Honda-SOHC
Other Stuff => Tricks & Tips => Topic started by: Lobo on February 25, 2021, 08:29:18 AM
-
1/2 pistons scored down one side of a 250cc 2-stroke, which runs on 100:1 premix. The rings are seized into the grooves at this point.
Salvageable? (not too fussed about performance, but need reliability (outboard motor).
Why the damage so localised?
Little / big ends all seem good, as are crankshaft bearings...
Ta,
Simon
-
I wouldn't be running a 2stroke on 100:1, i run my chainsaws on 40:1 even though recommend ratio is 50:1
-
Looks like straight localised overheating to me certaily more 2T oil as well as possibly considering change to fully synthetic brand would not hurt.
If it's a little used motor then fuel degredation might also be a factor with degraded fuel combustion products causing raised burning temperatures. Do you drain it right down and replace with fresh fuel when its out of use?
Is the recommended 2T ratio really that low certainly an increase will help in my opinion downside of too much oil is obviously more frequent decoking and plug fouling?
-
Lubrication breached, obviously! But reason is important to correct.
As others, is it really supposed to be at 100 to 1?
Presumably water cooled?
Have you pictures of combustion chamber, piston crown and spark plug to see colours.
What is the spark plug identification/grade etc.
Looks like exhaust side of piston?
-
... it’s a ‘new’ motor to me; and 20yo I guess. 100:1 is the correct mix, and indeed that sticker remains on the side of the engine.
Yep, water cooled - though now apart its arteries are seen to be a wee furred up and will be corrected.
The piston crowns indicate a slightly rich burn, as do the plugs, but tbh it’ll only run on choke at the mo (or throttle) - hence the tear-down.
Nothing looks horrible in there ....
All the ignition components / plugs now new, the carb galleries & jets cleaned through to an inch of their lives, float level ok, Reed valves look & ‘feel’ ok, and yet it continues to die if idle drops below choke-on speed, accompanied with a lot of shuddering & the odd back fire.
I’m kinda at last resort, and will change all the crankcase gaskets and seals; the piston issue is unexpected and new to the problem.
....if (unburnt) gases are leaking past the seized piston rings would this cause the no idle & backfire?
-
With rings stuck it will compromise most stages of running as it's effectively sucking (drawing in the cases) as well as holding compression. It's more or less like having leaky cases in a way.
Dilution of crankcase gases with burnt blow by effectively reduces the volatility of the next charge phase. This is what they do on exhaust gas recirculation systems in car engines, but only at specific load demands.
You'd need the rings working properly to make realistic assessment of the running as a prerequisite.
-
Thanks gents. (So far 🙄)
I started cleaning all the deposits off the cylinder block today... and found the cause. Some PO has inexplicably plugged a cooling gallery with a SS screw, and then, for good measure put putty about the whole lot... causing an interruption of water to the cooling jacket in a localised spot.
Intrigued I dremelled away all the putty, and removed the screw. So far, I can see no damage to the casting, indeed it ‘rings’ consistently everywhere I tap it with a screwdriver. But I’m bloody confused as this hole is within a boss, and is tapped (M6). It’s other end is the common cooling gallery, which feeds about 4 other such cooling inlets (varying sizes, shapes, tho’ none others with screw threads.)
Honestly I’d have sworn the head had never been removed as no markings whatever on the painted fasteners. That said, it don’t look like a factory mod!
Any ideas please? Could it be a ‘hot climate’ option.... ie to supply more water to the jackets? I’ve looked the internet for flow paths, cylinder block photos etc but keep coming up blank.
Semi planning on leaving the hole clear, and having a rebore with .25mm o’sized pistons. (though the scoring not deep so will first try a bit of 600 grit sanding.)
All thoughts & ideas welcome... it’s an engine that won’t see much use so not too critical wrt perfection)
Ta.
-
I can't think immediately of a competent reason to restrict the water way there.
Trying to determine exactly where that is in relation to the piston damage, it looks like it's the same place?
As it's been running like that, think I'd get the rings free and gently clean the "snots" off very lightly, then put it back together and run it to further assess and answer questions. Put it on 50 to 1 ratio and see what you've got then.
If you rebore and go with new pistons, then any initial problems with it may take those as well. I know it's not mechanically ideal to reassemble but will tell you a lot with very low risk.
As you'd expect in an engine like this, those combustion chambers in the head are focused on complete burn with low thermal problems, it shouldn't intrinsically be making too much heat, making the cooling jacket modification distinctly odd.
It should run pretty well with the pistons and rings cleaned, then give the confidence to go with rebore etc if it's debugged fully.
-
Forgot to add, yes I'd clean out the bodge in water jacket.
Reasoning that it can't run like that, all things considered, and if there is a problem subsequently it would then show up if you run it with the components you've got there already.
-
Thanks Nige...
Yep, the scoring (piston & sleeve) are exactly adjacent to where the putty / nil cooling was. I’m still at a loss why the bodge... common sense shouts ‘crack’ in the casting - except I just can’t detect any. Muddying it all is the boss (versus straight hole) and tapped thread.
(I pondered, ‘temp sensor?’ - but there’s no corresponding hole in the cylinder head)
I gingerly tried to extricate the lower / easier ring this morn, and after an hour of coaxing it had it slightly proud of the piston - but then snapped. Tbh, it was nowhere free; and even now a 10mm bit remains ‘welded’ in the groove.
And that was the easier ring...
So - I’m up for at least 1 new piston & rings. Decision time. Do I buy a standard piston and bung it all back, or rebore and go with 2x +1 size pistons?
The localised scoring is significant, ie easily felt with a finger nail. How this translates to severity / reliable idle - I don’t know. All the engine needs to do is push a 10’ tinny about the local waters for a couple of hrs a month; if the consensus is “she’ll be right” then I’ll be totally happy to just bung in a replacement piston and rings.
Water jacket and all galleries all now cleaned thoroughly of build-up; cholesterol levels down to zero.
Appreciate the help - thanks,
Simon
-
any chance of picking up used parts?
If it was mine, I would want to throw it together for minimal cash just to see if it is worth the trouble. For what you want it to do, spending money on a rebore and new pistons
is the last thing I would want....but...unless you enjoy rowing...you want it to be reliable
-
Is it possible to assemble the motor castings only and apply some water pressure to the cooling system and check for any leaks?
-
Cracking would be my fear too, but it does show evidence of corrosion removed material around that section of the barrel compared to all the rest of both barrel circumference. Maybe that's what they responded too in trying to add material.
You can reface the top surface by double siding tape onto glass sheet some abrasive papaer, then use paraffin to lubricate while you "sand" the surface. Something like 180 grit for initial cut to see how it cleans up.
Piston, can you get rings for it? You can carefully clean out the ring groove with a flat needle file to get the old ring to check clearance before re-ringing to get it running and assessed. If you haven't got a needle file, then you can glue some abrasive paper to one side of a lolly stick to gently clean the ring lands.
Worthwhile trying if you can re ring, alloy wheel cleaner (usually contains sulphuric/hydrochloric acid or similar) to see if it'll clean the carbon tar from bore walls.
Not an easy way through it, but with it not working now the piston is scrap anyway. If you can at least get it going competently like this, then you'll have the surety to go for a rebore longer term.
That's a huge amount of blow by on that piston, and likely to be significant in poor running, but most probably caused by the "modification" originally.
-
They usually suck in water and pass it through jacket, then straight out again.
After salt water they are usually purged with clean before storage, not sure if inland water ways would have the same need to clean.
-
Have you seen the c**p floating in inland waterways?
-
Thanks gents - some useful thoughts.
Appreciate the comments Nigel, will do as you suggest, and yes, I can just buy the standard rings. Should I leave the liner scoring as-is, or risk making the bore out-of-round by dressing said scoring out?
Like the idea of sealing it up and applying a hose - thanks ka-ja. The block is designed like a bloody colander - but I’ll give that some thought today.
Sump M, yea, that’s my thinking, though eBay very thin on used parts. I’ll go as K2-K6 suggests and reuse the old piston, but with new rings.
Yep Ken; raw sea-water has been cooling this 10HP motor all it’s life. (tho hopefully followed up with a fresh-water flush after each use)
That chip is significant, but I’m hoping the head-gasket crushable component will ‘take the strain’ sealing against the liner.
Cheers all, appreciated,
Simon
-
Were you using a TC-W3 marine oil as outboards run a lot cooler in the cylinders than bikes and it is designed for the cooler running engine. At 100 to 1 mix as used by a lot of Yam and Suzuki outboards it doesn't pay to use the wrong oil.
-
Thanks Moorey - tho’ the engine new to me and I’ve no idea of it’s history. From the localised blocked jacket cooling (exactly where the piston rings seized) and the fact that all else looks good internally (ie bearings / surfaces, piston skirts / crowns etc) I reckon the T2 oil delivery was likely not the issue.
I will however likely go 50:1 once I get it running...
Cheers.
-
"Appreciate the comments Nigel, will do as you suggest, and yes, I can just buy the standard rings. Should I leave the liner scoring as-is, or risk making the bore out-of-round by dressing said scoring out?"
If you can clean off the baked carbon/burnt oil then leave the surface as is. It's of course nowhere near perfect, but with rings free and cooling working it should be better than first appearance appearance would suggest.
-
The reason I asked about the type of oil which I notice you didn't actually answer is using non marine oil makes water cooled outboards
prone to seizing on the top ring.
-
Hi again Moorey. I couldn’t answer you type of oil as I’ve not bought any yet - the engine is new to me, as is the 20L tank of fuel mix it came with... which is what I’ve been starting it on.
I will indeed use marine grade 2T when the time comes. (I always thought Marine 2T was about the biodegradability aspect tbh!)
Nige, have ordered a complete piston set off eBay from Singapore at < £30 landed which I’m happy with - should be here in a week or so.
As per your suggestion sanded down the various faces this arvo & pleased with the results. Thanks.
Latest pics of the block - I’m still stymied at the threaded boss within the water jacket - I can’t imagine what kind of ‘thing’ it may fasten. Certainly exploded diagrams of the block don’t show / indicate anything. The head gasket covers it all up, further mystifying the query.
Ken - you might be right that the bodge was in lieu of a chipped pot... it is actually quite extensive. I scratching my head tho’ at how any PO might have been alerted to a (presumed) gas / water leak as outboards naturally eject (cooling) water out of the exhaust - let alone everywhere else. (Ie the leg & tell tale stream)
Thanks gents...
-
Certainly looks better now cleaned a little, nice work.
You always have that dilemma when trying to recover things like this as to just how much metal you can remove. It's just about there regarding getting a decent surface to seal with the gasket. I'd use blue Hylomar on both the metal and the gasket, coated and let them dry for 30 mins before assembly to propagate the best conditions for a competent seal with it.
Interesting point on the marine oil Moorey, like Simon I knew they used specific oil, but not the intricacy of why.
Thinking about it today, and was wondering if it could have had a sacrificial anode in that location within the water jacket to prevent the aluminium being eaten? Can find them like this https://www.anodeoutlet.co.uk/product/00831-tecnoseal-zinc-cylinder-head-anode-826887-for-mercury-mercruiser-outboard/ and looks a possibility in terms of what other's implement.
-
I'm sat here wondering what the small trench cut into the barrel housing is doing. It runs almost the entire length of the barrels but doesn't appear to have an exit just an entrance, shame the head is so crusty as it may be hiding the exit. The small mod and filler could be to deal with a leak from that trench, was going to say waterway but it could be something else, like an oil passage running next to the water jacket to keep the oil cooler, if there was a leakage between the 2 maybe the filler was there to stop it.
Any idea what that feature is connected to?
I was wondering too, but looks like the port you see could be a chimney out of a chamber running underneath that "trench". If water came in at t'other end then crossed the block around the exhaust ports, it could then come up the chimney and out via the heads.
Possibly it'd give quicker combustion chamber warm up (leaving barrels alone) to get it off choke faster.
-
Wow Nigel, I reckon you might have hit the nail on the head with your sacrificial anode - I’ll go about looking for a zinc M6 bolt thingy and fit it regardless. Thank you. (And for your thoughts on Hylomar).
Ken, that trench is a dead area as it were, and is not used in oiling / cooling - maybe just an “is what it is” of casting?
Anyways, the hot water from the block / jacket exits via the bulged area (LHS previous photo) up into the thermostat housing. From there it is it then channelled down the length of the cylinder head (the head gasket being one side of that channel) before descending back into the oblong hole (R of previous pic) and out into the exhaust. (From here it goes down the leg and out through the centre of the prop)
FYI the head not yet been cleaned up!
Thanks again gents, really appreciate this site.
Simon
-
Ta da! You should have been a detective!
Ordered...
-
Ta da! You should have been a detective!
Ordered...
Ha ha, good result then and answers the question of mystery bolt.
I had the good fortune at initial training during apprenticeship to have some very lucid outlook people teach me, significantly influenced by aaib farnborough. So in a way metallurgical detectives.
-
...Nige - you ever meet Phil Gilmartin (RIP) AAIB / Farnborough?
PM if ya did...
-
Unfortunately no Simon. I was elsewhere in RAE with mechanical engineering, but the whole place was informed by the investigative findings from there with knowledge and people that had worked within.
Essentially the failures being a foundation of all works in first teir R&D and subsequent developments to avoid those eventuality. I was toolroom which did alot of wind tunnel modelling in scale from roughly a micron up to about 5 meters.
Absolutely incredible materials knowledge base, and skills.