Milled Head .0100", Will It Affect Timing?

[86bmw325e]

Hey guys,
upon all your advice, I took my head to machine shop and had it cut ten thousandths. There is still dimple left and it is in the spec all data gives me for maximum cut. My biggest concern is the timing. Should I run the thicker BMW head gasket? or should I just use the stock size (already have one). I just don't want to get it together and have the timing off. I really don't want to buy an adjustable gear as this car is really just a good stock daily driver. Any advice is much appreciated.

[86bmw325e]

Bump anyone?
I emailed pelican and they said check with the machinist, but the machinist is an old school push rod guy and I don't think he'd know. I really don't want to put it together just to have to take it apart again

[MR 325]

You'll be totally fine with the stock gasket. Put her together! :D

[Cabriolet]

"There is still dimple left"
basically means you are good to go. taking 10/1000s off is very common.

[86bmw325e]

Awesome! Thanks so much guys, I was really worried about this. I just want this car to be a fun reliable daily.

[vpilarrt]

Doing the math:

IIRC the cam gear is about 5" dia or 2.5" radius
tan^-1(.01/2.5) = .23°

So your timing will be retarded by .23° which is not enough to worry about.

[Som]

Without confirming that math, I read somewhere while researching this that you need to double the cam degrees to get the crank degrees off since the crank/cam gear ratio (supposedly) is 2:1. So if your math is right, it would .46° off.

That said, I feel like I've read a few different people do calculations for this and the range (including yours) seem to be ~.5° - 2° off. I think the 2° was for running the head at min-height, though, which I don't think 0.010" gets you to (close though).

Other thing I've read (from forum people, so do your own research to confirm) is that the timing retardation doesn't significantly affect peak power, but just shifts the curve (retardation moving the peak torque later in the revs, advancing moving it earlier). That said, Chuck (cwbaader in this thread: http://spece30.com/forum/42-engine/6...nerds-question) says he actually did try adjusting the timing by 2° increments up to +/- 8° and it did affect the power. Not sure if different fuel octane levels were tried, though, to potentially take advantage of the advanced timing.

At some point I'll have to do the math myself (I'm rebuilding a head now) and see how if I can confirm any of those numbers.

Som

[vpilarrt]

Yeah I suppose you're right. Since the distributor is tied directly to the cam on M20s (not counting the 323i) and the crank turns twice as fast as the cam, the crank will be off .46° when the coil fires. Therefore the ignition timing will be .46° retarded.

[digger]

there is no cam sensor only a crank sensor, the ignition is with respect to crank position so ignition timing is unaffected

[Som]

Heh, what the hell is your problem coming in here and dropping knowledge!! :)

Glad you set me straight, though. For most things I was definitely thinking of valve timing, but I definitely hadn't separated the two since I was still thinking fuel type might affect it.

Som

[AndrewBird]

When I first read your title I thought you had .100" taken off. lol THAT will cause some problems.

To get an idea of how much .010" actually is, take 4 sheets of paper and stack them together. That's about .010".

[86bmw325e]

Thanks for all the help guys! Head on, all timed, just waiting on a few parts...