@doug

The 74 360 has been a real son of a bitch to tune.  I've got a bunch of epoxy in the transfer tunnels in an effort to reduce the crankcase volume.  That, among many other things, has helped jetting considerably.  In stock form it was trying to ping itself to death at highway speeds.  Constant, strong detonation.  In this case it didn't ping once, though I feel like the jetting was on the lean side.  I have more volume in the cases I need to fill, hopefully that will make for consistent and clean jetting at all engine speeds. 

Popping on decel is part of the same issue.  Large crankcase volume, weak transfer delivery under low throttle conditions.  In effect you have to crank the idle screw in further than usual to maintain an idle.  When I get the 500cc motor in and I have time to split the cases I'll stuff it the rest of the way.  As it is though, the usual 360 symptoms are certainly improved.

"Clearance was specified at .003" and that's what I asked the machinist to do.  In hindsight though I remember that this piston was originally intended for use in a water cooled application.  Perhaps .004" or .005" would have been a better choice."

^^ This. 

It looks like a mild but classic "four corner seizure"...  since there are zero signs of piston overheating the logical conclusion is the piston didn't have enough clearance. 

Another factor could be if original engine was a modern "chrome" lined aluminum bore?  In any case the piston cam grind shape might be a different design from what it would be for an air cooled iron sleeve two stroke?  Wiseco is pretty helpful, maybe a call to them would get some answers. 

It could even be the huge 360 bore likely gets "oval" lower down at high load.... transfers would tend to cool the sides of the bore. 

Speaking of FL350, I raced them for several years at Glen Helen, Mickey Thompson Stadium races, and desert...  the original FL350 engines had a similar seizure issue if you ran them hard.  Honda had a "recall" if you seized under warranty and replaced the cylinder for one with milder porting... and of course less power.  Lots of stories I could tell about those days...  fun times.

Anyway, great work you're doing!  Keep it up and thanks for sharing. 

The original application was at least iron sleeved.  I think the water cooling was the reason I went with the FL350 piston in the first place.

Even with the tight clearance, it ran great until I went for the highway cruise.  It survived the initial break in without issue, including a top end pull in excess of 80 MPH.  It ran very quiet, near zero piston slap.  Rather than increase the clearance with the new piston I think I'll just keep it under 75, increase the oil pump output somewhat, double chamfer the ports, up the pilot one size and hope for the best.

I'd just take a light file to the areas of the piston that show signs of sticking and remove a couple thou.  Probably would never have another issue.  

Old school racing thing was to remove piston and dress the high spots after running it a bit...  same thing here and you won't end up with piston slap removing a little from those areas. 

A '72 360 I restored, and sold to a friend did this exact same thing, on a 105+ degree temp day at 5000 RPM steady. This engine has been solid on the fresh bore/Genuine Yamaha piston done about 3 years ago with lots of riding since, and similar RPM's. Same thing, let it cool a bit, and rode it home. I took it apart and found scoring exactly as shown in yours, but with a little more metal transfer. It cleaned up nicely, and I fattened the main jet up a bit and she's a running just fine again, with just a little bit of piston rattle. We also put on a larger front sprocket to bring the R's down on those higher MPH needed runs. 

Richen it up a notch on the needle unless it blubbers bad. Cruising at 75mph, gosh mine would be flat stick with me lying on the tank at that. Doesn't look overheated or anything like that.

MarkT, "Lots of stories I could tell about those days... fun times".
Better start an FL350 post with pics. Sounds really interesting.

@RT325

I bet that would have avoided it.  I go between notch 1 and 2 on this needle.  Leanest notch runs clean at low revs, one step richer likes to break up a bit while cruising.  I spend a lot of time at city speeds so I have been feeling out the lean setting. 

Ideally, some day I'll get it to run consistently at all RPM with the same settings.  I'm convinced more crankcase stuffing is the fix.  We'll see.