Is a RAV4 4-Door 2WD L4-1998cc 2.0L DOHC (3S-FE) motor zero-clearance?
ie. if the timing belt breaks, are the valves likely to touch the
piston tops?

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Request for Question Clarification by siliconsamurai-ga on 10 Feb 2005 08:47 PST

this question makes no sense, can you carify so I can give a complete
and acceptable answer?

ANY engine which breaks the timing belt/chain will have the pistons
hit some of the valves.

The term zero-clearance doesn't mean anything in this context. all
engines with hydraulic lifters are technically zero-clearance between
the lifter and the cam and the mechanical lifter engines only have
clearances of .010 or so inches. The valve will normally move up to a
half inch.

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Clarification of Question by corvallis-ga on 10 Feb 2005 12:19 PST

Well put. I am from the old school; automotive self-taught from about
1958 on. I had understood that there were two type of engines. One had
enough clearance so that if you broke the timing belt/chain the valves
would not touch the piston tops. The other, I thought was called
zero-clearance; you break the timing belt/chain and the valves touch
pistons. This engine appears to be dual overhead camshaft, so no
hydraulic lifters as no push rods? It is impossible to find this info
in the Chilton's manual without spending hours. I am trying to save my
time and aggrivation; and provide my wife with facts. thank you

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Request for Question Clarification by siliconsamurai-ga on 10 Feb 2005 13:18 PST

OK, but I'm still not clear what would constitute an answer to your question.

I am a former sports car garage owner, heavy equipment mechanic, and
pit crew member SCCA. You may be correct about the terminology I just
never heard that and I never saw an engine where breaking the timing
chain was a good thing. Only an F-head or flathead engine could unsync
valve/piston timing without major damage.

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Clarification of Question by corvallis-ga on 10 Feb 2005 17:28 PST

I accept this as an answer. I am ready to cancel this question; how do
I get you your $20?

Hi, thank you for submitting your question to Answers.Google, I hope I
can provide the information you are seeking.

The term zero-clearance in regard to the valve train of an internal
combustion engine relates to the fact that engines with mechanical
lifters would normally have a small space open in the valve train
mechanism (normally between the top of the valve or valve cap and the
rocker arm) when the engine is cold. This is to prevent the valve
being held slightly open as the valve train expands with heat as the
engine runs.

On some overhead cam engines this is adjusted by careful measurement
and the placement of shims between the top of the valve stem and a
metal cap.

Most (probably all) engines with hydraulic lifters are what could be
termed zero-clearance because there is zero lash in the valve train ?
heat expansion is compensated for other ways.

The advantage of hydraulic lifters is that they are generally quieter
and don?t require periodic adjustment.

In any engine where the valve is moving above the piston (overhead
valve type) the need to create enough compression requires that the
piston come quite close to the top of the cylinder at the top of its
stroke. Valve timing (when it opens in relation to the position of the
piston) is such that the valve is never extended far into the cylinder
when the piston is near the top of the stroke.

Valve timing is controlled with a chain or a belt on most passenger
vehicles but exotics or race cars may use a solid gear train because
it is less likely to break or slip out of timing.

When a timing chain breaks or a timing belt slips, one of the valves
may be pushed far into the cylinder when the piston is rising and this
causes the valve to break the piston or vice versa. Depending on
circumstances and the particular engine, this will usually cause
either a major or perhaps a catastrophic failure.

You may perhaps have been thinking of the term freewheeling which is
sometimes applied to engines which have been carefully designed to
rotate without damage if the timing belt breaks. It is possible that
there will be significant damage even in these engines but the amount
of damage will usually be far less than that with an engine where the
valve would actually contact the piston at speed.

http://www.aa1car.com/library/ar594.htm

Manufacturers seldom publish information about which engines can safely freewheel.

I couldn?t locate any information relating to freewheeling
capabilities of this engine using Google or other search engines.

You could determine this for yourself by making certain that a valve
is fully extended, then carefully turning the crank (with timing belt
removed)and observing the results. I don?t recommend this but you
sound like an experienced mechanic and could probably do this without
much risk. Of course you would probably need to remove and replace the
head several times, using clay to let the valve leave a mark - just
measure the amount of clay left. You would still need to calculate
expansion of the valve and mechanism when hot.

When rebuilding an engine timing chains and the gears should always be
changed. With belts there is little wear on the drive ?gears? so just
replace the belt.

AllData (http://www.alldata.com/TSB/60/996041AE.html) lists a number
of technical service bulletins for the 1999 RAV 4 with that engine.

There are no engine-specific notices listed so there is no particular
problem with the valve timing mechanism on that engine.


Google Search Term: RAV4 4-Door 2WD L4-1998cc 2.0L DOHC (3S-FE)

Google Search Term: zero clearance engine

Further information:
64.78.42.182/sweethaven/MechTech/Automotive01/
default.asp?unNum=2&lesNum=4&modNum=2
www.musclecarclub.com/library/tech/engine.shtml www.autoswalk.com/enovin.html
www.autoshop-online.com/auto101/engtext.htm
www.type2.com/library/engineg/hydral2.htm

Google Search Term: broken timing chain
Why the change to belts
http://www.cartalk.com/content/columns/Archive/1999/April/01.html


Thank you again for turning to Answers.Google for help. I hope this
provides enough information to satisfy you. I don't believe you will
be able to locate reliable information about whether this particular
engine can freewheel since it may also depend on whether the engine
has overheated.

Actually you still have lifters, just no push rods.

sorry, ignore that about lifters, you have cam followers

I think we're mixing up the terminology here.  From the original
question, corvallis is asking "if the timing belt breaks, are the
valves likely to touch the
piston tops"?  This is termed an INTERFERENCE ENGINE.  Some cars have
interference engines; some do not: these are called FREE-RUNNING.  So
it is incorrect to say that "ANY engine which breaks the timing
belt/chain will have the pistons hit some of the valves."  You can
look at http://www.gates.com/downloads/download_common.cfm?file=GatesTBR.pdf&folder=brochure
and see that yours is NOT an interference engine, since there is no
"*" by it.  Toyotas generally aren't interference.  Look at the very
end of that file to see an illustration.

yes craig is correct