Piston scoring after rebuild.

[agent laraby]

ok so rebuilt the motor a few months back and after 700ks head gasket failed and the bike reminded me of my old BSA with oil pissing everywhere. it had been blowing white/grey smoke for the left pot particularly so I was considering valve guides given the intermittent nature of things. I also used the old seal for the oil gallery which had failed up inspection. so replaced that and another new Head gasket.

put new head gasket on set timing did a few ks to tune it and it ran fine but was low on compression. (still blowing smoke intermittently. )

So off with her head again and discovered that there had still been oil getting into the cylinder chamber. I removed the cylinders and discovered that I had a broken oil ring in the left cylinder (there was small broken piece was in the the piston ring groove so the broken bit hadnt entered the oil circuit.).
Ithink I installed it incorrectly with overlap on the end pieces. The other oil ring s fine I also found a fair bit of wear/scoring around the rear of both pistons on the skirt and slight glazing in cylinder.
which after only 900ks total is of concern. see pics.

I am wondering if the piston marks could be cause by play in the con rod bearings, they seemed pretty tight when I put the bottom end back together so not sure this is the cause, the re bore was done professional and they are new .75 Os pistons. the scoring in the pistons does not have any groves
I am going to rebuilt the whole thing including new valve guides and seals, I have another head on its way which I will get checked and machined if required to make sure it is right .I am also thinking of pulling down the big end and replacing the conrod bearings. does any one know which colour is standard size and is there a guide for tolerances to move to the next shell bearing size? And has anyone had any scoring like this after rebuilding?
Cheers DG

 

[NewHavenMike]

Well, theres couple of problems that couldve caused this...

-Piston and cylinder wall clearance was not correct.
-Improper break-in procedure, revving too high, overheating..
-assembly error, lack of oil. improper ring end gap/installation.

You could probably reuse those pistons and find another cylinder, just take a scotch-brite a buff out the piston skirts and get new rings

 

[agent laraby]

Thanks,

Well, theres couple of problems that couldve caused this...

-Piston and cylinder wall clearance was not correct.
-Improper break-in procedure, revving too high, overheating..
-assembly error, lack of oil. improper ring end gap/installation.

You could probably reuse those pistons and find another cylinder, just take a scotch-brite a buff out the piston skirts and get new rings

The cylinders have no scoring on them so they should reusable as well, the cylinders were bored by a someone who has a very good rep and he was given the pistons to match up so i think it would be the other options the guy who helped my seemed to think it was fine ti rev it pretty hard straight after rebuild saying it helped the rings seat. I am going to rebuild it myself and and run it in properly.

 

[NewHavenMike]

Its not good to free rev the engine just after a rebuild. You need to be riding the bike and have a load on the engine, not to mention a couple heat cycles. How did you break the engine in?

 

[sandmanred]

Is this contact is visible on the carb side of the engine only? Or at least more visible on the carb side than the exhaust side?

 

[agent laraby]

yep carb side only

 

[NewHavenMike]

Stock carb setup?

 

[sandmanred]

It's been some time since I had an engine apart but I think some contact or wear on that side of the piston is normal. When a cylinder is on the combustion stroke (from ignition at TDC to exhaust near BDC) the piston gets pushed to the carb side due to angle of connecting rod and the enormous down force from combustion. I don't see any serious galling just a little smoothing from contact. I wouldn't be too worried about it from what I see in your pics.

 

[NewHavenMike]

Kinda looks like it was washed though. Get a pic of the cylinders.

 

[agent laraby]

Hi guys thanks fir your responses bit of an update:
Further pull down revealed some more magic
Diagnosis: Failed valve guide broke oil ring with oil be forced bypassing and I noticed a small but not insignificant defect in the piston that must have bee caused by something metal, and the tip of a valve spring had broken off. Also carbs were dumping too much fuel causing wash down past the cylinder. The con rod end play was right on the limit for up down movement
plan:
- Cylinders are being honed as only had light marks, machine shop says they will be fine with very light hone.
- New pistons and rings are on their way.
- New valve guides and valves (new valves were installed but for 80 bucks AU I figure in case the play led to any damage)
- Secondhand head has arrived with another 34hp cam which is in much better condition than mine. So that is off to the machine shop to have the valve guides installed and the seats recut and valves new valves lapped. (The old head is ok but the replacement one is in much better condition). Thew new head i reckon has less than 30k on it the rockers are very good and valves are good but i will just go new with everything so wont use the valves. The springs are in spec in fact test as new.
- New con rod and big end bearings as there has obviously been a small amount of metal kicking around in the oil.
- and of course all new seal and filter kit.
So basically a full pulldown, clean and rebuild.
Carbies have had a new kit with original mik jets in and have been sonic cleaned.

Essentially I figured based on the info I had been provided that that rings and pistons, timing chain etc and would be fine. However not so, I should have just done the whole thing on the first place. Plus spend the right amount of time and process of running through the engine break in.

Essentially I was too impatient and should have done full rebuild from the start so by replacing everything and rebuilding the bottom and top end it should be right. the bike original was running well with only the signs of the valve guide problem but things had begun to deteriorate from there. I parked it as soon as it begun to be more problematic and am glad I did. I reckon it would't be long before it got catastrophic. Although a rebuild is a pain in the ass at least I know everything is replaced and sound. I have pulled down and rebuilt enough motors to know better but was bit cocky with this one and got owned.

 

[xschris]

Make sure to get new con rod bolts as you should never use old ones.

 

[NewHavenMike]

I rebuilt my entire engine and scoured the Earth for some parts. In the end, a needle bearing on the input shaft blew out and ruined the engine. I havent inspected the top end, I just removed the whole engine and replaced it with an engine I have never started.

 

[agent laraby]

That is bad news and after mine went south after a rebuild I feel your pain. Which bearing was it ?

 

[NewHavenMike]

It was the needle bearing on the transmission input shaft. The bearing sits right behind the clutch pushrod seal. Theres an oil passage that runs right to it. I believe the passage got clogged up. Everything was fine until after a few rides after an oil change. I decided to add some Zinc additive. Pretty sure it was Amsoil brand. Not Positive if that had anything to do with it though.