So it seems that it had been burning oil, again not excessively. Most likely through the exhaust valve. Either it’s not seating fully or a seal has gone. It was fairly easy to clean up most of the carbon on the top of the piston so I could then see that it was 0.75 over-size but what does ‘13’ reference?
By moving the piston to the bottom I could see four score marks. They are not deep, I cannot feel them with my finger but they are equal distance apart on either side of the sleeve.
Once the barrel was off I found where the scoring came from. The rings weren’t damaged from what I could tell. These score marks were not randomly spaced and did not appear to be from a foreign object. They seemed to fit the description of one type of piston seizure. When heat concentrates in the thicker boss of the piston pin which causes more expansion in that area. The symmetry of the marks is called a 45 degree seizure because the marks are usually 45 degrees from the piston-pin axis. I learned this from @Patuca and @Yamahaha, fellow members of the Yamaha XT500/TT500 Forum an invaluable resource.
Thanks to them it all made sense especially after I took the barrel off and saw the piston. It also goes along with my introduction of this project where I mention a possible seizure near Stuttgart on my road trip from UK to Austria.
“Those seizure marks are often found with high compression pistons most notably and non Yamaha pistons. The clearance needed come a rebore with these pistons is often a little more than the Yamaha quote, but are still completed at factory clearances. As memory serves the silica content of the metal these pistons are made of differs and as Patucas information, expansion of the thicker areas of the piston reduces clearance with seizure being inevitable.” (@Yamahaha)
Although the compression had felt good and I believed that the valve seat and/or seal was the culprit of the oil in the chamber. Lapping the valves didn’t solve it so I added valves and valve stem seals to the shopping list and had the seats reground.
If you are lapping the valves you should also change the valve stem seals. Cheap and easy to do while you are in there.
A lack of oil pressure may have been a contributing factor giving rise to extra heat in the cam box, so I decided on a thorough overhaul to the top end and oil distribution.
1. Thorough investigation and overhaul of oil pump, replacement of the oil return valve. I didn’t measure oil pressure but after replacing the oil it came out of the oil filter ‘bleed’ screw in a few seconds immediately after start up. Which suggests pressure was good.
2. Rebore, new Athena piston and rings 4th over-size.
3. Rockers, cam, valves are in another post here.
4. There is no vertical play in the con rod main bearing but a little in the horizontal, less than 1 mm. Gear box worked fine before so I decided not to split the crankcase or investigate the bottom end any further.
To check to see if the piston is genuine Yamaha, remove it from the con rod and once placed upside down the Yamaha name should be seen on the inside of the piston, pattern ones are not named unless from a brand name like Athena, Wiseco, Venolia or Protec etc.
Piston removed, definitely Yamaha.
The pin was not tight but no play in either the con rod or piston. There is sideways play in the con rod of about 1mm measured at the piston end. No play at all in the vertical.
A machinist also warned that if the scoring of the barrel is more than 0.125 then it will need a new sleeve as it is already 0.75 over-sized.
More advice picked up from @Yamahaha, is to use a light coating of EP90 gearbox oil when reassembling an engine as I it does not drip off even if left for months and helping on startup when fresh lubrication can take a bit of time to reach all parts.
Similarly on bearings throughout the bike and on the drive chain, apply a first light coating of copper slip prior to grease as even jet washing will not remove the copper element, always leaving a lubricated surface.
The new Athena piston arrived and then went off immediately to the machinist with the cylinder barrel for a re-bore to 4th over-size, 88mm.
Got the cylinder back after being rebored. He checked the camshaft and it’s within wear limits, the valve guides are also good.
Celebrated with a rattle 😆
I couldn’t resist assembling the cylinder and new piston kit as soon as I had all parts ready. I first inserted each ring in the barrel about 12mm down and measured the gap with feeler gauges. I read in one of the manuals that the gaps should be checked even if new, there may be a chance they need filing. I pushed them down using the piston so they were square, they seemed damned tight to me by eye. One of the compression rings was bang on 0.30mm, the minimum, with the other at 0.48. The oil rings (rails) were both 0.33.
I noticed when comparing the Athena piston with the original that the inlet cutout is different. The same width but the depth shallower with a shorter (11 mm instead of 16 mm) and steeper slope. The exhaust cutouts seemed identical. I assumed there was still the clearance in the chamber for the valve and piston not to collide.
The angle of the photos makes one look wider but they are in fact the same.
I installed the rings with the markings facing up but I can’t see that it would matter as they were uniform on both sides, That is to say square edged no bevels.
One intriguing thing. When the machinist measured the bore of the scored cylinder he found it was 87.65 and not 87.75+ as I expected for a 3rd oversize. I am now wondering if this led to the seizure in the first place.
Next: Oil Pump and Clutch
