4.0 a "non interferance" motor?

In the process of installing my new crank. I turned the motor over (turned the crank with a break over bar) in order to line my rods up with the crank, so I could put the bearings and caps on. I left my head on, and never removed the pistons.

As I was rotating the crank I kept waiting on one of the pistons to hit a valve that was open, because I did not have the timing set installed. When i was aligning the timing marks on the gears, I noticed I could spin the cam 360 degrees without the valves hitting any pistons.

Before any of you reply, yes i do have have the push rods, lifters, and rockers installed, I haven't even removed the valve cover since I pulled the motor. My manual actually says in bold letters "do not rotate the crank without the timing chain installed."

It doesn't appear to be an issue. Its good to know that if the timing chain breaks on a 4.0, or it "jumps" time, that there won't be catastrophic failure, the motor will just quit running.

I have no idea if the 4.0L engine is non-interference or not, but are you sure that there were even any valves open? If you have the timing chain off, the camshaft could have jumped into a position where most of the valves are closed and probably a couple that are just partially open.

Coming from a long history of Mitsu 3.0L V6 DOHC engines, I'm very familiar with the joys of broken timing belts

The 4.0 is a non-interferance motor.

I did a little research, and among american made motors, most of them are non-interfererance. I thought only a few were not. SBC, the Lt-1 in my camaro.. both non-interferenace. It's mostly japanese cars that are interferance motors. With their fancy DOHC's ect..

gradon said:

The 4.0 is a non-interferance motor.

Click to expand...

I guess this would explain how I don't have an electronic rev limiter, but the valve float around 5100 acts like a rev limiter, yet I didn't destroy any valves finding that out.

whats does non interference mean?

Rambopants said:

whats does non interference mean?

Click to expand...

my words:
basically an interference engine is where the valves and the pistons are very close together, basically saying that there is no valve reliefs in the pistons...so if the timing belt (or chain) decides to break or jump, it destroys internals frantically. Non-interference is a better set-up to where you have those reliefs and the gap is much larger so that if that belt (or chain) decides to jump or break, you likely can salvage most of the internals.

Wikipedia:
Depending on the design of the engine, the piston and valve paths may "interfere" with one another, and incorrect timing in their movements may result in the piston and valves colliding. (Such designs are also called "interference head" or "interference engines", and include virtually all diesel engines. Conversely, non-interfering engines, such as the Mazda B engine, are called "free-wheeling" or "non-interference" engines.)
In interference designs, regular service is especially important as incorrect timing may result in the pistons and valves colliding and causing extensive engine damage and therefore costly repairs. The piston will likely bend the valves or if a piece of valve or piston is broken off within the cylinder, the broken piece may cause severe damage within the cylinder, often also affecting the connecting rods. Some manufacturers, such as Nissan, have switched back to timing chains for the majority of their engines because of the breakage problems associated with belts. However, in some newer engines, timing belts are designed to last the effective life of the engine.[citation needed] Other manufacturers, such as Toyota, use a mix: timing chains on their interference engines and timing belts on their non-interference engines.
When a timing belt is replaced, care must be taken to ensure that the valve and piston movements are correctly synchronised. The usual failure mode of a timing belt is stripped teeth (which leaves a smooth section of belt where the drive cog will slip) rather than an outright snapping of the belt, which is very uncommon.[citation needed] Correct belt tension is critical - too loose and the belt will whip, too tight and it will whine and put excess strain on the bearings of the cogs. In either case belt life will be drastically shortened.


I know that it doesn't explain non-interference, but all it is the opposite of interference.

Non interference means that the even if the timing chain breaks, the pistons still won't hit the valves. In other words, even with the valves at the high spot of the cam and the piston at the high spot of the crank, the pistons don't hit the valves.

Some motors (usually not American) are designed for higher performance where the interference aspect is considered an acceptable trade-off.

In aplications that high compression is also required. The Piston if at TDC and the Valves open would hit causing damage to the piston, rods, cam shaft, ect...

I like the explanations you all have because I suck at that part! hahahaha.