The back story
I had the cylinder head of a 1995 E320 straight six machined to remove a bow from end to end after overheating.

My machinist checked his online data base and warned me that getting this particular head flat would require machining very close to the recommended limit of material removal. I asked him what impact that would have? "It is going to change the timing," was his reply.

Finally the Question!
How much does machining the cylinder head alter the timing? Is there a direct ratio? (X degrees per 0.001 inches of drop in the head relative to the block kind of thing?)
How sensitive an issue is this for this particular motor? None at all? Disastrous?

Is there a corresponding rule (of thumb, or otherwise) for offsetting the impact of machining the cylinder head? Can it be (should it be) compensated for by changing the relationship of camshaft and crankshaft gear teeth (e.g. reduce/increase number of links between gears)?

Has anyone successfully dealt with this issue? Or even know for a FACT that it is/is not an issue?

Many Thanks as always,

Okay,first things first. Yes, it will change the timing but not in the simple "twist the distributor to fix it" way you might be expecting. Reducing the thickness of the head will have the effect of "lengthening" the timing chain (the chain won't actually be longer but the gears' centers will be closer together resulting in the same effect) which will be retarding the valve timing. The amount the crank to cam gear is changed will determine if you will need an offset key. You won't know until you've assembled the engine.

There MIGHT be a formula which could be used but you have too many variables involved (stretch of chain, wear of gears, etc.) to calculate with any guarantee of precision.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Not enough to worry about. Worst case you move the power band up by a couple hundred RPM. If you replace the chain when doing the head it will gain back a few degrees of stretch. I wouldn't call it a "must do" but if the miles are up there it might not be a bad idea.

__________________

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

I'm curious, though. What is an offset key? What are the signs that one is needed?

Does the failure to fire up after reassembly mean that I'm looking in the wrong direction? (Both cams were set correctly using pins in the gears when the chain was reinstalled.)

The engine will run with the cams quite a bit off, albeit poorly. I would not attribute a no start to cam timing unless they are nowhere in the ballpark.

An offset key allows the cam or crank gears (as the case may be) to be indexed + or - a couple of degrees. Without going into a lengthy discussion why, you dont need one.

__________________

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

When a head over heats, it doesn't only bend on the bottom, but also on top were the cam bearings are.
The head should be straightened before machining, otherwise your cans will be forced into a bow.

__________________

76 240 D. Bought in 1998 for $25.
85 300D. Got it for free with a bad engine. ( Sold )
60 Unimog 404. What was left of it, was given to me. Now powerd by 617A.
88 560 SEL. Bought without engine and trans. Now powerd by 617A.
67 250 SE. Cuope. For resto or sale.
64 220SE. For resto.