3AC -- "Interference Engine"???

[Rumrumm]

A mechanic buddy told me that the 3AC is NOT an interference engine, meaning if the timing belt let go driving down the freeway that the engine would freewheel with no piston/valve damage. True or false???

[danzo]

That's correct. I have heard that on older non-interference engines that have alot of carbon buildup on the pistons it may be possible to slightly bend valves. Haven't seen it, don't know if it happens. If you broke your t-belt just line up the marks as per the FSM and the motor should run again.

[keith]

Somewhere on this site, I posted pictures that I took of the clearance between the valves at max open and piston at TDC. There is plenty of clearance. I used play clay on top of the piston at TDC, clamped down a head without a head gasket and turned the crankshaft a full 360. If you put in a high performance regrind crankshaft, you could turn it into an interference engine, but stock, it is not.

[Petros]

The valves clearing the pistons are not a problem, but I have seen the piston hit the head when the bearing clearance was so large (due to running it without oil) that it allowed the piston top to smack the head. I have also seen a number of times when exhaust valves have melted, broken parts off, or even lost their whole valve head (and damaged the piston badly) because of running the 5 deg BTDC timing. Exhaust runs cooler with 10 deg spark timing.