So. I'm in the process of rebuilding my engine...again. I'll get right to it. Its a 7mgte from an 87 supra. After it was torn down, I saw cylinders 1 and 6 both have cracked/seperated ring lands. The other 4 pistons seem fine. The really weird part is when I bought it, it had the exact same problem, pistons 1 and 6 with cracked ring lands. I didn't think much of it, bought new pistons and rebuilt it.

Does anyone have ideas to why it would happen twice...on the same pistons each time? I get the feeling it has to be heat related. My thought is that the ring gap should be a bit more, but they were within spec when built...help?

 

I haven't seen a 7m in a long time, but does the intake have equal length runners? Your heat idea could be right, in that those two cylinders might be running the leanest compared to cylinders 2-5, and 3-4...

Like I said, it's been a long time since I've seen a 7m, but on a friends 12v cummins, we're planning on making a dual feed intake horn to help equalize the airflow to the furthest from the stock horn. Instead of one horn feeding all 6 cylinders, we'll have one feeding cylinders 1-3 and another feeding 4-6. When we rebuilt the motor, we saw more heat damage on the furthest cylinders from the intake (2, 5 and 6, 1 was damaged beyond recognition) so hence my thinking that your heat idea is right...

 

Yep usually cracked ring lands has to do with a lean condition which causes the extra heat or detonation. Are you running OEM pistons or aftermarket ones and what about your boost levels? Usually cracked ring lands have to do with upping the boost without tuning for it.

 

it had oem pistons when I bought it...2 were cracked. I bought some aftermarket pistons and used those. It was definatly running lean for a while when I first built it. I had to learn the ins and outs of all my gauges and what they do. Once I figured out how to use my wideband afr and apexi safc together it helped. My egts were in the normal range for this engine. 1300 cruising on the highway, 1400-1450 pulling a hill or under steady boost. Factory boost is about 6-7 psi, with my mods it runs about 12 max. Tuning with the safc every 1000rpm to keep the afr proper.

 

So I am guessing that when you bought those new aftermarket pistons that you got some forged ones and I am sure you are running high octane fuel. It sounds like you are having detonation issues rather then a running lean condition, I know that this is pretty common on the 7mgte when raising boost at all. I am no expert on the motor and would suggest hitting a Supra forum or the like that have the experience, all I have to offer is try a stage or two colder spark plugs and maybe run a little richer.