The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: David Tinsley on 08.07. 2024 17:37
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55 GF SA
One of the pistons looks as though it has been through some strife - overheating?
Anyone got any pointers on what would cause this so it does not happen again? Timing off maybe?
Thanks,
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Not seen that pattern of damage before. The little marks show something has been banging about in the cylinder. What are the bores like? I see its only +20 so you have some room to play with.
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I have not measured the bores yet, but visually they look fine, also no damage inside the combustion chamber on the head that I just cleaned up this morning. Hopefully I will get away with just new pistons.
I have never seen that kind of damage either so a bit curious. Sure someone will come along and know what the probably cause was.
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New pistons seems a bit OTT just for that damage.
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I did think I could dress the damage out, but would it still be prone to hot spots causing further issues?
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Always hard to tell from just looking at a picture but it looks like the damaged spot on the left is a indent and it looks like a foreign object embedded in the piston on the right. If it is a object of some kind you need to try and work out where it came from, have a very close look at the valve and guide and the inlet tract , carby etc. Definitely replace the pistons.
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What do the valves look like? If it’s a bit of metal thats been knocking about marks should be found on those.
Could a previous owner have installed the pistons already damaged?
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What do the valves look like? If it’s a bit of metal thats been knocking about marks should be found on those.
Could a previous owner have installed the pistons already damaged?
Valves look fine (apart from being worn), took them out today.
I think I am going to replace the pistons, but was interested in seeing what people though about how it got in that state. I was concerned it might be heat damage, but on closer look it does like debris found its way there and caused that damage to the crown
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I would absolutely replace those pistons. Its impossible to tell how far though the damage goes and there are almost certain to be cracks. If the crown cracks and breaks up it will fill the engine with metal flakes and almost certainly take out the bottom end.
Please saw the piston in half through the damage and let's see what is going on.
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What do the valves look like? If it’s a bit of metal thats been knocking about marks should be found on those.
Could a previous owner have installed the pistons already damaged?
Valves look fine (apart from being worn), took them out today.
I think I am going to replace the pistons, but was interested in seeing what people though about how it got in that state. I was concerned it might be heat damage, but on closer look it does like debris found its way there and caused that damage to the crown
It’s possible its casting flaws, not likely but possible, the pic attached is from my holed B44 piston, I reckon due to a flawed piston and not detonation etc. So flaws can look like pretty much anything. It was also cracked from rings to bottom of skirt.
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It appears that the piston crown has been machined and become too thin.
I know the valve pockets are much larger. Just a guess but you can compare the pistons with original ones.
Cheers Klaus
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I would also suspect 'foreign bodies', judging from the light peppering all over the crown. You can also see it has been turned down at some point (although for all I know, which is nothing, they were finished like that to get the concave shape).
I wouldn't worry too much now about the 'Why and how?' but would definitely replace the pistons, check very carefully for missing bits of casting around the valve guides and inlet tract, and also get the bores measured accurately top to bottom all ways up. No point spending good money on new pistons for worn bores really, although I'm sure we all have from time to time (says he guiltily). (Are there any areas of the crown that are faintly magnetic . . .?)
It also looks as if there's some marking on the upper surface of the top compression ring, but no matter now.
I'd just look forwards - rebuild with good bits, ensure carb and timing are set up correctly, and then be pretty confident it will all be OK.
In for a penny in for a pound frankly, with the crank etc to start the cash outflow. Ouch, but will be worth it Dave.
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I would also suspect 'foreign bodies', judging from the light peppering all over the crown. You can also see it has been turned down at some point (although for all I know, which is nothing, they were finished like that to get the concave shape).
I wouldn't worry too much now about the 'Why and how?' but would definitely replace the pistons, check very carefully for missing bits of casting around the valve guides and inlet tract, and also get the bores measured accurately top to bottom all ways up. No point spending good money on new pistons for worn bores really, although I'm sure we all have from time to time (says he guiltily). (Are there any areas of the crown that are faintly magnetic . . .?)
It also looks as if there's some marking on the upper surface of the top compression ring, but no matter now.
I'd just look forwards - rebuild with good bits, ensure carb and timing are set up correctly, and then be pretty confident it will all be OK.
In for a penny in for a pound frankly, with the crank etc to start the cash outflow. Ouch, but will be worth it Dave.
Yes, the whole engine is now disassembled and ready for cleaning and examination. Then hold my breath and start writing cheques! But it will be worth it.
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In a situation like this there are a few decisions to be made, all have place and in the end it depends on that good ol' cost/ benefit ratio.
For a good reliable runner and keeper, a full strip new everything (echo "OUCH") does it. At the other end of the scale, used pistons and rings or even a weld repair to that crown (looks like a hole?) gets the bike back in use in the short term. Like Mr G says, going for new pistons really need a nice fresh bore for peace of mind and good performance.
Those pistons look like the dished top low compression type. Have a look for makers' trademarks and numbers inside. BSA pistons will have the piled arms. Plus 20's here in all probability AE Hepolite. Flat top pistons, (Original Hepolite ref 11062) and a 356 cam give a more lively ride, but now we are adding substantially to the costs. In any case now the motor is apart completely we need to pay attention to the crank in all its glory....more expense. And that itty bitty oil pump.
Major decision. Always fit an air filter!!!
That top ring may be worn to a T profile, a sure sign of ring and piston groove wear and a recipe for burning oil, as the up and down movement of the ring in the groove transfers the oil film of the bore into the combustion chamber.
Swarfy.
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In a situation like this there are a few decisions to be made, all have place and in the end it depends on that good ol' cost/ benefit ratio.
For a good reliable runner and keeper, a full strip new everything (echo "OUCH") does it. At the other end of the scale, used pistons and rings or even a weld repair to that crown (looks like a hole?) gets the bike back in use in the short term. Like Mr G says, going for new pistons really need a nice fresh bore for peace of mind and good performance.
Those pistons look like the dished top low compression type. Have a look for makers' trademarks and numbers inside. BSA pistons will have the piled arms. Plus 20's here in all probability AE Hepolite. Flat top pistons, (Original Hepolite ref 11062) and a 356 cam give a more lively ride, but now we are adding substantially to the costs. In any case now the motor is apart completely we need to pay attention to the crank in all its glory....more expense. And that itty bitty oil pump.
Major decision. Always fit an air filter!!!
Thanks Swarfy,
I am going to do a full rebuild properly, pointless being this far into it now and trying to scrimp on things. I want to do this once and do it right. I am happy to stay with the low comp pistons and the original camshaft looks OK. I am not going to be wringing its neck and happy to pootle along at a speed that befits my age!
That top ring may be worn to a T profile, a sure sign of ring and piston groove wear and a recipe for burning oil, as the up and down movement of the ring in the groove transfers the oil film of the bore into the combustion chamber.
Swarfy.
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any damage to cyl head ? did you check the bottom of the valve guides any broken pieces ?
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Surprisingly the cylinder head has no damage and the guides appear OK, but worn, so getting the head overhauled.
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Something fell down the plug hole?
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very strange damage to that piston, its definitely had something bouncing around in there which has caused all the smaller indents, I dont understand why there is no indications on the head or valves though, that makes no sense - unless as Swarfy suggested the piston was reused from elsewhere.....it also looks on closeup as if there is "something" still stuck in the RH hole (maybe?), maybe the small indents were from another earlier usage and these 2 major failures have occurred in this application??
Certainly not reusable IMO.......!
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I'm wondering if there was detonation and a weak mixture that softened the piston crown then a soft spot came out at TDC on due to the piston acceleration. Its a very long shot but it could explain why the edges of the damage are above the surface. The other explanation is like an amour piercing shell that leaves a raised edge which would imply debris but where from?
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i know , the mystery ball bearing from worty's engine has been hanging about with that piston
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Does the hole nearer the centre of the piston correspond with the spark plug electrode. Have you checked the plug for damage.
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I did wonder if it was related to the plug position, but it isn't. I fitted new plugs and chucked the originals away. The new plugs are still fine.
When I get a moment I am going to saw that piston in half across the damage and see if it gives us any more clues as to what happened.