some advice would be greatly appreciated , i have replaced the timing belt
on my 3.4 dohc engine(94 grand prix)and having the old belt break all timing
points were lost,thier is no valve damage(thank god)but the timing
marks(yellow paint on the cam sprocket teeth)appear to be wrong,probally
someone else before worked on it,i have had the car scanned at a repair shop
because no matter what i did it still appears out of tune,they said all is
good with the exeption that it is just out of tune.the cams are not right ,i
am sure and the mechanic okayed that my intermediate sprocket,crank marks
are right with tdc on no.1,so the trouble is soley with the cams not
right,upon many people "guiding me"internet help and a few manuels,i have
removed the plenum,cam carrier covers,and have the "flats"on the camshafts
facing up and locked down,i have loosened the sprocket bolts on the front
cams(closest to front of vehicle)so they are freewheeling,upon putting belt
back on and tensioned,i was instructed to rotate assembly by hand 360
degrees(back cams with hold downs off and front still locked down"sprockets
freewheeling"then after doing this,tighten front cam bolts down and release
the cams to turn with belt,i stopped at this point,but am i on the right
track so far????i don't really understand why the front has to be
freewheeling for 360 degrees ,then released to complete timing????the book
also then mentions something about after this procedure,the back cam flats
will be 180 degrees different then the front flats???this also i don't
understand,if all flats are face up ,upon 360 degreese,wouldn't they be in
The flats on the cams locate that particular set of cams to TDC.
You first set the rear bank of cams to cylinder #1 at top dead
center, you then rotate the crankshaft 360* (and simultaneously
the rear cams 180*) and then line up the front set of cams to
at top dead center (cylinders 1 and 4 are at top dead center at
the same time).
GM -could- have put the flats on the front bank cams 180*
opposite from where they did, my guess is that this would have
necessitated extra tooling during the cam grinding process.
FWIW, Subaru uses a similar timing belt install sequence as your
upon rotating crank 360*,as stated in manuel with back cams "flats facing
up"and (front cams "flats facing up"and locked down with cam tool,sprockets
freewheeling)everything lines up of course upon it's full rotation including
the flats on the rear cams again facing up(the cam flats go 360* too,not
180*)so upon releasing the front cams to turn with ass. ,the flats on cams
are always facing up on 360*,i got the idea that the proper timing involves
front cam flats be opposite of back flats(180*) but this doesn't work out
with what the manuel says,so at this point i am very confused,there is too
much work to put everythung back together to just try it out,i need to be
sure the cams are tuned correctly why it's all apart,i know you know what
your talking about. so just what am i doing wrong or reading wrong on the
configuration of this????
and then line up the front set of cams to
i fully understand what your trying to say,but the manuel of course doesn't
say or convey that point at all,it only makes sense to me that they should
be that way. the no.1 cylinder is at TDC when i time the back cams and in
the complete finished job i assume that the back bank flats are going to be
facing up where as the front bank flats are going to be facing directly
down(RIGHT???)to accomplish this i would only turn the crank 180*(back cams
to would be 180*)then release the front cams to accomplish the differential
180* between front and back cam timing. let me get this straight(the back
cam bank "flats up"should be flats up when no.1 cylinder is at TDC"piston at
top of chamber, RIGHT?? (front cam bank)should be flats up at TDC
"piston at top of chamber" on the no.4 piston, RIGHT???this then should
position cam flats at opposite ends. bob
What manual are you using?
For the most part, manuals make the assumption that the reader
understands the basics of valve timing.
This would be once all sprocket bolts have been tightened (cams
properly timed) and the crankshaft has been turned 2 full
No, the crankshaft rotates twice for each revolution of the cams.
For each 360* rotation of the crankshaft, the cams rotate 180*,
it takes two full rotations (720*) of the crankshaft to equal one
full (360*) rotation of the camshafts.
If you marked the sprockets with paint before you started and
lined them all up like this O- -O O- -O
( O= sprocket, - = paint mark)
tighten sprocket bolts on rear cylinder head with tool installed
on cam flats, both cylinder heads.
TDC #1 O- -O O- -O
Rear flats are up, front flats are up
remove tool from rear bank, rotate crankshaft 360*
(rear cams are turning, front sprockets are slipping on the cams)
You now see this at TDC #4
-O O- O- -O
Rear flats are down, front flats are up.
You tighten the sprocket bolts on the front cylinder head.
Remove tool from front cylinder head, rotate crankshaft 360*
(front and rear cams are turning)
You now see this at TDC #1
O- -O -O O-
Rear flats are up, front flats are down.
Rotate crankshaft 360* again
You now see this at TDC #4
-O O- O- -O
Rear flats are down, front flats are up.
You should never see this when #1 and #4 at TDC
O- -O O- -O or -O O- -O O-
Front cams versus rear cams should always be 180* out of phase
with each other.
Clear as mud, 'eh?
okay i have been thinking on these lines now for awhile,and you time and
patiance has helped me greatly thank you,the thing i think that has been
evading me and confussing me is that i am assuming that one whole turn on
the crank(immediate sprocket following the same)from say 6:00 to 6:00 was
the 360* turn,this in turn did nothing but bring the flats around to the
same point,not turn them 180*,i am now thinking that the piston is turning
tdc twice in that complete cycle and that this is considered 720*,that when
my mark comming from the 6:00 position to the 12:00 is what 360* really
is,which of course would put the cams in the opposite way of the
other(180*),am i correct now in that thinking that a complete cyle turn on
the main crank/intermeddiate sprocket is 720*,and half a turn is 360*,i know
this may seem like a dumb question,but no i have never in all my years of
working on cars messed with cams(rather then changing timing chains on the
regular engines)or older type distributers.let alone a DOHC engine.
i have tried and tried this,and unless something is just giving me
brainfarts,this just doesn't work,you are saying basically what the manuel
says,but this just doesn't happen,
for each rotation of the crankshaft(360*)the mark goes from 6:00 to 6:00,all
the way around to it,s starting point,the back cams"flats up to start
with"rotate a full 360*back to the flats up position too,two rotations on
the crank just simply turns the cams twice too,i am going nuts trying to
figure out how a 360*crank turn produces just a 180* on the cams,they
don't,this is a 60* engine,i don't know if that makes a difference,but i do
know that the only way to get the cams 180* is to turn the crank only from a
6;00 to 12:00 position,the piston does rotate twice to tdc position in a
full 360* turn. just what the heck is going on???
On Fri, 04 Aug 2006 00:50:45 GMT, "robert -wanda fox"
Nope. Read what he wrote. When you turn the crank 360*, the rear cam
turns 180*. Then you lock down the sprocket on the front cam.
Turn the crank another 360*. The rear cam has now completed 360* -
flats up, while the front cam has completed a 180, flats down.
If the piston starts at tdc, it will return to tdc once, at the end of
the 360*. But the piston will return to tdc twice in a firing cycle.
once on the compression stroke and again on the exhaust stroke.
this is where i am confussed,when i am turning the crank 360*(back cam
turning with crank starting with the flats up and front cam freewheeling and
flats locked yet)at the end of the 360* rotation the cam flats are facing up
having made a 360* rotation also,they are not 180*,so i can't release the
fronts at this point since all flats are facing up and would all turn 360*
and continue to be facing up too.
i understand now that the piston will make two travels to tdc in a 360*
turn"compression stroke and exhaust stroke (right??)
but no matter what happens,the cam also turns 360* at the same time the
crank is turned 360*,it doesn't take two cranks to turn the flats back up to
thier starting point
Then you lock down the sprocket on the front cam.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.