Another Question about Cam Timing (please read it)

[osidak]

Is it possible to get the cam/crank shaft 360 degrees out of sync? I ask this because I set the cam up and and installed the rockers. Adjusted the valve gap on #1 and gave it a spin. I came up with 1.06mm

Per BA I was shooting for 1.1 to 1.4 mm so I planned on setting them at 1.3mm. I went to make my adjustment on the cam shaft and it slipped. I set it back up and then all I got .60 mm movement. What would have caused such a large difference.

When the cam slipped I rotated the engine clockwise 360 degrees (I think) and then set the dot on the cam to 12 oclock. From that point on I get the .60mm reading.

Help

Osidak
78 911sc

[J Richard]

It dosen't take much movement to get a substantially different reading on the gauge.
Do you have the chain tensioner sprockets blocked up solid to take out any chain play? You have to keep turning the motor in the running direction or you will add play back and give you a false reading. Taking out all the play in the system is why you need to rotate the motor through once the initial setting is done. Given the firing order I think you would have a valve/piston jamb if you were out a full 360d on one of the cams. I once made the mistake of setting the cam timing ahead by the specified setting. The cam was closing instead of opening and I was getting the right measurement but on the wrong side of the lobe. It actually ran with a bit of a knock, thank god none of those precious parts became aquainted with each other...

It's not a big deal, just start at the beginning and check each step over again. If you are at Z1 rotate the motor through the firing order and watch the intake valves, they should follow in sequence, then you know you are ok.

------------------
93RSA

[This message has been edited by J Richard (edited 08-29-2001).]

[GIBSON]

I think the point about slack is a good one. I keep a set of mechanical tensioners to allow me to keep the chain tight, seems to help the readings. There are devices sold which do the same thing.
Which cams are you timing? S? They are probably the most difficult to time as such a small difference in the setting can cause a big difference in lift. Keep trying.

Bruce Herrmann
73911S

[Don in Redondo]

I am in the middle of a very challenging cam timing operation, involving a 2.0 to 2.2 top end upgrade with a high rise Solex cam and larger valve diameters, all leaving very little clearance between valves and piston cutouts during valve overlap at TDC.

I just learned a very valuable measuring technique from one of the pros, which is to simply use the rocker foot screws of the #1 and #4 cylinders to measure the valve to piston clearance, one turn of the screw equals appx. .040" of clearance. Using this method you can optimise the clearance relationship between intake and exhaust valves on very tight valve clearance setups, of non-stock assemblies. Sure beats the often referenced plastigage method that requires very time consuming disasembly and reassembly.

[Peterfrans]

I am fairly sure that it is not possible to have the cams 360 degrees out of sync. As it is a four stroke engine, the fact that the cams turn at half the crank speed will give the same result if you turn the cam another 360 (with the crank standing still). Most important in making sure that your valves will not hit the pistons is that both cams are in sync.

After the cams are set, other items requiring timing like your distributor and for instance a MFI pump can be set 360 degrees wrong

Maybe the problem is that your dial indicator is bottoming out half way.

Good luck

------------------
Peter
'70 911S Targa

[This message has been edited by Peterfrans (edited 08-30-2001).]

[osidak]

Thanks for the info, I will again try tonight. and let you guys know how it is going. Any other suggestions are welcome.

Oh the cams are stock sc cams, so nothing real wild about them.

osidak