Jonny Posted September 3, 2022 Share Posted September 3, 2022 If not clear I’m talking about the dimpling at the top above the valves. Quote Link to comment
bluedog59 Posted September 3, 2022 Share Posted September 3, 2022 Could be a bit of detonation, anything on the piston crown ? Quote Link to comment
johnr Posted September 3, 2022 Share Posted September 3, 2022 is it on every cylinder? Quote Link to comment
Jonny Posted September 3, 2022 Author Share Posted September 3, 2022 36 minutes ago, johnr said: is it on every cylinder? No just this one Quote Link to comment
coombehouse Posted September 3, 2022 Share Posted September 3, 2022 It looks like damage from something that has got into the cylinder at some time in the past. If the piston doesn't have matching damage, it's probably been fixed in the past. 3 Quote Link to comment
Jonny Posted September 4, 2022 Author Share Posted September 4, 2022 14 hours ago, bluedog59 said: Could be a bit of detonation, anything on the piston crown ? Yes there are some corresponding marks on the piston Quote Link to comment
bluedog59 Posted September 4, 2022 Share Posted September 4, 2022 It looks like it has started to have a nibble at edge of the crown and I would guess that it's just started on it's way down to the rings. Do the other cylinders show any damage in the same area or do they have a slight (normal) carbon build up ? Quote Link to comment
Gixer1460 Posted September 4, 2022 Share Posted September 4, 2022 Difficult to see but next piston seems blacker so either it's richer or this is leaner / timing too advanced? Seems focused on exhaust side - detonation usually seen all over unless particular hot spot! Quote Link to comment
FJD Posted September 4, 2022 Share Posted September 4, 2022 A stone might have banged the dents in piston and head Quote Link to comment
dupersunc Posted September 4, 2022 Share Posted September 4, 2022 I'm going to say detonation, if it was foreign object damage it wouldn't be localised to one part of the piston. The exhaust side is usually where det starts. 1 Quote Link to comment
Joseph Posted September 4, 2022 Share Posted September 4, 2022 14 minutes ago, FJD said: A stone might have banged the dents in piston and head That would mean running pretty big main jets in the carbs Quote Link to comment
george 1100 Posted September 4, 2022 Share Posted September 4, 2022 It looks like detonation Quote Link to comment
bluedog59 Posted September 4, 2022 Share Posted September 4, 2022 7 minutes ago, FJD said: A stone might have banged the dents in piston and head I think that's very unlikely. When the squish band is working correctly there is usually very little colour on the piston crown (as there's minimal mixture there to burn). Ignition timing ? Is there any sign of damage on the corresponding piston in the crank throw ? The one which is 180deg to the damaged one. Hot spot to promote a bit of detonation ? It is right next to the exhaust valves. I would have a look at your squish clearance across the cylinders. Is it a stock engine or has it had some work done ? Quote Link to comment
coombehouse Posted September 4, 2022 Share Posted September 4, 2022 (edited) It looks like there might be a few similar marks on the other side of the piston too. There are corresponding marks on the head. Hard to see clearly on a phone. I have seen similar when a spark plug fell apart but the damage was much more jagged. Edited September 4, 2022 by coombehouse More info 1 Quote Link to comment
dupersunc Posted September 4, 2022 Share Posted September 4, 2022 29 minutes ago, coombehouse said: It looks like there might be a few similar marks on the other side of the piston too. There are corresponding marks on the head. Hard to see clearly on a phone. I have seen similar when a spark plug fell apart but the damage was much more jagged. Good spot. The marks on the inlet side do look more like FOD. Quote Link to comment
Jonny Posted September 4, 2022 Author Share Posted September 4, 2022 1 hour ago, bluedog59 said: I think that's very unlikely. When the squish band is working correctly there is usually very little colour on the piston crown (as there's minimal mixture there to burn). Ignition timing ? Is there any sign of damage on the corresponding piston in the crank throw ? The one which is 180deg to the damaged one. Hot spot to promote a bit of detonation ? It is right next to the exhaust valves. I would have a look at your squish clearance across the cylinders. Is it a stock engine or has it had some work done ? All the other pistons and parts of the head seem fine. The engine is a 1230cc so not stock. There is far more carbon build up on the other pistons. Might be worth saying that the reason the engine is in bits is I had a leaky head gasket and that is the cylinder that oil was leaking into. Quote Link to comment
Jonny Posted September 4, 2022 Author Share Posted September 4, 2022 (edited) 45 minutes ago, coombehouse said: It looks like there might be a few similar marks on the other side of the piston too. There are corresponding marks on the head. Hard to see clearly on a phone. I have seen similar when a spark plug fell apart but the damage was much more jagged. Yes there are marks on both sides although worse on the exhaust side. The piston shows matching damage but much less Edited September 4, 2022 by Jonny Quote Link to comment
bluedog59 Posted September 4, 2022 Share Posted September 4, 2022 Ah, big bore kit upping the compression ratio then a bit of oil being drawn in raising it a bit more (oil being non compressible) and bingo, detonation ! 1 Quote Link to comment
bitzz Posted September 4, 2022 Share Posted September 4, 2022 What does the valve seat look like? Quote Link to comment
Gixer1460 Posted September 4, 2022 Share Posted September 4, 2022 (edited) 8 hours ago, bluedog59 said: When the squish band is working correctly there is usually very little colour on the piston crown (as there's minimal mixture there to burn). Eh? Squish is used to force mixture into the centre of the chamber so that is where the best burn occurs. Edited September 4, 2022 by Gixer1460 Spelling Quote Link to comment
bluedog59 Posted September 4, 2022 Share Posted September 4, 2022 22 minutes ago, Gixer1460 said: Eh? Swish is used to force mixture into the centre of the chamber so that is where the best burn occurs. Sorry, I thought it was obvious that I was referring to the squish band area of the piston crown. A case of knowing what I meant but not wording it clearly. Quote Link to comment
Jonny Posted September 4, 2022 Author Share Posted September 4, 2022 5 hours ago, bluedog59 said: Ah, big bore kit upping the compression ratio then a bit of oil being drawn in raising it a bit more (oil being non compressible) and bingo, detonation ! Thanks! That’s good to know! Quote Link to comment
imago Posted September 5, 2022 Share Posted September 5, 2022 That doesn't look like detonation damage to me, the dimples are too deep, small, and uniform in shape. It looks like some bits have been drawn in or left in. Something small and metallic and hard I'd say. Too round for ring bits, but it could be anything really. The other thing that makes me say material in there is that nearly all the damage is on the low side of the piston. When it's all closed up and in the frame anything in the combustion chamber drops to that side. It's where you see the rust if an engine's been sitting with water in the bore for example. 1 Quote Link to comment
TonyGee Posted September 5, 2022 Share Posted September 5, 2022 It could be a tiny stone or something !!!!! how many times have you seen bits of crap round a spark plug ? and when you take it out the crap falls in to the combustion chamber 5 Quote Link to comment
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.