jeff jackson

Bill...if you think about it a minute...if a valve is off its seat, and under lobe "lift"...if the lobe were to suddenly move "in EITHER direction"...the only possible consequence could be the valve snapping shut against its seat in the head...no way could it slam "OPEN"...into the piston crown. OR, am I missing something "obvious" here ???

Garth S

While either the intake or exhaust cam may be so positioned on its lobe that spring pressure takes over and slams it shut, that is only one cylinder that is now out of time with the crank - what is happening in the other three cylinders of that bank? This is where contact is likely ....

jeff jackson

Garth...if the flywheel lock is in place...and the crank is "immobilized"...how is any contact not existant prior to the movement of the cams....going to occur if the cams move, but the crank doesn't ???Or...did I misread this post, and the crank wasn't secured at TDC with the flywheel lock ??? If THAT was the case...it makes me wonder how in heavens sake did he break loose the crankshaft pulley bolt without turning the engine over ???

Garth S

Jeff,
I think you've just defined an interference engine - from the reverse view: if the crank is stationary ( at TDC in this case) and one bank of cams rotate more than 'x' degrees, valve piston contact is likely to occur in one or more cylinders - if not, why would it be considered an interference design?
The rational of the TB service to be performed at 45 deg BTDC is that this is the 'sweet spot' that allows free cam rotation without piston contact.
Does this permit a full cam rotation? - I don't know - but the respective piston heights wrt the deck height @ 45 deg certainly permits more cam rotation than a slipping lobe can generate.
Maybe we're thinking of different things ....

ErnestSw

Garth,
If the crank is at 45 degrees there's still no free lunch as far as cam rotation is concerned. I think the whole point of 45 degrees is that the cams are off all lobes, not that interference can't occur. I've been in on six or seven timing belt jobs, all of which were done at 0 degrees. The passenger side cam tends to fall off a lobe, but we're talking about half a tooth, not enough to create a problem.

jeff jackson

I'm just saying that if the cams are at a given location...at TDC, and valve spring pressure causes an unexpected seating of the lobes under open spring pressure, certainly the other lobes along the same camshaft are going to move as well...but...with valve spring pressure on the seats, at no more than 90#...I seriously doubt any damage due to piston contact will occur on that bank , or will be of any significance...unless you are saying they go "full open"...as a result of the degree of movement for the valves under spring tension closing to their seats. Certainly NOT enough to bend a valve stem...OR damge a piston crown in the "opening coincidence"...perhaps a nick...but, if a valve "opening"...due to lobe movement occurs, and this valve were to come in contact with a "stationary-fixed" piston due to the use of a flywwheel lock...doesn't it stand to reason, that the same valve spring compressing to snap the valve open...would "give" against a stationary resistance ??? and allow the valve to "reverse compress" the spring once its coil pressure had been "exceeded" ??? Getting kind of hypothetical here, but I don't think "catastrophic" damage to the vlve, seat, or guide, would result from this described instance.. Ever the optimist I am....

ULpilot

Woah, came back in today after doing it and found 13 new posts. Great stuff, thanks for all the input. It did work out pretty well- I sort of 'walked' the belt tooth by tooth until each cam was in the right spot (thanks to help of my mom too). I didn't have any other notches- only the TDC red and my blue sharpie mark for the 45 degrees that I put on earlier on my own. After many times turning it over (by hand) they still line up. No explosions (yet) so I'm hoping for the best!
Again, thanks a bunch!
-Simon

docmirror

Good show Simon, I think you're in the clear. Don't forget to check the tension in all the excitement. Thanks to Mom too!

Doc

Brian 944

Let us know if you need help with the tension. I did mine when it was on the engine stand, it took a couple of tries, the 90 Deg bend thing is a very inaccurate way of doing it, with the E-Bay Click tool, good results are possible. I would recoment removing the Plugs to do it proper.

Ted928

When the engine is at TDC, the only pistons at the top are #1 and #6. #7 and #4 are at bottom and the rest are at mid stroke (engine order is 1-3-7-2-6-5-4-8; 1-4 are on the passenger side, 5-6 the driver side).
The passenger side cam wants to spin back a little since the #4 exhaust valve is starting to open (low on the lob) while the #2 intake is high on the lob so it is not causing rotation. Since #1 valves are all closed since it is at the start of its power stroke, there is no impact.

The driver side cam would have to spin forward in order for the #6 intake valve to open up and strike the piston since it is starting its intake stroke. Although the #5 exhaust valve is open, it is high on the cam so this cam should not move.

You should be fine Simon.

SharkSkin

Simon, glad to hear you got it sorted out without too much drama.

BTW, damage is done by pistons -- with their greater leverage as mentioned -- hitting immovable valves and exerting force in a direction that they are not designed for. OTOH, if a valve should move like the lobe slippage described above, it's going to be moving on it's intended axis and tend to just stop if it contacts a piston, because there are no off-axis forces to make it bend. Now, there are lots of variables and few real absolutes in what I've said above, but if it were me I would have a much smaller load to clean out of my coveralls in your situation than if the crank were run into the valves. Either way the contact is to be avoided, but usually a cam slipping like that is no big deal. Do a compression check if you're worried about it.

UKKid35

I can confirm that I have repeatedly rotated the cams with the crank at 45 BTDC. Absolutely no problem whatsoever, but please note that this is on an S4 not a GT which has higher lift cams.

FlyingDog

I don't know why everybody freaked out on you. You just did a 16V timing belt change. The purpose of the 45* mark is only so that you can completely rotate the cams without the valves hitting the pistons. At 0* the cams can slip off the lobes without any risk of damage. It's only after you go over the next lobe that you could have a problem.

jebdog, not all 16V are non-interference and not all 32V are interference. According to PET, the vibration dampers with 45* marks started in US cars in 1983 with the M28.19/20 which are non-interference.

SharkSkin

Matt, are you saying that Simon ended up following the 16V procedure or that he has a 16V?

FlyingDog

Following the procedure for a 16V, which is safe for a 32V.