Questions? Looking for parts? Parts for sale? or just for a chat,

The WD Motorcycle forum

WD Motorcycle forum
Start a New Topic 
Author
Comment
View Entire Thread
Re: Piston Damage

I couldn't see how a circlip would cause that damage when it's circular, and the damage was oval, and there were no scrapes on the bore. I said originally it had melted and this piston definitely has as it's blown past the rings. A circlip can't cause the damage on this piston. They do look like the same cause, that's why I posted the link and I have never seen a problem like it. Is there any damage on the bore on your piston Steve? Has the oil scrapper ring melted? If so, that's what happened to the end of the circlip on the other post. The gases are getting down the side of the piston and the gudgeon pin area is the first "open" space for it to get to and so heating and melting the piston there.

email (option): horror@blueyonder.co.uk

Re: Piston Damage

I disagree. The circlip is steel and the piston soft aluminium. It doesn't get hot enough to melt steel. When a circlip comes out it wears the piston away over time and will break through to the rings and the damage then escalates. Eventualy the steel bits of circlip and ring will eventually break through into the combustion chamber and hence out the exhaust. There is damage to the top and bottom of the bore in both these instances. Detonation usually damages the top of the piston.

Re: Piston Damage

Fortunately I'm not taking my M20 to Normandy this year. I'm loving riding my WD/G at the moment. But I think it might be a good idea to lift the head for a butchers and treble check the timing when I get back. Ron

email (option): ronpier@talk21.com

Re: Piston Damage

the top land scoring will occur on the hottest side of the piston and/or where there is most ali. (around wrist-pin bosses) as its caused by the alloy expanding with the heat and trying to seize on the cylinder wall.

Common problem on any engine running hotter than normal - reducing the the top land diameter is often needed to stop this on a racing engine.
Really all the top ring land does is protecet the top ring from the combustion heat.

If you place a straight edge vertically on a good piston, you can see that the ring lands are all slightly smaller diameter than the bore.
Smaller dia at the top (hottest), reducing towards the oil-control ring.

Surprised the top ring has not shattered - has it gone all 'chewing-gummy' and malleable. Even Cast iron rings can go 'soft' and lose thier tension when overheated.

Piston rings rotate round the piston (in thier grooves) at 1/2 engine speed (or was that 1/4 - its a while since i learned this fact) - if prevented from rotating, hot gases can work on the rings lands around the ring-gap - resulting in wire cutting.
sods law says that a pair of ring gaps will line uo when the rings lock up.

used to see this all the time when drag racing on high nitro.
Nitro was banned but it later re-appeared when nitrous became the rage.
Intresting to see it on a SV ..............

It was a 'trick' to pin rings in a Side-valve (like a two stroke) to try and get the rings to seal when the bore goes out of shape when running.
Just make sure thay are pinned 120deg from each other if you do this - and yes i have seen them pinned with the gaps in line - because it was easier that way????

FUEL - it is/was common to add parafin to early SV engines as it makes the fuel more like the original stuff (less volatile) and the engines run cooler.
I have never run any myself, but I reckon 5-10% parafin would be worth a try.
I'll try and dig out the web-page i found this info on...

Re: Piston Damage

Some interesting potential points amongst these replies.

In Ian's reply he touches on mixture strength, yes, the carb slide is quite worn, so possibly the mixture could be on the weak side, though the plug always indicates its about right every time its's removed.

As regards gearing, it's currently running an 18 tooth engine sprocket (1 tooth down on the standard 19 for the year) with a relatively lightweight Watsonian Avon Sports sidecar and doesn't seem particularly laboured in any way. Up until a few months ago I were running a heavier sidecar previously on a 17 tooth sprocket and it required more gear changing with that.

Fuel shouldn't be an issue as I always run it on super unleaded, again like Ian says, considering it should run on crappy Pool petrol that should be overkill. I don't normally use any additives, though occasionally I'll add a capful of (allegedly) proper Nato spec. upper cylinder lubricant. Though I hadn't added any on this journey and had filled up 3 times over the weekend (Shell, Jet and Sainsburys, each time super unleaded).

I'm fairly certain that the timing was correct having been set with the head off, though with a steel rule, not a depth micrometer (though ex-fitters can use these fairly accurately!?!) and rather than relying on tension on the fag paper method I've alway used a detached piece of a .0015" feeler gauge which I position at approximately the 10'oclock position, this starts to drop anti-clockwise when the points just start to open, and is aways checked a few times for consistency, though as noted by Ian there is subtly different positions. On this point I remember talking with a guy who'd just ran an M21 outfit with a sidecar in the Paris Peking Rally in the 1980s? and he claimed they ran even more advanced at 9/16" BTDC rather than the standard 7/16" so potentially it may not be as super critical as it's made out to be. Does anyone know how it was set at the factory on new bikes? If it was so critical I suspect BSA would have been inundated with warranty returns after allowing owners to set it with fag papers and giving them a lever to fiddle with while riding!

I set it to the standard 7/16" BTDC settings. I didn't think to check it before dismantling this time though.

As regards the circlips in this piston, they were the standard wire type (2 ear) and one was still fully in its groove, the other was only half there, partially in its groove still and on that side it had been rubbing a reasonable amount on the bore (see pic).

In response to Nigel's query about the state of the top ring, it was still fully springy, so sponginess to it. All the remaining bits of ring appeared to be free apart from the scraper ring which had been trapped and eroded by the melting action. Both it and the second ring appear to have been cut through by the hot gasses, but it seems almost as though the gasses haver been acting from the gudgeon pin area upwards rather than coming down from the combustion chamber (otherwise how would they get past the relatively unscathed top ring), Sounds weird I know, as why would there be such hot gasses below the piston, unless they are coming from the top and it acts like a bullet wound where the area of impact is relatively small but more severe around the point of exit. Surprising that although the rings appear to have been burnt through yet the remaining circlip, which was in the direct vicinity, was unmelted.

The top of the piston was undamaged though there is a fair carbon build up, though whether this happened after the damage started to occur or after is questionable, as I'd not needed to have the head off for about 5000 miles (not even needed to grind in the valves!)

At the end of the day maybe when these bikes were made, in normal usage there wasn't the opportunity for running constantly at fairly high speeds for long periods as there is on today's dual carriageways and motorways and in this case I've just been too cruel on an old bike, which didn't complain enough and still got me home without too much fuss!

I'm sure there's still more input to come on this!

 photo Bore Damage Valve Side_zpsgd2k8zoo.jpg

 photo Bore damage near side_zpsxzt9bju3.jpg

email (option): steveplumb1@sky.com

Re: Piston Damage

Steve you were parked opposite me at Market Harborough, shame you had all this problems.

Re: Piston Damage

Ian C. - Which bike were you then? I should have picked up another of TT John's head gaskets too while I was there, mine ended up pretty distorted with very stretched holes!

email (option): steveplumb1@sky.com

Re: Piston Damage

The 'Beijing' M21 used standard ignition timing...I can vouch for that as I was instrumental in preparing the engine...

I don't accept the 'near enough is good enough' view of ignition timing at all...

These bikes are tolerant to incorrect settings but that doesn't mean they are running at their best if the ignition timing is 1/8" out for example...They are running, that's all...

I entirely accept that there could be a better setting than the original that may reflect the different fuels etc. of todays world...However, that is not a basis on which to randomly set the ignition 'near enough'...

Experimentation with measurable confirmation of the effects is the only way to establish a better setting if there is one...Until someone does that then I consider setting to the laid down figures in the most accurate way possible is the best course of action...

If the timing is set differently every time work is carried out in that area and a similar attitude is applied to the carb settings, pretty soon it won't be able to define the reasons for anything...

It might do to consider why BSA chose 7/16" for the M20 and 3/8" for the B33 for example...A pointless exercise if a + or - 1/16" (for example) tolerance is deemed adequate...

If there was any engine that should not have the timing incorrectly set it is an aircooled side valve single which is running routinely with a smaller margin for error due to its high operating temperatures...

Next thing we'll be reboring to + or - .020".... ...Ian

Re: Piston Damage

Steve my bike was the non standard M33 with the girder forks 8 inch brake and black and red in colour.

Re: Piston Damage

Yeah, one of my favourite looking bikes there, that B33 motor (or did I see a ZB31 engine number?) fills the frame just lovely and that black painted brake plate meant it doesn't look out of place at all. I'm commuting on a nice '47 XB31 while the outfit's off the road, but the mag's condenser's on it's way out so it's touch and go if it gets home without a cooling off (OK in the morning with no traffic on the roads) - s'pose I should nick the M21's mag for the interim period, or better still, get it sorted once and for all as I've been meaning to get around to it for the last 3 years or more!

email (option): steveplumb1@sky.com

Re: Piston Damage

The problem he has been having seems to be connected to non UK made pistons, it happened to the same make of piston that he used last time the same thing happened. I'm wondering if it could be connected with the wrong type of circlip, not necessarily being of the wrong type, but maybe the wrong type of circlip in conjunction with the gudgeon pin, namely the fact that most types of wire circlip pistons have a gudgeon pin with a slight chamfer on the very end of them, as the gudgeon pin tries to pass over the clip, the chamfer on the end forces the circlip harder into its groove, sometimes the pin has a circle machined I the end to indicate it is the chamfered type of pin and is only to be used with wire circlips usually these have no ears on them, maybe the problem arises by using the wire circlip with the ears rather than without?

email (option): davmax@ntlworld.com

Re: Piston Damage

Steve thanks for that, a PO had converted the engine to a 500 its in the V5C as a M33. It was a bit of a rust bucket when I bought it, held together with roofing bolts or so it seemed. Regards the condenser could you not fit a Bright Spark condenser? They seem to have a good review.

Re: Piston Damage

Yes we forgot he had a JPP piston in it.
No experiance of these but am very suspicious of any of the lower priced pattern parts out there.

The australian (very close to China) JP piston is known to be crap in B50/B44's - very heavy and prone to cracking/sudden failure in MX.
Bearing in mind that MX is probably the easiest form of motor racing on engine parts (not G/box though ...).

Hepolite pistons were not that good, but better than pattern stuff.

CIRCLIPS;
If a round wire circlip - clip off any tags/tangs intended to make them easy to fit and if there is no chamfer on the wrist-pin, grind a 1-1.5mm chamfer, by hand using a bench grinder or something - looks terriable but will jam the circlip into the groove if there is any side load - which there should'nt be?

Square section 'Seeger' type cirlips also benefit from having the holed tangs cutting off, as these can/do break off. The wrist pin does need to have a minimal chamfer for these. Bit of a wrestle fitting them and needs a removal groove Dremeling into the piston.

IGNITION TIMING;
I would not expect a worn engine or something of our vintage designed to run on fuel completely different from what we run, to run at its best on the 1940's timings.
That being said, you do need to start somewhere and as Ian says, a SV particuallly run under load (sidecar) for many miles is running near its thermal limit - it does need accurate ignition timing - whatever that may be?
Magnetos are great, but a sod to set accurate igntion timing and do have a habit of drifting after many miles.

FINALLY, one thought - i use an Enduro style (TrailTech) speedo which also have RPM and engine temperature - the latter via a washer under the spark-plug.
I have found this temperature guage useful for setting the B25 ignition timing and jetting - you know when you have it wromng when the temperature (whatever it normally is) rockets!

Re: Piston Damage

I shouls have written JE piston from Australia (damn, new glasses!!)

i like problems like this, reminds me of my former 'real' job.

looking again at the piston and bearing in mind that the top ring was intact, some of the damage around the g/pin holes could very well have been caused by the end of a circlip, getting trapped.

Looking at the btm pix it looks as if round section, wire circlips were fitted, is this correct?

A sq section 'seeger' circlip, when end loaded has the effect of 'punching' the aluminium supporting it, from the piston boss area - giving a granular broken appearance.
I dont see any of this.
Similarly if a (holed)tang breaks off, it causes sharp ended damage - as it is sq in section and sharp edged.

A wire circlip when it comes loose, causes a much 'softer', more rounded damage.
More like the pix.
What happens after the ciclip comes loose is then in the hands of the god's - generally the g/pin pushes against the cylinder, causing wear damage, then as circlip is worn down, it then traps between piston and cylinder and goes walkabout.............
Cutting the end bit off and bevelling the g/pin ceartainly prevent wire circlips coming loose.

A major question is why the side-loading on the g/pin in the first place (this has been commented upon many times) and was the wire circlip the correct size, damaged, or what?

Regarding sizing, i have seen metric circlips (generally all you can get) fitted into imperial bores - giving the wrong diamter and wire thickness to produce a loose fit.

My B25 was like this, luckily it was never run - a circlip meant for a 16mm g/pin had been supplied, a B25 has a 17.5mm g/pin, not only was it loose on the diameter, but the wire diameter was also too thin.
I struggled to find the correct circlip and eventually fitted PTFE plugs as i had the material and lathe.

Re: Piston Damage

Sorry piston is JCC (not JPP, as I originally posted). Wire circlips were fitted with 2 ears, the ones supplied with the piston and the grooves were of the correct rounded type for these circlips. They seemed a good snug fit at the time of fitting.

As for excessive side loading on the gudgeon pin, I'm fairly confident that the loading was on both sides, it's just one circlip parted company sooner than the other (the hottest side nearest the valves), as both sides had suffered similar damage. I've since checked the fit of the gudgeon pin in the small end bush and that is a bit on the loose side (not overly sloppy, but could be better!), whether this has any bearing on the issue? It also partially rules out the possibility that the piston was rotating on the gudgeon pin rather than the gudgeon pin rotating in the small end bush as it is designed to do. I say 'partially' as the pin's fit in the piston could have become excessively looser due to expansion if running hot.

I'm definitely fitting non-eared circlips this time and checking for a suitable chamfer on end of gudgeon pin. I've ordered another pattern piston as nothing has come of my enquiries for a NOS piston, both via the forum (thanks to those who've looked) and directly to some other potentials sources but need the bike back on the road quickly.

I have a spare cylinder available to use and I intend on having the existing one re-sleeved (as it was already on +.060") and be more actively looking out for a NOS piston to use with that cylinder.

I'm also quite tempted by the idea of fitting a temperature sensor for long runs to give me warning of impending disaster (the company who make the TrailTech speedo's NigelP mentioned also make just a smaller temperature sensor only which takes readings from a washer which simply fits under the plug).

Thanks to all who've suggested anything which may help in the future.

email (option): steveplumb1@sky.com

Re: Piston Damage

Hi Steve

Very interested in that temperature sensor.

I too do long runs under load. However, what would be considered dangerously hot for a SV?

Regards

Pat

email (option): sacombsashtrees@hotmail.com

Re: Piston Damage

There are other cheaper versions of temp guage as well - look under Karting on Ebay.
Make sure you run and that you ask for a 14mm plug temperature sensor washer.
You are on yr own if u run 18mm plugs!
You can 'ease' a 12mm washer out to 14mm using a tapered rod or a ring sizing tool(as in something that goes on your finger).

No reason to think that the piston moving on the g/pin is a problem - many, many vehicles actually press fit the g/pin into the con-rod and let all movement be in the piston - dont need circlips then...

Convert any wire circlips into 'tangless' type by simply clipping them off with wire cutters - even without removal groove they can removed using something sharp (dividers) or thin (feeler guages).

Re: Piston Damage

If you don't have complete and undamaged circlips on that piston then a bit has broken off and jiggled around for thousands of miles eroding the piston on every stroke. Note that the damage is on both sides of the pin, the bore of which provides a convenient passage for the offending bit of circlip. Only ever use NEW circlips of the correct type, been there, done that etc.

Re: Piston Damage

what can happen is that the circlip is pushed against the cylinder until it wears paper thing, then it can go walkabout - that's why you can never find anything or any witness marks of bits bouncing about, leaving you wondering if you ever fitted the thing in the 1st place!?

Nieuwe pagina 1