, tell whoever does it exactly what you want. Do not assume they will know the correct clearance. Write it on top of the piston with a felt tip pen if you have to. The guy you hand it to in the front of the shop is probably not the one doing the work.
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