question about "interferance" engines

If an interferance engine breaks a chain/belt and the valves crash into the pistons, besides bending the valves, does this also frequently do damage to the wrist pin/con rod bearings or other lower end parts? The enginge in question is a 16v saab b202 enginge, evidence of valve contact on 3 of 4 pistons. I suspect that the head has been replaced.

Depending on RPM I have seen interference engines NOT need the pistons replaced... I have also seen them destroy the tops of pistons... every case is different.

^^^^^^What he said^^^^^^^^

I lost a belt in a X1/9 when trying to start it and all that was damaged were the valves.

I have seen both extremes. On my first fiat the belt slipped a couple of teeth and I needed to do was replace a couple of valves.

My second fiat came to me with a dead engine from where the timing belt tensioner let go. When I popped the head, I discovered that the entire thing was a boat anchor. The tops of the valves had been broken off and they were embedded into the top of the head, the pistons, AND had deeply gouged the cylinder walls.

I have seen both extremes. On my first fiat the belt slipped a couple of teeth and I needed to do was replace a couple of valves.

My second fiat came to me with a dead engine from where the timing belt tensioner let go. When I popped the head, I discovered that the entire thing was a boat anchor. The tops of the valves had been broken off and they were embedded into the top of the head, the pistons, AND had deeply gouged the cylinder walls.

Yep, what they said. I've never seen a VW engine do more damage than just valves(except for maybe minor nicks in the piston tops, but not bad enough to replace them), but every make is different. I think a lot of it has to do with the diameter and maliability of the stem. If the valves just bend, you're usually alright, but if the head of the valve breaks off, it's going to do a lot of damage.

My neighbor's Volvo 940 lost the valvetrain when the water pump died. He was on the interstate and the car just lost power. I think they just replaced the head.

What about non-interference engines? What is the difference (or rather, why are they different)? I understand that the 1.8 liter Miata engine is a non-interference engine but I can't understand why.

Non-interference engines, like the 4AGE, don't push the valves open enough to ever risk being in contact with the pistons (or each other). So, you could put the piston at TDC and crank the cam through 360 degrees and the valves will never touch the piston. Thus, if the belt breaks, you are (usually) OK.

My step-son had a idle bearing let loose on his Talon. It jumped a couple of teeth but managed to not destroy the vavles by pure luck. I bent 4 exhaust valves on a 22R when the plastic timing chain rail broke and wedged between the chain and lower sprocket. There's a metal one in there now.

OH! I get it.

Yeah I had a '91 Passat 16v that had the tensioner seize up while going down the interstate. Well that caused the belt to be eaten up from the backside and gradually slip, instead of an instant break or the instant stripped teeth around the crack shaft pulley. When the cam stops suddenly, only the valves that are open are going to get it. When it slips gradually, all eight intake valves get bent wide open. I actually had to beat all eight valves back in with a hammer to be able to have a little more spring travel to get the keepers out, then beat them out of the guides. Oddly enough, after all that the guides were still well within spec and the pistons only had a little carbon scraped off the top. Installed eight new valves and she was good as new.

confuZion3 wrote: My neighbor's Volvo 940 lost the valvetrain when the water pump died. He was on the interstate and the car just lost power. I think they just replaced the head.

If you are in the US, somebody is confused. No US-spec 940 had an interference engine. When the timing belt goes, the car stops. that's it. Replace belt, drive another 120,000 until it fails again :)

Thanks guys, my engine has only minor divots in the piston tops. I will be be slaping on a new head tomorrow!

Evan_R wrote:

confuZion3 wrote: My neighbor's Volvo 940 lost the valvetrain when the water pump died. He was on the interstate and the car just lost power. I think they just replaced the head.

If you are in the US, somebody is confused. No US-spec 940 had an interference engine. When the timing belt goes, the car stops. that's it. Replace belt, drive another 120,000 until it fails again :)

It's an old car. It definitely is a 940, and it definitely needed a new head. They attributed the failure of the engine to the failure of the water pump. The water pump drove the timing belt.

Now, I have heard that on non-interference engines with dual cams, you can still run into issues when one cam stops right before the other causing the valves to smack into each other. Maybe that's what happened?

HappyAndy wrote: Thanks guys, my engine has only minor divots in the piston tops. I will be be slaping on a new head tomorrow!

Good to hear!

I was wondering the same thing because a friend at work has a 99 Honda CR-V that had the timing belt break. I think I can pick up the car pretty cheap,fix the head,and flip it,but I have never done this job,and I have no idea what to expect in a honda engine like this. I would assume the pistons are fine,but you guys have me rethinking that assumption. Would this be a good buy if I could get it for maybe $2k or so?

confuZion3 wrote:

Evan_R wrote:

confuZion3 wrote: My neighbor's Volvo 940 lost the valvetrain when the water pump died. He was on the interstate and the car just lost power. I think they just replaced the head.

If you are in the US, somebody is confused. No US-spec 940 had an interference engine. When the timing belt goes, the car stops. that's it. Replace belt, drive another 120,000 until it fails again :)

It's an old car. It definitely is a 940, and it definitely needed a new head. They attributed the failure of the engine to the failure of the water pump. The water pump drove the timing belt. Now, I have heard that on non-interference engines with dual cams, you can still run into issues when one cam stops right before the other causing the valves to smack into each other. Maybe that's what happened?

OK, I stand corrected... sort of. there was a 940 with the B234F 16-valve engine. that engine is interference indeed. However, no Volvo gas engine uses the timing belt to drive the water pump. (Some Volvo Diesel engines do.)

More than likely, the water pump belt broke, the car overheated, and the head warped.

Some of this depends on the car.

One of the best buys on the market is a Subaru interference motor that has lost it's timing belt. Most common- WRX (or other) in a front end collision.

The plastic timing gears are on the front of the motor and almost always collide with the radiator in a front-ender. They never last. Conventional wisdom says the motor is now toast because it is an interference motor.

I've picked up several of these dirt cheap.

This motor (the 2.0T) is referred to as an interference motor, because valves can collide with each other, not because interference is possible with the pistons. 4 different motors I've picked up have needed nothing more than 1 or 2 valves replaced.

some times even light hit will Pinch the top ring land it depends on the piston MFG and how close to the top the ring is.

44

SVreX wrote: Some of this depends on the car. One of the best buys on the market is a Subaru interference motor that has lost it's timing belt. Most common- WRX (or other) in a front end collision. The plastic timing gears are on the front of the motor and almost always collide with the radiator in a front-ender. They never last. Conventional wisdom says the motor is now toast because it is an interference motor. I've picked up several of these dirt cheap. This motor (the 2.0T) is referred to as an interference motor, because valves can collide with each other, not because interference is possible with the pistons. 4 different motors I've picked up have needed nothing more than 1 or 2 valves replaced.

I'm confused? how can the valves ever hit each other when the belt breaks? This is a DOHC engine correct? is the clearance between the valves so close that when the belt breaks, one cam stops first than the other continues for about a quarter turn more and thats all it takes to make the valves collide but it never hits the piston?Wierd.

Yes exactly valve hit each other as they come in at angles to the cyl

My wife (girlfriend at the time, actually) had the very good fortune to be driving her '87 Corolla FX16 GTS when the timing belt failed. Me being a Honda type, I just figured she was really, really lucky 'til I read the post up above. I always learn something here...