Callumgw wrote:nice summary Damo,
I never remember which is 1 and which is 4. The second from the dizzie, third from cam belt is the excessively damaged one and the burn crown is right next to the dizzie.
The first ring is a long way down from the crown, I'd be surprised if it pinched. There is spalling on the skirt of one of the other pistons and that ring had failed there, so that still consistant with your approach. That piston looks clean and nice on the crown though....but had a valve mark
Could the cam be badly ground? with too much height and too sharp an open radius?
C
All good.
The front of the engine (#1) is at the balancer, and #4 is closest to the gearbox in 99.9% of all engines.
You will (should) also find that the little dimple mark in the pistons will be oriented toward the front of the engine, and the stamped numbers in the rods should face the exhaust side of the engine. These two a sure fire way to tell if a hack has assembled the engine or not.
I pulled down one of my engines which had a fairly heavy valve to piston episode, and it had pinched and broken 2 out of 4 piston's rings.
And another spares engine I had, had a semi melted piston and the rings from one had smashed the side of the piston land like someone had gone it with a cold chisel. Another two had fragments of rings no bigger than 2mm fall out like grains of rice under the 'pinch'.
That will only happen with cast rings though.
A cam with very steep closing ramps will tend to bounce valve more reddily with soft springs, but the cam you have is a stocker isn't it?
The only time I have had it happen is when the box used to jump out of fifth under full load, gets a buzz of 8k and the damage is done. And that was just with old springs. The other one I had was when I used a head from a burnt wreck. The high temperature of the engine fire had softened the tensile steel, and the first big over rev it had it bent a couple of valves.
Also, if the block has been decked a couple of times and the head skimmed, you end up with less 'room to move' as far as having a later closing exhaust valve or clearance in the event of a over rev bounce.
You can actually measure the amount of valve clearance you have with the engine fully assembled, by winding the engine over to 5BTDC, and slowly screwing down the exhaust valve adjuster till you fell the valve head touch the piston. My engine machinist recommends no less than 60thou (1.5mm) to be safe. For my combo, I had to do some whacky things to get my 1.5mm!
Also too, when I was suggesting a sticking wastegate, I was implying when the turbo had been heat soaked and at a good few hundred degrees after a continuous boosting, that the wastegate becomes sticky and when snapped shut on the down shift, temporarily sticks closed.
Damo