iani Posted June 18, 2022 Report Share Posted June 18, 2022 My engine started losing oil in large quantities via the dipstick & the catch can breather, I had the car recovered home and, as the leak was so bad, decided to whip the head off (I have this down to a fine art after 3 times in 2 weeks!) It turns out that two pistons have decided to melt, possible causes would seem to be either too much ignition advance and/or the increased CR of the new head I fitted after the first one developed a crack. Now I have to pull the motor and get it checked over & new pistons fitted, more time, expense and off road time, this has not been a good project. Quote Link to post Share on other sites
Peter Cobbold Posted June 18, 2022 Report Share Posted June 18, 2022 A combination of over-advanced spark, too high CR and lean mixture of too-low RON might do that, eventually. But if it happened in few miles I'd suspect stuck piston rings and excessive blow-by. Rather than pull the engine, remove the sump to get the pistons and check the bores. Peter Quote Link to post Share on other sites
iani Posted June 18, 2022 Author Report Share Posted June 18, 2022 9 minutes ago, Peter Cobbold said: A combination of over-advanced spark, too high CR and lean mixture of too-low RON might do that, eventually. But if it happened in few miles I'd suspect stuck piston rings and excessive blow-by. Rather than pull the engine, remove the sump to get the pistons and check the bores. Peter This is in a GT6 Peter, sump removal is doable but not so easy, in the long run I think I am best off pulling the motor. This happened within 100 miles of fitting this head, I didn't notice any issues when I had the head off last week. Quote Link to post Share on other sites
john.r.davies Posted June 18, 2022 Report Share Posted June 18, 2022 No sign of burnt through, or even any melting of the pistons you show. A melted piston would cause a smoke screen behind your car. I know, I've been there. Excessive blowby is the reason for your oil loss. The cause? You had a cracked head? Block damage? Certainly you have poorly sealing piston rings. Quote Link to post Share on other sites
iani Posted June 18, 2022 Author Report Share Posted June 18, 2022 28 minutes ago, john.r.davies said: No sign of burnt through, or even any melting of the pistons you show. A melted piston would cause a smoke screen behind your car. I know, I've been there. Excessive blowby is the reason for your oil loss. The cause? You had a cracked head? Block damage? Certainly you have poorly sealing piston rings. The top pic shows where piston crown has been reduced on the exhaust side on No5 piston, the lower pic shows the beginning of the same on No2. I had a lot of smoke coming from the engine breather, I can't say if it was also coming from the exhaust. This is with a replacement head, not the one with the crack. Quote Link to post Share on other sites
Nigel Triumph Posted June 19, 2022 Report Share Posted June 19, 2022 Sorry to see that Ian. If it wasn't for bad luck, you wouldn't have any luck at all with this engine. This was a new engine as I recall, with a hot cam and triple Weber DCOEs. I agree that you will need to pull the engine to investigate the extent of the damage and rectify. The biggest question is why did it happen? Without knowing why, there is clearly a real danger it could happen again. You know I like to ask questions! - How many miles had the engine done since it was built? - What running in regime did you use? - How were carbs set up? Could the mixture have been weak? - Could there be air leaks on the induction runs to the affected cylinders? - Ignition timing, how much static advance and what was total advance including centrifugal or mapped advance curve? I would be inclined, after repairing the engine, to run it in for at least 250 miles then go for a rolling road session to optimise fuel and ignition settings. Good luck, please let us know what you find. Nigel Quote Link to post Share on other sites
Mk2 Chopper Posted June 19, 2022 Report Share Posted June 19, 2022 What were your plugs like? If it could have melted a piston, your plugs should be destroyed also. Gareth Quote Link to post Share on other sites
iani Posted June 19, 2022 Author Report Share Posted June 19, 2022 Thanks for all the replies, I have had a call with Tony Lyndsey-Dean this morning and we believe we know what has happened. I had my car tuned up on a rolling road & jetted accordingly, when I swapped the head over last week I should have changed the jets to allow for more fuel, I didn't do this and have the damage to show for it. No5 piston is worst with No2 just showing the beginnings of damage, given that this has happened in sub 100 miles the fuelling was way too lean. Tony & I will hone the bores & fit a new set of pistons, I will also change the chokes & jets to match the new head, will book onto RR ASAP afterwards too. Quote Link to post Share on other sites
Peter Cobbold Posted June 19, 2022 Report Share Posted June 19, 2022 Ian, Fit a UEGO in the exhaust so you can read the AFR while driving, you dont want leaning-out, there lies detonation and ring damage. Peter Quote Link to post Share on other sites
Andy Moltu Posted June 21, 2022 Report Share Posted June 21, 2022 On 6/19/2022 at 9:21 AM, iani said: Thanks for all the replies, I have had a call with Tony Lyndsey-Dean this morning and we believe we know what has happened. I had my car tuned up on a rolling road & jetted accordingly, when I swapped the head over last week I should have changed the jets to allow for more fuel, I didn't do this and have the damage to show for it. No5 piston is worst with No2 just showing the beginnings of damage, given that this has happened in sub 100 miles the fuelling was way too lean. Tony & I will hone the bores & fit a new set of pistons, I will also change the chokes & jets to match the new head, will book onto RR ASAP afterwards too. That's pretty drastic for a simple head swap - must have flowed a lot more air to lean out and do so much damage in such a short time. You wonder if the initial set up was too lean and the head swap has worsened it? Or is the fuel pump not delivering enough fuel and causing the leaning off at high revs? No pinking I presume. Quote Link to post Share on other sites
Peter Cobbold Posted June 21, 2022 Report Share Posted June 21, 2022 153624 bent disy shaft, leading to over-advance on 5 and 2, and knock ?? Quote Link to post Share on other sites
John L Posted June 21, 2022 Report Share Posted June 21, 2022 If you have fitted new pistons, what sort of size bores have you allowed for the new pistons? When you say the pistons have melted, do you mean that they have picked up on the cylinder walls? Do you have any pictures yet please, just interested. John Quote Link to post Share on other sites
iani Posted June 21, 2022 Author Report Share Posted June 21, 2022 7 hours ago, Andy Moltu said: That's pretty drastic for a simple head swap - must have flowed a lot more air to lean out and do so much damage in such a short time. You wonder if the initial set up was too lean and the head swap has worsened it? Or is the fuel pump not delivering enough fuel and causing the leaning off at high revs? No pinking I presume. No pinking. I was originally running a Huco pump, it wasn't delivering enough fuel so I swapped it to a Facet redtop unit, fuel pressure was constant then. Fuelling had been set up on a rolling road, the chokes had been opened up a little and the jets changed, the head on there now has been opened up a lot more than the first one however, it will flow a lot more. Quote Link to post Share on other sites
iani Posted June 21, 2022 Author Report Share Posted June 21, 2022 6 hours ago, Peter Cobbold said: 153624 bent disy shaft, leading to over-advance on 5 and 2, and knock ?? This is a 123+ dizzy, fairly new, no knock heard. Quote Link to post Share on other sites
iani Posted June 21, 2022 Author Report Share Posted June 21, 2022 1 hour ago, John L said: If you have fitted new pistons, what sort of size bores have you allowed for the new pistons? When you say the pistons have melted, do you mean that they have picked up on the cylinder walls? Do you have any pictures yet please, just interested. John Engine is still in the car, hoping to remove it this weekend and will be investigating the following weekend. Quote Link to post Share on other sites
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