Piston/barrel dramas

[SS90]

Interesting thread.



No doubt qualified people know this, and its OK to ask "is the rod bent???" smarter to be able to suggest an easy way to check.

Signs of a bent rod or other alignment problems are diagonal wear marks across the piston skirt. Attachment 246931



This page talks about broken pieces of clip crossing to the other side of the piston [ATTACH]246925[/

I have had a short glance at this, but I am still unable to see any images even remotely like the damage shown in Jellywrestlers images, can someone point me in the right direction... Smarter to suggest ways of checking? Thats your opinion, this guy clearly has ability to remove and fit his own liners (no torque plates used, which is an engineering " No No particularly when you are dealing with a long stroke air cooled cylinder with a thick cast iron liner), so I think it is safe to assume that he has the ability to determine if how much force he is using to remove a gudgeon pin would be bending a rod (I have never personally seen a rod bent from this... I have however recently seen web based text suggesting it) and I have had to use heat and high force when removing gudgeons at different times.

I believe to bend a rod, you would have to use some force, and your average skilled home mechanic would recognize this at the time.

I HAVE however, seen damage from cylinder machining faults, on an engine run for about 20 dyno runs, and showing signs not too dissimilar to Jellywrestlers pictures.... However, the wear was no where near as much as he has, but similar in position for sure.

[bucketracer]

One of my suggestions was, a broken clip has caused the damage, and a bent rod is only one of the reasons a clip can be broken or ejected.

put forward as being caused by a bent rod... My question would be, is the rod bent???

A more telling question would have been " is there a clip or part of one missing?".

[Woodman]

It does not take much force to bend a con rod at all. The bent rod thing may be that the big end and small end are not in alignment. This can be either laterally or it could be twisted meaning the small end will travel along the gudgeon pin and back every stroke. Couple this with not much rod to boss clearance and possibly the rod is pushing the piston against the cylinder wall each time it reaches tdc or bdc.

The rod could also be perfectly aligned but the rebore could have been done on an angle which has similar symptoms. Easy to check all this on a conventional engine where you can assemble it and turn it over and see whether the small end "walks" along the pin as you turn it over. Hard to check on a motorcycle engine though.

BTW these are just theories from the pics that I can see. So don't start a shit fight.

[husaberg]

One of my suggestions was, a broken clip has caused the damage, and a bent rod is only one of the reasons a clip can be broken or ejected.

A more telling question would have been " is there a clip or part of one missing?".

I think the foreign corespondent has missed the engine has just been rebuilt bit. It was obviously rebuilt for a reason by person unknown for unknown reasons.In fact I wonder if ther e could be some form of out of parrellel in the case or bearing and or sleeve as well. Like I said earlier everyone always looks for a smoking gun when it could be a whole series of problems.

[jellywrestler]

took the front barrell piston off and here's the comparison between the two...

[bucketracer]

Good pictures, and plenty of shrapnel there.



Does your damaged piston look like picture 1.1.2 in the PDF, it does to me.

This is my take on it, the damaged piston is starting to develop the classic melted piston look in the piston pin area from debris being shaken up and down. Eroding the piston and marking the cylinder at the end of the pin travel as they slam to a stop at each end of the stroke. The clip and debris were not able to escape the piston cutout and were held there and shaken up like they were in a cocktail shaker.



This all started with the piston pin clip coming out. The clip was trapped in the piston cutaway area and couldn’t escape or become trapped in a transfer port like it might in a 2-Stroke. As it was shaken violently up and down it eventually broke up the oil control land and ring releasing debris that marked the bore, the marks are both sides as the debris can travel to the other side of the piston through the hollow piston pin. The piston pin did not leave the classic tramlines, maybe there was not enough time or its ends just did not dig into the bore.

TDC BDC

The shrapnel damage evidence, is in the symmetry of the markings on the cylinder wall matching the piston cut outs, on both sides and at TDC and BDC. Curved at TDC matching the pin cutout in the piston and straight at BDC matching the straight bottom of the piston cutout. And finishing at the exact extremes of the piston cutouts travel. I expect if you put the piston in the bore the damage will exactly match the cutouts at TDC and BDC.

The clip may not have been fitted properly or was being hammered by the piston pin due to a miss alignment problem and ejected, further investigation of the damaged piston can reveal which.



If there is a miss alignment problem it will show in a diagonal wear pattern on the damaged piston. Typically the top on one side and the bottom on the other side touches the bore and there is diagonal wear lines between them on the thrust sides of the piston skirt. With such a low mileage it may be hard to see but if its there, there should be some indication.

I would be quite keen to see photos of the thrust sides of the pistons if possible.

[Crasherfromwayback]

took the front barrell piston off and here's the comparison between the two...

Some pretty serious heat going on there!

[Grumph]

Is the more severely damaged side, the plug side ?

[Buckets4Me]

Piston Damage

On both sides of the piston, the end of the piston pin bore has suffered serious damage. In some places the damage reaches up as far as the ring zone (Fig. 1).



When the piston was removed there was no longer a circlip in the retaining groove. It had jumped out during operation and broken into fragments. Although the second circlip is damaged, it was still installed in its groove when the piston was removed. Due to the lack of retention, the piston pin has wandered outwards during operation towards the cylinder running surface. Clearly the front face of the pin has prolonged contact with the cylinder running surface. As a result, the front face is worn to a convexed shape (Fig. 3). The running pattern of the piston is formed very asymmetrically.








just a scrape from the web (may or may not be relavent) but the damage was not caused from heat

[Chambers]

(no torque plates used, which is an engineering " No No particularly when you are dealing with a long stroke air cooled cylinder with a thick cast iron liner)

A No No?????

No doubt Torque Plates are a great idea and I am a bit of a believer. But I don't think they are that common and there has been a lot of good engineering done without them. I don’t recall them being required by my 1948 copy of Modern Motorcycle Mechanics that hails from the era of the Vincent or any of the more modern motorcycle workshop manuals that I have seen.

If there are any factory workshop manuals for common modern road bikes of any era or even an exotic one that insists on a Torque Plate during rebuilding/reboring/reconditioning I would truly love to see a quality post about it complete with a PDF scan and/or a link or two.

[Chambers]

Does your damaged piston look like picture 1.1.2 in the PDF, it does to me.

I expect if you put the piston in the bore the damage will exactly match the cutouts at TDC and BDC.

Two very good points.

Point-1) Does the damaged piston look like the 1,1,2 one in the PDF??

Point-2) And does the marks in the bore on both sides match the piston cut outs at TDC and BDC??

Hopefully Mr Jelly will take a look and tell us.

This is very interesting. as point-2 in particular could make or break Buckets theory.

[Gigglebutton]

Does your damaged piston look like picture 1.1.2 in the PDF, it does to me.

The shrapnel damage evidence, is in the symmetry of the markings on the cylinder wall matching the piston cut outs, on both sides and at TDC and BDC. Curved at TDC matching the pin cutout in the piston and straight at BDC matching the straight bottom of the piston cutout. And finishing at the exact extremes of the piston cutouts travel.

{if I am right} I expect if you put the piston in the bore the damage will exactly match the cutouts at TDC and BDC.

A 7th Form school boy doing the research and asking the right questions, puts us "Real World Experts" to shame, one way or the other, its good work and thinking Bucketracer.

[speedpro]

Another thing to look for is the piston pin circlip grooves being distorted where the gudgeon pin may have been trying to push it out. I've seen it where the grooves were nearly twice as wide as original.
Teflon buttons are great. So much easier to get in and out, as long as they don't fall in the sump when you lift the barrels off.
I made a 40mm thick surface ground torque plate for my turbo engine barrel. The bloody engine reconditioners took the barrel off the plate to bore it, which unfortunately for them I noticed. I had it measured by some real engineers in their climate controlled measuring room. The bores were all straight and parallel, individually, but the cylinder block had a slight twist, hence the big plate to bolt it to. The end result was the two bores had a very slight slope forward and the other two sloped ever so slightly backwards when it was all bolted down on the plate.
Grooves in the barrel from a pin getting out the side of a piston probably won't be even because of the offset. The gudgeon pin hole in the piston will not be in the centre.
The damage is definitely caused by a bit of debris rattling around.

[Buckets4Me]

Granted, but from real world experiance, my opinion is that When a cirlip comes adrift (as has been suggested) it, more often than not quite quickly ends up lodged between the piston and liner, leaving the offending item jammed in place, rather than the somewhat miraculous situation where it rattles between the two points, seemingly Perfectly in rhythm, leaving even marks at TDC and BDC.

how would the clip get between the piston and barrel? As the ring normaly scrapes the oil off the barrel and the gas from under the cylinder head cant get past either.

Remeber this is a 4 stroke not a pesky 2 stroke scooter with ports everywhere for the clip to jump into

there is however a big cavern where the little end pin is that the clip can rattle around in

and being that it will be draged up and down the barrel anywhere up to 8000 times a min. there is a good posibility that it will leave lots of marks up and down the liner (specialy at the top and bottom of the stroke)
also anyparts of the piston or rings that it can brake off then start to traveling up and down with it (also causing damage)
this damage will be more at the top and bottom of the stroke (where everything changes direction)

as for even marks at tdc and bdc
logic says the piston goes up and down more or less the same amount of times leaving more or less the same amount of damage at both the top and bottom of the stroke

[TZ350]

It does not take much force to bend a con rod at all. The bent rod thing may be that the big end and small end are not in alignment. This can be either laterally or it could be twisted meaning the small end will travel along the gudgeon pin and back every stroke. Couple this with not much rod to boss clearance and possibly the rod is pushing the piston against the cylinder wall each time it reaches tdc or bdc.

The rod could also be perfectly aligned but the rebore could have been done on an angle which has similar symptoms. Easy to check all this on a conventional engine where you can assemble it and turn it over and see whether the small end "walks" along the pin as you turn it over. Hard to check on a motorcycle engine though.

Yes, agreed, rods are easily bent and if there is any miss alignment you can see the pin walking when looking up inside a reco car/truck motor while turning it over.

Another way of checking for a bent rod is to slide a feeler gauge around the top of the piston at TDC, a bent rod will show up as being tighter on one side than the other. This can be done on the Vincent if the barrel is held down.

You can also check for broken rings this way too as the tip of the feeler gauge will drop into the gap where a piece of ring is broken away. You can also check the other compression and oil rings too by pushing the feeler gauge down past each ring in turn then sliding the feeler around to see if there are any gaps.