Valve lash on exhaust strok

[JJ Green]

I’m new to Porsche ownership
I have a 1974 911 (it a parts bin special)
1974 2.7 Engine, 78 sc shell, 1971 901 4 speed transaxle

I have had the engine running and it ran ok
Took it out to fix the gearbox, and while I was at it I started tidying up the engine, I toured the head studs, some were very loose but they all came up to spec.
Then I thought I should check the rocker adjustment. I set them all on the compression stroke but I noticed that on the exhaust stroke that they have a lot of lash (1mm) or more
I can not find any info on what they should be but, I know it’s not right.
Has anyone else encountered this before?

[ErrorMargin]

Valve lash is set at TDC for each cylinder. The lash at other points in the stroke will vary and is not something to worry about.

Here is a good video on the topic: https://youtu.be/6ciw3mDjz0k?si=JrwfgRRiEtJknkAS

[JJ Green]

Thanks, my assumption was that the values would also be 100% closed on the exhaust stroke at tdc and should measure the same.
Before I adjusted them they were all very tight so there was very little lash even on the exhaust strok and I thought maybe there was a can tensioner issue

[stownsen914]

The standard is to set at TDC. It sounds like you were trying to set them at another point. This should work (if done correctly, when the rocker is riding the back side of the cam), and be within close tolerance. 1 mm is way too much variation and shouldn't happen.

It sounds like what may have actually happened is valve lash was inadvertently set at some other place in the cam rotation cycle (like overlap, for example) when the valves were partially open. I can't think of any other reason to get differences of 1 mm.

Best bet is to set at TDC as recommended.

[Bill Verburg]

Originally Posted by JJ Green

I’m new to Porsche ownership
I have a 1974 911 (it a parts bin special)
1974 2.7 Engine, 78 sc shell, 1971 901 4 speed transaxle

I have had the engine running and it ran ok
Took it out to fix the gearbox, and while I was at it I started tidying up the engine, I toured the head studs, some were very loose but they all came up to spec.
Then I thought I should check the rocker adjustment. I set them all on the compression stroke but I noticed that on the exhaust stroke that they have a lot of lash (1mm) or more
I can not find any info on what they should be but, I know it’s not right.
Has anyone else encountered this before?

There are 2 xTDC's
TDC compression stroke: here both valves are closed w/ the rocker lobes on the back(lowest) side of the cams
TDC exhaust stroke: here depending on cam spec both valves are off their seats, The spec for a base cis 2.7 is 2.4-2.8mm @TDCexhaust w/ .1mm clearance, ie there is no play

TDC compression for cyl #1 is marked on the crank pulley

[dannobee]

Someone adjusted the valves during valve overlap and not on TDC on the firing stroke. Easy way to look is to pop off the distributor cap and see where the rotor is pointing.

Your 1mm clearance is nearly exactly what we'd time CIS cams at, 0.8mm.

Readjust the valves.