AKnarrowback
Well-Known Member
Ok, the back ground story.
I have a 98 XP LTD with the 947.
I bought it last year with water damaged pistons, rebuilt it 1st over with oem pistons. On the 7th ride the air box fell off, water got in and scored both pistons.
Rebuilt the top end at the start of this summer using Weiscos and bore job 2 over (88.5 mm). Took it easy on the break in, mixed some oil into the gas, ran two full tanks through and gave it it's first full throttle pass and lost the mag side piston. There was clear damage that looked like water had gotten into the motor and smeared the intake side. Checked the water separator and it was 3/4 full of water and crud, found a good amount of water in the mag carb as well. I realized I had done the entire break in with the fuel selector set on reserve and had sucked all the crud that might have been in the gas tank through the carbs and motor. Lots of water after the separator.
The mag piston was water damaged, the pto piston had evidence of a four point seizure. Both cylinders honed clean and I was still in clearance to run 88.5mm pistons again.
Up to this point everything I could attribute to water damage smearing one piston. However, the 88.5 Weiscos measured 88mm at the skirt after they were pulled out? No bending, cracks or wear on the areas that were undamaged from the water. Two tanks of easy riding and the pistons did not maintain 88.5mm but appeared to have "shrunk". The other odd thing was the pto piston and the four point seizure marks when I was very gentle in the break in, long warm ups, no stressing the motor, how did the marks get there?
So, fast forward two weeks and I have another set of 88.5 weiscos installed with a clean hone job on each cylinder, premixed the tank, did the warm up 10 minutes, shut off to let it cool, another 10 minutes all without bringing the throttle past 3000 rpms while sitting in the water on the trailer. Took it easy on things for another two tanks and went to give it the first long full throttle shot and it shuts down again after 10 seconds.
A compression check with the snap on gauge shows 140 mag, 135 pto. Pto had a light smear in line with the ring gap of the top ring.
The carbs were rebuilt last year with all original mikuni parts, needle and seats, pop off set. The motor pressure tested good before taking it apart after the second shut down with the weiscos, held 8lbs for over 5 minutes and stopped the test without seeing any real pressure loss. Gas was twice filtered super unleaded.
The thing that stood out like a sore thumb was the amount of glazing both cylinders had after each of the failures this season. Both times there were two tanks of run time on the fresh hone job, premixed tank as well as oil injection. I threw in pictures of what I found after the second failure last weekend. Another standout was the amount of blow by on the piston faces.
I have never had a motor rebuild fail during break in. This is the first time I am getting my butt kicked and could use some opinions here.
Doing a lot of reading and research in the last few days and found lots of info saying glazing can be a result of unburned fuel/oil elements embedded in the cross hatch as a result of not having enough heat in the burn. Others say it's by taking it too easy on the break in or holding the throttle in one spot for too long during break in. Also have read many views on break in procedure.
I take it easy on my new pistons, but many views say to let things warm up good, take it easy for a few minutes, with varying throttle, but then to be more aggressive and use some energy to push the rings out to avoid glazing.
With things being the way they are I have one more chance to try before I have to give up on this machine for the summer. I touched things up with the hone, piston damage was almost nothing and Group K pointed out that slight scuffs can be carefully smoothed out with fine wet/dry sand paper, so I did. The rings are in good shape, have been run for two tanks but are staying on the pistons for this last try. Motor pressure tested good again and I'm putting it in the water tomorrow.
My thinking is the gentle approach to the break in didn't work. I was going to try the more aggressive approach tomorrow, a good warm up, but higher rpms with a full throttle run sooner than later.
The glazing has me perplexed?? I have only seen this much in motors that are years old, not two tanks. The marks in both cylinders, right side, are the upper ring alignment pin/gap location. The ring set was for an 88.5mm piston but was at the max for what the shop manual said the gap should be. We did do a inner micrometer check of the cylinders and they were bored to match the pistons. Water Temps are around 45 to 48 degrees F that I'm running in.
I have a 98 XP LTD with the 947.
I bought it last year with water damaged pistons, rebuilt it 1st over with oem pistons. On the 7th ride the air box fell off, water got in and scored both pistons.
Rebuilt the top end at the start of this summer using Weiscos and bore job 2 over (88.5 mm). Took it easy on the break in, mixed some oil into the gas, ran two full tanks through and gave it it's first full throttle pass and lost the mag side piston. There was clear damage that looked like water had gotten into the motor and smeared the intake side. Checked the water separator and it was 3/4 full of water and crud, found a good amount of water in the mag carb as well. I realized I had done the entire break in with the fuel selector set on reserve and had sucked all the crud that might have been in the gas tank through the carbs and motor. Lots of water after the separator.
The mag piston was water damaged, the pto piston had evidence of a four point seizure. Both cylinders honed clean and I was still in clearance to run 88.5mm pistons again.
Up to this point everything I could attribute to water damage smearing one piston. However, the 88.5 Weiscos measured 88mm at the skirt after they were pulled out? No bending, cracks or wear on the areas that were undamaged from the water. Two tanks of easy riding and the pistons did not maintain 88.5mm but appeared to have "shrunk". The other odd thing was the pto piston and the four point seizure marks when I was very gentle in the break in, long warm ups, no stressing the motor, how did the marks get there?
So, fast forward two weeks and I have another set of 88.5 weiscos installed with a clean hone job on each cylinder, premixed the tank, did the warm up 10 minutes, shut off to let it cool, another 10 minutes all without bringing the throttle past 3000 rpms while sitting in the water on the trailer. Took it easy on things for another two tanks and went to give it the first long full throttle shot and it shuts down again after 10 seconds.
A compression check with the snap on gauge shows 140 mag, 135 pto. Pto had a light smear in line with the ring gap of the top ring.
The carbs were rebuilt last year with all original mikuni parts, needle and seats, pop off set. The motor pressure tested good before taking it apart after the second shut down with the weiscos, held 8lbs for over 5 minutes and stopped the test without seeing any real pressure loss. Gas was twice filtered super unleaded.
The thing that stood out like a sore thumb was the amount of glazing both cylinders had after each of the failures this season. Both times there were two tanks of run time on the fresh hone job, premixed tank as well as oil injection. I threw in pictures of what I found after the second failure last weekend. Another standout was the amount of blow by on the piston faces.
I have never had a motor rebuild fail during break in. This is the first time I am getting my butt kicked and could use some opinions here.
Doing a lot of reading and research in the last few days and found lots of info saying glazing can be a result of unburned fuel/oil elements embedded in the cross hatch as a result of not having enough heat in the burn. Others say it's by taking it too easy on the break in or holding the throttle in one spot for too long during break in. Also have read many views on break in procedure.
I take it easy on my new pistons, but many views say to let things warm up good, take it easy for a few minutes, with varying throttle, but then to be more aggressive and use some energy to push the rings out to avoid glazing.
With things being the way they are I have one more chance to try before I have to give up on this machine for the summer. I touched things up with the hone, piston damage was almost nothing and Group K pointed out that slight scuffs can be carefully smoothed out with fine wet/dry sand paper, so I did. The rings are in good shape, have been run for two tanks but are staying on the pistons for this last try. Motor pressure tested good again and I'm putting it in the water tomorrow.
My thinking is the gentle approach to the break in didn't work. I was going to try the more aggressive approach tomorrow, a good warm up, but higher rpms with a full throttle run sooner than later.
The glazing has me perplexed?? I have only seen this much in motors that are years old, not two tanks. The marks in both cylinders, right side, are the upper ring alignment pin/gap location. The ring set was for an 88.5mm piston but was at the max for what the shop manual said the gap should be. We did do a inner micrometer check of the cylinders and they were bored to match the pistons. Water Temps are around 45 to 48 degrees F that I'm running in.
Glad you got it back on the water.
