[TrialsRider]

Looks like it never had a chance to seat the rings,




Ah,Ha! you are missing the clutch retaining ball it must have split in 2 and been left rattling around inside the wrist pin.

 

[XB33BSA]

A chunk of piston alloy wouldn't erode the cylinder wall to that degree. How would a piece of cast aluminum small enough to come from that eroded area and small enough to travel through the inner diameter of the pin cause 4 divits that size? If there was a chunk of piston pin missing, I would assume he would have mentioned it.

of course you are right and after i wrote that i then seen your images from the bmw
and of course that image explained the whole thing
i have seen wear on wrist pin ends from clips
there can be also some strange harmonics at high rpm especially a 180 crank 4t twin
it has a rocking couple as well as a uneven firing
wrist pin clips should be installed with the tails vertical
i think for a race engine telfon buttons are the way to go
a clp cannot escape if its not there
here is the harmonics i speak of they kick in about 9500rpm on my 350
same engine as the 250 just smaller bore in he 250
about 38 seconds in there is the harmonics

 

[Cyorg]

Yes.
Finally got a talk with the guy who fixed it. ALL 4 circlips was intact when he dimantled it. The one seen in the photo can't be removed, alloy melted it in place.
So the circlip theory out and no signs of piston ring parts in the wrong place.
Anyhow the bike is assembled again. Ready to start except one part has disappeared, the clutch retainig ball. Will see if I can find it or find one to buy.

If you want, you could post a few more pictures of that offending piston. Especially one or two showing the circlip retaining groove on the opposite side of that piston shown in your previous post. The more photos the better...

 

[Mike 40M]

After some trials, somewhat higher magnification, opposite side (where circlip could be removed

 

[Cyorg]

Hard to tell anything further from that photo. There do appear to be some marks at the edge of the groove and in the groove itself that look odd. As in how did they occur if the clip was still in there? If the clip has been removed in the past, they sometimes get pick marks, but if done properly and the clip is removed using the area that is relieved (for that purpose) there shouldn't be any marks in the groove. How does it look to you ( using strong readers or a magnifying glass)? Are there any marks in the groove that couldn't have occurred if the clip was still there? Although.... if the clip wasn't in there, I would expect more damage would be likely and maybe more blow by combustion crap in the groove given the condition of the rings etc. It would also contradict what your engine guy said.
Bottom line as I see it. Either something got left in there during the last rebuild, although it would take some doing to get something hiding in that area.
If I took that engine apart ... there wasn't any shrapnel and both clips were installed, I'd be wondering if some previous owner threw it back together with those pits in the wall because they didn't want to spend the money on a cylinder.... but didn't you have the engine apart at some point before this?
How carful was your engine guy when he disassembled the engine? Is he positive the clip was in there? Always good to start with a clean bench and stuff rags in the case around the rods before you pull the cylinder completely.... even if you are planning on splitting the cases. Better chance of catching whatever was in there assuming anything was left of it. If he didn't split you cases, hopefully it's not hiding in the bottom of the cases.... although odds are are it would just lay there.

 

[Mike 40M]

What I know of it's history. I think it was set up for racing about 16 years ago. NE3 cam which is a not extremely hot one, Reinforced cam chain. Ported. 30mm Mikunis. Electronic ignition. The rest mostly standard. We raced it two seasons as it was. Then the engine was apart in 2016. Notes says cylinder cleaned and honed. Top piston ring was broken at this cylinder. Both top rings was replaced. So it has two race seasons since then. Everything seemed ok except some blue smoke from the other cylinder until last race when it came in blowing heavily from the engine breather. The guy who made both these strips have many years experience in racing and maintaining these Hondas, so I assume that he knows what he's doing.
Thanks for all inputs inputs I've got on this fault. I do believe now that some foreign thing must have been tumling around inside the gudgeon pin. As it is hard to tell what it was, I think I'll call it a gremlin stuck in there. Did these marks trying to get free and finally vanished in the woods, yelling free, free, at last free.

 

[Cyorg]

Then the engine was apart in 2016. Notes says cylinder cleaned and honed. Top piston ring was broken at this cylinder. Both top rings was replaced. So it has two race seasons since then. Everything seemed ok except some blue smoke from the other cylinder until last race when it came in blowing heavily from the engine breather. The guy who made both these strips have many years experience in racing and maintaining these Hondas, so I assume that he knows what he's doing. .

Given the season, I'll try not to sound like a grinch, but.... I'm not sure it's safe to assume he knows what he is doing. As previously mentioned, the photo of the cylinder wall shows a finish that was very poorly done. Even for a first timer done at home in the garage it would be considered a fail. The repeat ring failure suggests that the ring lands were worn beyond the service limit or perhaps there were problems with ring end gap that caused ring flutter, but ring land problems are more likely. Things that should have been measured and addressed. The top of that liner looks strange and makes me wonder if there is a bit of a ridge that the top ring is smacking into. The brokers rings from the latest occurrence could also likely be from the ring ends snagging those two upper pits or shrapnel flying around in the pits. Cylinder wall finish is critical if you want to squeeze any sort of HP and longevity out of an engine. It's worth spending some time doing some reading on the subject. It will fill pages and you will get all kinds of opinions. One of the best sources is Grumpy's Performance Garage. You can find current info there and what was "true" 30 years ago isn't necessarily true today.

 

[Teazer]

That is a really unusual wear pattern. It appears that the pin has repeatedly hit the liner at BDC and TDC which are the times in a cycle where the rod is more or less stationary. That in turn suggests that the pin must have moved laterally (side to side) to impact both sides and then to not create a groove down the liner.

So maybe it is related to a harmonic effect of some sort. What sort of revs does it run to and did the rider notice any particular revs that it used to vibrate worse than others?

 

[TrialsRider]

After some trials, somewhat higher magnification, opposite side (where circlip could be removed
View attachment 96319

Can you almost push the wrist pin through the hole now with the cir-clip still in place?
Possibly once everything is flaming hot

 

[Cyorg]

That is a really unusual wear pattern. It appears that the pin has repeatedly hit the liner at BDC and TDC which are the times in a cycle where the rod is more or less stationary. That in turn suggests that the pin must have moved laterally (side to side) to impact both sides and then to not create a groove down the liner.

So maybe it is related to a harmonic effect of some sort. What sort of revs does it run to and did the rider notice any particular revs that it used to vibrate worse than others?

But.....if you look closely, the pits appear cratered from being repeatedly struck with something small, the pits are not rounded in a way that you would expect if it was the pin and Mike said there weren't any witness marks on the pin. Granted the pin is harder than the wall and would wear less. The pits also get deeper as they get closer to the end of the stroke, so an undamaged pin running at right angles to the bore couldn't make pits that look tapered outward like that.

 

[Teazer]

But.....if you look closely, the pits appear cratered from being repeatedly struck with something small, the pits are not rounded in a way that you would expect if it was the pin and Mike said there weren't any witness marks on the pin. Granted the pin is harder than the wall and would wear less. The pits also get deeper as they get closer to the end of the stroke, so an undamaged pin running at right angles to the bore couldn't make pits that look tapered outward like that.

Agree that it's not actually the pin moving side to side and impacting the walls. That was before my first cup of coffee.

The damage to the piston looks like a clip came loose, but it's still there. A typical clip failure produces a groove from top to bottom as the pin broaches the liner. Inb this case it's something that's happening when the piston reverses direction which is what I was exploring earlier.

What does happen at direction reversal is that inertia becomes an issue as loads are reversed and that raises a question of where the holes are in relation to pin and ring positions. If it's at a ring height in the bore, that would suggest that a ring is causing the damage. We normally think that rings rotate and would not consider the rings as a possible cause of the issue, but if the oil ring was pinched or had excessive clearance, a ring issue might be postulated. It's easy to speculate that once teh damages started and the ring was unable to rotate that the process would continue to deteriorate, but what started things going south?

Perhaps a ring was slightly damaged and was unable to rotate. That is most likely a handling/fitting issue. And why only on one side and not the other? That suggests that it was less of a harmonics issue and more of a mechanical damage issue.

It's possible that the ring groove was excessive on one piston and that allowed a ring to rock at TDC and BDC which in turn allowed the end of the ring to ding out a tiny amount of metal each time it reversed. Another possibility is that the expander ring was either overlapped or incorrectly fitted.

Mike: Do the divots align with the oil control ring in terms of position from top of the liner?

 

[Cyorg]

Don't forget there are 4 divits. I think the answer to you question to Mike about the divits relation to the oil ring will be yes ( but that would only apply to the top 2 divots.) The reason I say that it is... if you go back to the first photo showing that piston, the pin boss is eroded right up to the oil ring so therefore the top of the divits would line up with the bottom of the oil ring Y/N? I did wonder about a ring getting snagged when the barrel was put on, and that would normally leave fairly distinct witness marks, but thinking about it... on my second cup, some of the witness marks, if any, could be erased by the erosion between the pin boss and the oil ring. Only problem with that theory is that the oil ring and second ring were fine according to Mike. If a top ring was snagged during assembly and broken off, you would think there would be some trail either across the two lower ring lands or in the cylinder wall starting from the very top. Breaking a chunk of compression ring off during installation and having it find its way into the pin area seems unlikely. You'd think it would just head south into the case. Not trying to be contrary and sometimes it takes more than one set of eyes to sort these things out. Plus it's fun figuring them out when it's not your engine and your not paying (sorry Mike).

 

[Teazer]

If the two rings in the oil ring had been set with ends 180 degrees apart, that could explain both sides, but with them apparently close to each other, that would create wear at top and bottom on the same side as the ring gap and the other side would be clear of damage. If Mike was the liners or old barrel, he should be able to measure the top of the top divot and bottom of the lower ones and determine which part of the rings that correspond to at TDC and BDC.

That might, and the emphasis is on "might", shed more light on the cause and mechanism involved.

 

[Cyorg]

From what I'm seeing in the photos, the pin boss is eroded downward (below the pin), I'm pretty sure that when measured, the lower extent of that erosion on the piston will lineup with the lower edge of the pit in the wall at BDC and vice versa at the top. Unless I'm missing something, all that can tell us is that something was rattling around in there. Based on the depth of the pits, if either the oil ring or second ring played a part, then there would be damage evident on the rings.

 

[Teazer]

From what I'm seeing in the photos, the pin boss is eroded downward (below the pin), I'm pretty sure that when measured, the lower extent of that erosion on the piston will lineup with the lower edge of the pit in the wall at BDC and vice versa at the top. Unless I'm missing something, all that can tell us is that something was rattling around in there. Based on the depth of the pits, if either the oil ring or second ring played a part, then there would be damage evident on the rings.

Yeah, but that doesn't explain why the wear is most extreme at BDC and TDC......

 

[Mike 40M]

The distance between the wear marks are about 68mm. Stroke is 50.6mm and wrist pin diameter is 15mm. This should indicate that something hard has hammered the marks in both piston and cylinder. As both circlips was in place, it must have been something else. The top ring was in a dozen pieces, some of them only 2mm long. If a piece could have went down without leaving any marks on the piston, it could have caused the damage. There was no signs of the problem before the last race. In the race the engine did some 170,000 revoloutions at between 8500 and 11500rpm, which might be enough to cause the damage.
Now I'll try to figure out why a RG250 has one of the pistons and head looks like it has been sandblasted.

 

[Cyorg]

Yeah, but that doesn't explain why the wear is most extreme at BDC and TDC......

My theory on that is in post 30. The eroded area on the piston boss which is more or less tapered, hurls the debris against the wall when the piston changes direction and that creates the craters seen in the bottom of the pits.

 

[Cyorg]

The distance between the wear marks are about 68mm. Stroke is 50.6mm and wrist pin diameter is 15mm. This should indicate that something hard has hammered the marks in both piston and cylinder. As both circlips was in place, it must have been something else. The top ring was in a dozen pieces, some of them only 2mm long. If a piece could have went down without leaving any marks on the piston, it could have caused the damage. There was no signs of the problem before the last race. In the race the engine did some 170,000 revoloutions at between 8500 and 11500rpm, which might be enough to cause the damage.
Now I'll try to figure out why a RG250 has one of the pistons and head looks like it has been sandblasted.

You could start by......Google images of piston detonation and see if you find a photo that matches.

 

[Teazer]

The RG is easy. Detonation for sure. What's less easy is the cause. I have a client RD350 that melted down and it wasn't obvious what happened. It's tempting to tear it down but first I did compression and leakdown tests and it would not hold pressure. Using a spray of soapy water the bubbles started to appear in all sorts of places - enough to cause a problem. Then I checked the ignition timing and it was slightly advanced on the good side and even more advanced on the bad side. In addition there was difference in height between the two barrels leading to ineffective squish on one side. It's all those small things that add up.

If it's not too late, try a compression test and leakdown and see where the ignition is set - as best as you can before you start to tear it down. If it's too late for that, double check everything from jetting and fuel flow to ignition timing and squish.

 

[Mike 40M]

Thanks for advice on the RG. I must have been lucky as I've not seen detonation marks like that before. Sadly I can't check because the engine is already in pieces because one big end bearing had seized. Doublechecking looks like a very good idea.