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"Interference" vs "Non-interference"


imp

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Anyone willing to take a crack at this? I've been trying to imagine why, in this age of OHC engines, anyone would offer an interference engine? Obviously, both types exist. I picture the valves, 2 of them by example, "using" up a certain amount of combustion chamber volume, into which no piston dome should ever intrude, but it's very small, just a few cubic inches, compared to cc volumes of 40cc and more.

If crankshaft and bearings are operating where they should be, the pistons always come up to the same place in their cylinders. Above and around that top area is the combustion chamber into which the valves protrude when open. Good design would disallow piston tops from interfering there. Yet, they do, often, causing various broken parts.....dead engine.

Happens when valves go "out of time" for whatever reason, but shouldn't if clearances are designed in: valve timing is then unimportant. What basic facts am I missing?
 


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410Fortune

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Im not sure why they do it I have often wondered the same thing
Most small engines I work on have plenty of clearance between the top of the piston and the valves even if the timing chain snaps
Why do automakers still build interference engines? Especially the engines that use timing belts designed to be replaced every 60K miles.
Seems like they want the repairs
Its like a punishment, you buy our car, you ignore the 60K timing belt change interval (dealer servce $$$$ maker) and now you need your engine rebuilt (dealer service money maker)
Sounds alot like our Gen II explorer sway bar links.................they knew those things were not gonna hold up, the dealers make money on those things just like McD's makes their money on soft drinks...............
 




MrQ

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Maybe it has to do with combustion chamber size and compression? Smaller chamber, higher compression. When the the chamber get small enough the valves intrude into the piston area.
 
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mgknarf

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Anyone willing to take a crack at this? I've been trying to imagine why, in this age of OHC engines, anyone would offer an interference engine? Obviously, both types exist. I picture the valves, 2 of them by example, "using" up a certain amount of combustion chamber volume, into which no piston dome should ever intrude, but it's very small, just a few cubic inches, compared to cc volumes of 40cc and more.

If crankshaft and bearings are operating where they should be, the pistons always come up to the same place in their cylinders. Above and around that top area is the combustion chamber into which the valves protrude when open. Good design would disallow piston tops from interfering there. Yet, they do, often, causing various broken parts.....dead engine.

Happens when valves go "out of time" for whatever reason, but shouldn't if clearances are designed in: valve timing is then unimportant. What basic facts am I missing?
is the 2004 ford 4.0 engine non interference?
 




410Fortune

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imp

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Maybe it has to do with combustion chamber size and compression? Smaller chamber, higher compression. When the the chamber get small enough the valves intrude into the piston area.
This is true. But the VOLUME generated by the valves' movement amounts to only a few cubic inches, out of many needed for correct compression ratio, say, 50cc, or about 3 cubic inches, it would seem the combustion chamber volume could be built around the valves' intrusion into the cylinder. L-head design, no problem.
 




410Fortune

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Ill say it, Subaru sucks ass
there I said it
DAMN interference engines

VW is up there on that list too.......
Ive dealt with heads on Subaru and VW engines and I wish I never had to...........to make things worse some of them have a rubber damper on the crank balancer and the timing marks can be WAY OFF = new heads now have bent valves after one crank from the starter.. Yeah fun!
 
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imp

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Ill say it, Subaru sucks ass
there I said it
DAMN interference engines

VW is up there on that list too.......
Ive dealt with heads on Subaru and VW engines and I wish I never had to...........to make things worse some of them have a rubber damper on the crank balancer and the timing marks can be WAY OFF = new heads now have bent valves after one crank from the starter.. Yeah fun!
Many, if not most (all Fords I've seen were) front crank vibration dampers have two steel rings bonded together by a rubber interface. With extreme age, they do loosen and shift. Bad news. Aftermarket dampers may do away with the bond, I don't know for sure.
 




MrQ

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4.0 sohc is NOT interference engine = piston will NOT hit the valves

4.0's are interference. I have seen after the engine jump timing and the piston smacked into the valve.

@2000StreetRod has pictorial evidence in this post.
 




410Fortune

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well holy crap
I tried to be helpful with a google search and was fed incorrect info.......big shock!!!!!!!!!!!!!!!!!!!!!!!!!!
FAKE NEWS!!!!!
 
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MrQ

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Considering how critical timing is to these engines for piston/valve clearance, it would have been prudent of Ford to verify the quality of their tensioners and guides before signing off on the design. But they didn't and here we are.

So if you have jumped timing don't try to start the engine or the next "thunk" you hear might be your piston saying "Howdy, pardner!" to your valve. :wave:
 
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mgknarf

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Considering how critical timing is to these engines for piston/valve clearance, it would have been prudent of Ford to verify the quality of their tensioners and guides before signing off on the design. But they didn't and here we are.

So if you have jumped timing don't try to start the engine or the next "thunk" you hear might be your piston saying "Howdy, pardner!" to your valve. :wave:
So replaced all my guides and chains. I turned the engine by hand and had 25 lbs. compression in all 6 cylinders. Is that a good sign?
 




MrQ

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25lbs is low, but turning the engine over by hand will not generate enough of reading to check one way or another. As long as you timed the engine correctly you can use the starter to check compression. You should see between 190 and 160 psi, but it's more important that each cylinder be within 10% of the others (10% is a good rule of thumb, some say less others more). exe: (1) 180psi, (2) 175psi, (3) 170psi, (4) 175psi, (5) 160psi, (6) 130psi. Your outlier is cylinder 6. Take the average of your highest readers (in this case cyl 1 through 4 =175psi), multiply by .10 (18psi) and subtract from the average (157psi). if your lower numbers are less than that you are looking at a bad cylinder(s). Well at least that is how I do it.

Edit: Correct difference is 10% not 20%. Changed post to reflect this.
 
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mgknarf

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25lbs is low, but turning the engine over by hand will not generate enough of reading to check one way or another. As long as you timed the engine correctly you can use the starter to check compression. You should see between 190 and 160 psi, but it's more important that each cylinder be within 20% of the others (20% is a good rule of thumb, some say less others more). exe: (1) 180psi, (2) 175psi, (3) 170psi, (4) 175psi, (5) 160psi, (6) 130psi. Your outlier is cylinder 6. Take the average of your highest readers (in this case cyl 1 through 4 =175psi), multiply by .20 (35psi) and subtract from the average (140psi). if your lower numbers are less than that you are looking at a bad cylinder(s). Well at least that is how I do it.
OK thanks the engine is out of the truck right now so that's why I was turning it but hand. I'm putting in back in this weekend. fingers crossed
 




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