RE: Piston - valve kissing

[Michael Harris <michaeldch@domain.elided>]

>Subject: RE: Piston - valve kissing
>
>I'm going to say this one more time, if the timing belt tensioner(s) are
>properly installed and adjusted by THE book, the engine turning backwards
>will NOT cause the timing belt to slip or jump teeth!  Any none believers
>out there?  Fred Di Matteo  AROC Technical Advisor from Fort Myers, Florida
>

Hi Fred:
Perhaps I am the lone dissenter, but I must disagree to some extent with
your assertion that it is acceptable to turn the V6 crankshaft backwards. =
=20
It is coincidental that this issue should arise now, since just this
weekend, I changed the rear giubo's in my '85 GTV6 and ran into this
problem.  I did rotate the crankshaft backwards while loosening all the
bolts securing the giubo's when suddenly the driveshaft locked up.  I
suspected what had happened, & tested this by attaching a socket onto the
front of the crankshaft, removed the sparkplugs (to make turning the engine
easier) and gingerly trying again to rotate the engine.  It would only turn
in one direction, and was completely locked at a given point in the other
direction.  I assumed that this meant that the belt had slipped and now one
of the valves was contacting the piston crown.  I can really see no other
rational explanation for this behavior.  I must admit that I could not
verify that the cam sprocket had slipped since with the cam/crank
misalignment, I could not rotate the engine to the point where the
reference marks would have made verification possible.   Instead, I removed
the belt and roughly estimated the correct orientation between the crank
and the cams - which proved sufficient to allow the engine to be rotated
(with the belt now on) to TDC with the cam timing marks aligned (and I set
the timing accurately).
The engine is a newly rebuilt 3 liter with S cams and a thermo-mechanical
tensioner.  The belt is 2 months old, has 5000 miles on it from when the
tensioner was installed & the tension set on the belt (by the book).  It is
possible that my belt might have been a bit loose since I have noted that
they do tend to stretch initially during the first few thousand miles (as
illustration, Porsche requires that their 944 series cars have the belts
re-tensioned 2000 miles after a new belt is installed and I have noted a
similar need in the Alfa engine).
Just to be on the safe side, I'll always make sure that the crank is only
turned clockwise - I just don't think it's worth the gamble, the expense &
time consumed in re-setting the timing or worse yet, replacing a few
crushed valves=85
Cheers
Michael
1985 GTV6
GTV6 Web Site:   http://www.gtv6.org

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