325ix timing belt question

[okjohne]

Hello everyone,

I'm working on my 1991 325ix coupe. it has 243k and as i was turning the key to off, the timing belt snapped. I decided to see if i got lucky and replace it along with the water pump. when i went to align the timing i was surprised to find the cam mark only three teeth off the Crank. I was able to get the cam to move into place without any clanking. Here is the question. Does the crank rotate twice for every time the cam rotates once or visa versa? Can the marks be lined up but the car out of time due to the different rotations?

Thank you in advance,

John

[superj]

crank turns twice for each rotation of the cam. the marks can be off 180 degrees. if it snapped and the engine was still rotating, at a minimum you have bent some valves.

the crank has to compress the incoming fuel mixture on one rotation and then it has to expel the burned mixture on the next rotation. the valves only have one lobe that opens and closes to let in and exhaust the mixture so the cam has to do one full rotation each time the crank does two or you would blow the fuel charge out the exhaust before it ignited. there is some overlap in the chamber to allow for proper scavenging of the exhaust mixture and help with combustion but it won't mess with what you are asking

[95BMWIC]

If you are open to advice - I would go ahead and pull the head and replace valves and valve seats... 243,000 miles - you need to do this anyway.

Plus now is the opportunity (when everything is coming off) to replace seals, belt, pump and plane the head without having to do it later when something else fails.