Holed Piston

[tricky1962]

I've just got back from a jaunt in France. I had 500 trouble free miles on French autoroutes at a steady 70mph terminated by a holed piston, a 1cm (0.5") diameter hole in the centre of the crown. No unusual noises heard until about two seconds before I lost the cylinder.

Now I understand that usual causes are weak mixture or advanced ignition, but the crown, cylinder head and plug condition indicate that all was running well, possibly on the slightly rich side. The only fault I've found so far is what I would consider significant free play in the little end bearing/con rod interface. The free play is hard to measure, but I would estimate +0.25mm.

So my question is, could wear here produce a holed piston?

Cheers all

[Alan H]

Only if the piston was getting so close to the spark plug that it was sparking to the piston.
Did the plug still have the electrode on?
Weak mixture (caused by many things - cracked/loose inlet rubber, weak mixture (running on reserve with one carb feed not filling the carb correctly), timing out slightly, loose points.
0.25mm/10 thou is quite a lot of free movement. Are the oil feeds to that pot OK?


Or just bad luck.

[tricky1962]

Cheers

Plug arcing to the piston crown is an interesting idea. The plug looked fine-ish - I had to do several miles on the other cylinder just to get somewhere safe and have a look. Electrode still in place. I'm pretty sure the mixture wasn't weak and checked the timing before setting off.

I'd had a lot of problems with bubbles in the lines until I bought those new check valves off ebay about 1500 miles ago.

Do you know the thickness of the squish band in a standard cylinder? Is 0.25mm a significant chunk of it?

I hope it's bad luck, but then am not happy about going anywhere too far. I've got Carole Nash European Recovery, but caught up in the mad dash home at the end of the holidays and it took me 14 hours to get home from Dover - 75 miles and no sleep for 36 hours

[Alan H]

The 500 doesn't have a squish band as such, but the head to piston gap should be over 10thou, even at the edges of the piston. Looks like a teardown and clean the crankcase out (you don't want any ally ex-piston bits in the bottom end) and get some new little end bearings to see if it's the gudgeon pin, little end needle roller bearings or loose piston/gudgeon pin fit. If the pin is OK in centre and ends where it goes in the piston, and the bearing is OK too, then it'll be a new conrod which means a crank job. Hopefully not but if so then there are others on here that can help.
I'm sure I already asked, but where are you in the UK?
If you stick it on your profile, I'll not need to ask again when I forget again!

[tricky1962]

In answer to your last question - Sarf of you mate! - Eastbourne.

New little ends on their way. Going to flush the bottom end out with diesel and see how well the new bearing fits in the con rod and with a new pin. If that's slack, then a new con rod will be on order

[Alan H]

In answer to your last question - Sarf of you mate! - Eastbourne.

Bloody hell - just a bit!!

New little ends on their way. Going to flush the bottom end out with diesel and see how well the new bearing fits in the con rod and with a new pin. If that's slack, then a new con rod will be on order

Fingers crossed. Just make sure you get it all out. Check in the exhaust for loose stuff too as some will have got out that way (hopefully most of it.)

[Coyote]

I see you mentioned new check valves from eBay. There was some discussion a while back about those things. The big issue is the springs and how much pressure it takes to open the valves. That information is a total unknown. I would say without hesitation those valves are what nailed you. Not enough oiling or none at all.

[Suzukidave]

I would suggest a leak down test after you get the engine back together , i have seen tests that air leaks show up in the strangest of places . Are you running a stock air box ?

[Vintageman]

When you were cruising that steady 70 mph for a period of time a few things come to mind right away.

You were holding the throttle at some fixed position 1/8/ 1/4/ or 1/3, etc. open for a long period. If you recall that to be true check the jetting circuits in use for that position for dirt or for example if 1/3 – 2/3 see if needle jet inside barrel is pitted. Many are when due to decades worth condensation corrosion. If so it can cause a lot of restriction to fuel flow and you run lean. Float height import in all cases too.

If you were running under 1/4 still check jetting, but now a subtle air leak such as cranks seal may lean you out more readily. Did bike idle well and steady? All Exhaust pipes had even flow? If not another clue seal may have been leaking to a point of harming.

Also remove fuel line and verify fuel flowing fast enough and long enough to keep up with demand. Finally check gas cap to ensure it is venting. If not after a while tank builds up a vacuum slowing gravity feed fuel flow to carbs.

My experience if you had throttle open more than a bit, engine putting out a fair amount of power you were lacking fuel flow and leaned out.


If your bike had many thousand of miles on piston and plug was a little too hot ( and fuel a lttile too lean) it slowly erodes aluminum to a point if finally holes through

Good luck

[tricky1962]

Thanks for all the responses.

Just to reiterate, I doesn't look to me as though the bike was running lean in any way.

It is possible that lack of oiling caused the wear - would such wear cause a holed piston?

Oil consumption for the trip was one litre in 700 miles (2.8 US pints I think). That works out at 40:1 so reasonable consumption, and if oil was restricted to one side of the engine, I would have thought that the other side would have smoked a fair bit more - not the case here, so I'm standing by my check valves for now.

[Vintageman]

Usually a holed piston too hot too lean ... can't shed heat fast enough (air cooled). fuel cools too so a little more is better than a little less in air cooled engines. (Wrong plug e.g. too long).

My advice is to never hold 2 stroke at a fixed throttle pos for too long unless you are very sure. that spot/running condition may be be too lean for power being generated and amount wasted as heat only. And if a lot of cruising expected run one step cooler plug that ride.

On average riding, throttle varied often (serveral jet circuits in play) things can look OK.

Also fuel feed restricted by petcok or poor tank vent can cause the same... too lean and takes a while so again thing typically look ok until you pull for a long time gulp gulp the fuel up

Have you ever looked into the barrell of your needle jets for pitting? Need good light and good eyes to see it

If you were lacking oil you would see a lot of skirt damage too. based just on post info so far I am evisioning just a finger size hole above plug and little skirt damage in comparison

do you have pic of piston?

[tz375]

Insufficient oil causes the piston skirt to get hot and as that heat raises the crown temperature it can lead to a hole but teh skirts would show very heavy signs of seizure.

When the top blows because of lean mixture or advanced timing, it can lead to seizures but often teh skirts look almost fine. That will probably pint you in the right direction for a cause.

When a 2 stroke melts down at low throttle openings it is usually because it is lean on the needle/jet and not on the main and it doesn't usually show up like a lean overall mixture because at other throttle openings it may be OK. It's also common to melt down after a bit of a thrash as the motor gets hot and then we shut off the supply of cool fuel and air and shut down the oil as well.

Please post pictures of the plug, pistons and cylinder head.

[tricky1962]

If I've done it right here are pics of piston crown, skirt, underside and cylinder head.

http://s1062.photobucket.com/user/trick ... sort=3&o=3" onclick="window.open(this.href);return false;
http://s1062.photobucket.com/user/trick ... sort=3&o=1" onclick="window.open(this.href);return false;
http://s1062.photobucket.com/user/trick ... sort=3&o=2" onclick="window.open(this.href);return false;
http://s1062.photobucket.com/user/trick ... sort=3&o=0" onclick="window.open(this.href);return false;

I don't see signs of weak mixture, so am leaning towards play in the little end brought the piston crown too close to the spark plug at the point of ignition, or my timing marks are inaccurate. Still not sure if little end play can have this effect, but it seems the most likely.

I have purchased a dial guage to check the accuracy of the timing marks once the engine is assembled.

Also, taking on board Coyote's comments, I intend to reassemble the engine, then check the oil flow to both cranks by loosening the banjo bolts and compare the oil flow between both cylinders, then run the bike for 1000 miles, whip the cylinder off and look for free play in the new little end.

Any thoughts??

[Vintageman]

OK that (what I was thinking) does not look like lack of oil prob to me.

In fact maybe too much and you had a carbon build up (decoke maintance shceduled service may have been missed... leaky oil line check valve)

How many miles on that piston? If many miles I think the plug has been a little too hot I think the T500 bounce between a NGK 7 and 8 (cooler) too much carbon and a tadd too lean to help cool

All the above may have been a slow problem over time (which it looks to me) versus a sudden melt down. Should be very fixable for you

If small end bearing that bad you would here pinging like noise particularly when you rev up and let engine balance bewteen pull and coast

I assume your timing was OK. If it was OK but, you had not checked in a while they (points) typically wear towards retard not advance.

These are old bikes air coolded need proper fuel to help cool

And this is liquid cooled -> https://www.youtube.com/watch?v=DKKudxE1wGA" onclick="window.open(this.href);return false;" good advice... go up a step in your pilot jets if you run fast smaller throttle

Thanks for sharing this with me

fun stuff

[tricky1962]

Thanks for the quick response. Last decoke was 7000 miles ago and since then I have done city riding and very slow (50mph) highway stuff only. This was the first long trip, so got it up to 4000 rpm and left it there. Don't know how many miles the engine has done, but the bores are good and the clock said 10 000 miles when I got it.

Since the last decoke, when I first got the bike, I had had problems with bubbles in the oil line for about 4000 miles so may have been running the oil pump too open, or if wear had occurred this is when it might have happened.

Plugs are NGK B7HS and I've had no problems with oiling up of the plugs.

As to noises, I know what pre-ignition sounds like on a four stroke, but the only noise this engine/gearbox would make was an intermittent loud rattle between 2500 and 3000 rpm whether accelerating or coasting. At 4000 rpm it sounded just fine. This was not consistent and I put it down to something to do with a worn kickstart mounting. Not sure now.

Riding like that Saab guy is not going to make me popular on European highways - everyone is using cruise now and I would be making enemies. Clearly I need to stay off the highway, but that would have added 3 hours to my journey on this occasion and I wouldn't have made my destination on time.

Going up a size on pilots would richen the mixture, so are you suggesting I was too lean because I had too much oil, rather than not enough petrol?