Question : Interference Motor?

[SuzukiGhostRider]

Settle something for me , please? the 2.8/3.1/3100 are NOT interference motors. Correct? Someone doesn't want to believe me, and I TRIED to explain to him, that the only GM motors I'm aware of being interference motors are OHC , not OHV. I even called my GM dealership and spoke with a tech and the parts guy about it.
Just wanted to see what some of you 60* guys thought.

[geoffinbc]

They ARE interference motors. A common destruction of a 3X00 motor is a snapped cam causing the pistons to take out the valves. It all starts with the LIM gasket going bad and the valves hitting the piston.

Pretty much all new GM engines are interference motors including the diesel ones.

[3400-95-Modified]

Thank you betterthanyou

And why should I believe something that is wrong?

I've built 3 motors now of the OHV era, and all of em if the cam chain is not on you can not turn over the motor unless the rockers are removed.

I also work on a GM 350 in a race car, SAME principal and also an Interference Motor.

[1988GTU]

Sadly, they are interference. You'd think with the MPFI engines having 8.8:1 Compression it would not be.

[SuzukiGhostRider]

Ok, someone besides Jon, explain this to me then please. I used to be a GM mechanic, and this is the first time I'm hearing this. I even called and talked to a few GM employees today ( a tech, and a parts guy,BOTH in the field a long time) at length. BOTH said I was absolutely correct in thinking the only interference motors GM ever put out were the OHC ones, and generally they used a belt with exceptions like the Quad which used a timing chain.

[1988GTU]

When the T-Chain breaks and the crank is still rotation... That will happen. I would have been grateful if that didnt happen when the chain snapped.

[Mr_Efficiency]

I dont know of any GM motors that ARENT interference motors, i know my 2.2 ohv in my cavalier is and so is the 60* v6 like others have said.

I've only ever seen one motor that wasnt and interference motor and it was a 2.2/2.5 in the dodges in the late 80's early 90's

[geoffinbc]

If a motor has low compression and a low lift cam you may end up without interference. But it is usually not the case. Combustion chambers are small and valves are close to the deck and lift is almost always above .400 inch. All this helps promote breathing, swirl in the chamber and a descent compression ratio. Interferance or not really has nothing to do with where the cam sits. In either engine design the clearances between the pistons and the valves during operation and not much different.

[SuzukiGhostRider]

But , you would normally only worry about smashed valves in say, a 3.4 OHC, or a 2.3 Quad, right?

[1988GTU]

I dont know of any GM motors that ARENT interference motors, i know my 2.2 ohv in my cavalier is and so is the 60* v6 like others have said.

I've only ever seen one motor that wasnt and interference motor and it was a 2.2/2.5 in the dodges in the late 80's early 90's

The Iron Dukes found in the s10's are non interference.

[geoffinbc]

But , you would normally only worry about smashed valves in say, a 3.4 OHC, or a 2.3 Quad, right?

Of course. Typically OHC engine see a lot more timing belt/chain breakage due to the fact the thing is so damn long. When it stretches just a bit they start to slap around. Most do have tensioners but they only work to a point. People usually forget about regular maintenance and just let the thing explode. Look at a Honda for instance. They recommend changing the belt ever 100,000kms. But a 60 degree V6 you may never change the chain or perhaps do it because the front cover gasket is leaking. My Trooper motor has 280,000kms on it and I still feel no need to dive into it to change the chain out. I will do it when I rebuild the motor and make it a 3.1L.

[3400-95-Modified]

Bottom line a GM 3100, 3400 3500 and 2.8's and 3.1's are still all interference... It doesn't matter which one you would worry about more...

[2k1Impala]

The Iron Dukes found in the s10's are non interference.

And don't forget, they were also in Citations! Doesn't matter that they are non-interference because there is no timing chain in those engines, the cam is geared off the crank. Beautiful setup, actually.

[geoffinbc]

Yup the duke has got to be one of the most reliable engines out there. Basic and built strong. Mercruiser used the engine design with its own variations to do marine duty as well.

[Wiz85]

I was worried about if our motors are like that or not, because I have wondered that if I hypothetically floated the valves at top end or redline, would they hit? I figured yes they are interference mainly because there aren't any valve reliefs in the pistons. My roommate accidently hit his rev limiter once in his bmw, floated all exhaust valves and bent them. Cause: weak valve springs. I'm just worried that with 140,000 miles on my 3100, if I hit above 5600 rpm. Will there be damage?