Is The FS Engine An Interference Engine ??? ... Well It Could BE !!!

pcb

The Diagram Dude
:
2002 MP5
After searching for an answer to this question for over two years, I"ve found an answer that I can live with.....

Thanks CTt3P5 for helping me shed some objective light on this hornet's nest of a topic.



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I'd love to see the actual tolerance stackup on that engine- it's got to be REALLY close for bearing wear to mean the difference between non-interference to interference...
 
What does it mean for an engine to be interference or non interference? Is it when the valves hit the pistons when the t belt goes? Or is it something more than just that?
 
What does it mean for an engine to be interference or non interference? Is it when the valves hit the pistons when the t belt goes? Or is it something more than just that?

Yep that's all it is. When the valves hit the piston when the t-belt goes.
 
Pretty sure I had a piston touch the valves, mostly because the rod broke though

Yes the fs is interference, if anyone says otherwise they should take it up with the people that bent valves when the tbelt broke. There have been quite a few so its not like it was just one person
 
Pretty sure I had a piston touch the valves, mostly because the rod broke though

Yes the fs is interference, if anyone says otherwise they should take it up with the people that bent valves when the tbelt broke. There have been quite a few so its not like it was just one person
and there have been more than quite a few that the T-belt broke and nothing happened.. i think its just hit or miss with these engine.. some get lucky n some dont
 
Like a spun rod bearing lasting 3 weeks on hour long interstate trips runing 75, i will never know how it lasted that long.These motors are worse for hit and miss than eBay turbos
 
I worked on a P5 once that had a snapped timing belt. Replaced the belt and it ran just fine. I say they are non-interference.
 
The way I'm understanding it, is that every new FS engine is non-interference,.. however with time and wear the tolerances on either end of the connecting rods become increasingly sloppy the the pistons can "throw" a little bit further as the slack builds.

That extra travel is just enough to slam the piston into a valve or two.

I'm figuring things like not changing your oil regularly and driving the snot out of your car can have a major impact on the tolerances of the rods.

When I started researching t-belts and the parts needed, all the parts stores listed our FS engine as interference, so I just went ahead and did my t-belt because I couldn't confirm that it had ever been done. (as it turned out, mine had already been done)

I posted a question about a year ago asking if anyone had ever had engine damage due to a snapped belt. I couldn't find ANYONE who said it happened to them.

http://www.mazdas247.com/forum/showthread.php?123823967-Is-the-FS-an-interference-engine

So,... engine damage due to a broken t-belt appears to be VERY rare. The decision is yours,... roll the dice.
 
The FSDE is non-interference. As with anything mechanical and spinning at 4000 rpm there will be the odd exception to the rule.
 
All parts places list the FS engine as Interference,... mostly to cover their butts I guess.

I'm pretty sure Tweety's engine has evolved into an interference engine.

(that long stretch without an oil change may have helped the evolution a great deal,... liquid sandpaper running through everything)
 
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Oh yes mine was very poorly maintained. But the simple fact is is that multiple people have bent valves due to timing belt breaking

You can take a chance or not
 
interference or not you can still bend valves and break stuff when the t-bekt goes, these are non interference engines referring to haynes and mazda manuals, the difference in an interference and a non interference is in an interference the t belt keeps the valave from hittting the pistons and thats its main job. in a non interference it is just keeping the time and if it breaks stuff can still get bent, just a lower chance
 
I've seen them jump timing and bent valves at idle.

wow this is an old thread lol

I've broken my timing belt twice now. Once was just above idle as I was leaving a stoplight, maybe 1500rpm. New belt and all was well for about 65k miles. 65k miles later the spring gave out and timing belt #2 went snaparoo. This was cruising at about 4000RPM. Again, new belt and all is well with no issues.
 
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