On 5/27/2014 8:40 AM, Morgans wrote:
>
>
> "Leon" <lcb11211@swbelldotnet> wrote
>
>> Interference heads?
>
> That is the term for engines that are designed without clearance to
> avoid the pistons from smashing into the valves if the valve train skips
> time or stops while the crankshaft still keeps going for a short time.
>
> This pistons come up and smash into the valves, usually breaking or
> bending valves, and/or smashing holes in pistons and other knarly stuff
> like that. You want to make sure you change the timing belt early on
> those engines, because if you wait til they break, you have an
> expensive, heavy paperweight.
Actually the term is interference engines. The heads do not necessarily
have any thing to do with valve to piston clearance unless the heads are
milled beyond factory specs to increase compression. It is the lift of
the cam that causes the piston to hit the valve should the timing
belt/chain break or the timing gears strip, and the valve is in or near
it's fully open position. Non interference engines do not have enough
cam lift to push the valves far enough open to interfere with the piston
as it approaches TDC whether the valve should be open or closed.
And FWIW before the days of RPM limiters it was not unusual during
excessive RPMS for the valves to float, meaning the valve springs were
not strong enough to push the valve shut fast enough after the cam lobe
relieved pressure on the valve train. In these cases, and even with the
timing belt/chain, and gears in proper order the pistons could come in
contact with the pistons. In those cases it was more desirable that a
push rod got bent vs a valve being bent. With OHC engines you don't
have as many moving parts and valve/piston damage is more likely.
Since we were talking about the ease of removing oil filters, spark
plugs, external parts, etc, I was a bit puzzled why an interference
engine would present a problem with being able to replace anything on an
engine.
I'm pretty sure there might have been some confusion as to what
interference an interference engine actually presented to some one
working on it. For the most part there is no difference in the engines
as far as convenience to the person servicing or replacing parts inside
the engine.
Now having said all of that, I retired from the automotive field some 20
years ago at 40. Up until then I never ever heard the term interference
heads. That may have changed in the last 20 years.
I at one time was the service sales manager for an Oldsmobile dealer.
We had to be careful with terminology when taking with the factory reps.
Ironically if you talked about the repair of paint on a vehicle with a
factory rep the repair description never included the word paint. The
repair involved color coating the affected panel. ;~)