M20 maching specs

[AlphaE]

I looked and couldnt find solid reliable answer.

How much can i take off an 885 head before,

1) there is valve/piston clearance issues using an oem thickness gasket?
2) i have use an adjustable timing gear?
3) before i have to use a fatter gasket? similiar to 1 but a bit different. Not concerned w interference as much as, what the most you can take odd and still use oem... maybe it is the same question after all.

this is on a 88 supereta. so im not 100% on if the pistons have the same height in relation to the block as a regular "i."

please only reply w first hand experience, not "i heard of a guy who..." If you did it and it worked lemme know.

Thanks!!

and the head is off my car now, so fast replies would be nice.

[jlevie]

There are limit dimples on head and you can deck the head to those without incurring valve to piston contact. If you do that you will have to use an adjustable cam sprocket to restore cam timing (it will too retarded) or use the thicker "repair" head gasket.

[AlphaE]

thanks, i new about the dimples. those dimples are DEEP. But how much before i have to worry about cam timing? numbers please.

can i take off 5 thousands and use oem w/o worry? can i take 7? 10? 12? i know they make the .012 thicker gasket. what if your takin off less than that? Just run the OEM thickness?

thanks

[SkiFree]

The reason you're not going to find a straight answer is because it does vary. If somebody gives you a firm number, treat it with EXTREME caution. Valve seat height can also affect piston clearance.

[jlevie]

There are three possible OE head gaskets (std, +1.75mm, & +2.05mm). Cam timing will acceptable with the standard gasket until 1.75mm (0.069") have been removed from the head. At which point you use the +1.75mm gasket. In a like manner you restore acceptable cam timing after removing 2.05mm (0.081") with the next thicker gasket.

On a race engine I mill the head to the minimum thickness to gain a bit of compression (~.3 points). Then I use an offset busing in the cam sprocket to put cam timing back where it should be.

[AlphaE]

jlevie THANK YOU!! NICE! thanks for all the answers! i have some small corrosion on what seems to be a very straight head (atleast w a staright edge and feeler gauge, machine shop will check it out). I all ready have the OEM gasket. the preliminary look says 5-6 thou off the head would take care of it. So from what im hearing i can run the OEM w that little amount off?

I understand there are many variables that make it hard to give a solid answer.

"There are three possible OE head gaskets (std, +1.75mm, & +2.05mm). Cam timing will acceptable with the standard gasket until 1.75mm (0.069") have been removed from the head. At which point you use the +1.75mm gasket. In a like manner you restore acceptable cam timing after removing 2.05mm (0.081") with the next thicker gasket."

ALLDATA (which sucks) says mentions a .3mm gasket??? that exist?

[AlphaE]

This is my corrosion. It's maybe 4-6 thou deep.

[jlevie]

I have yet to see a used had that would not benefit from a mild (~0.005") surfacing cut. So I always have that done to a used head. And while the head is at the machine shop it will get a valve grind, guides checked and perhaps replaced. And of course new seals. I'll have the cam profiled and if worn it will get reground or replaced. Oh yeah I'll have the shop check to springs to see if any are soft.

[e30trooper]

a bit off topic. i have a 2.7i set up , eta block and milled 885 head. Head was milled .010 to bump compression loss on the eta block. No that I have blow by on 6th cyl i might toss in a b25 block and call it a day. Now my worries at valve clearance.. The head wont be getting sent to the shop since the Milling , valve grind/adjustment and all of the above where done theres no reason.

The question, will I have any issues with this milled head on a b25 block? Since we are in to talk of milling/decked heads

[AndrewBird]

Just put to things into perspective, .005" is roughly the same as the thickness of two hairs. We aren't talking very much material here. I HIGHLY doubt BMW built these engines with less then even .030" clearance between piston and valve, but I could be wrong. Generally the faster things are moving near each other, the more clearance you give them. I would be interested to see what stock clearance is. Someone should clay their stock motor and measure it.

[AlphaE]

so they ened up takin 11 thou off... Thats gonna still work w the oem gasket correct?

[Cabriolet]

the easy answer is see that divot in your pic. it's near the outside of the casting. thats how deep you can take off without needing to get fancy and retarding the cam gear or getting a thicker gasket. use the OEM gasket and not VR.

[AlphaE]

as deep as that divot w/o fatter gasket?? or gear... i kinda doubt that..

Ok real oem shows a 1.75 and 2.05. 1.75 must be stock thickness. eeuroparts shows the 2.05 as the +.3 fatter one. This all makes sense w all data now since they call for .3 (.012 thou) over gasket as well..

So i think there are only 2 options for gaskets oem 1.75 and 2.05 (+.3)

anyone confirm this..

[digger]

an adjsutable gear wont harm a stock engine as long as it is not a crap piece and setup right so worth it IMO. Then check the clearance between head and piston and piston and valves. the reason to check is that what if the block has been skimmed? the reality is if you take off 0.27mm there should be enough reserve in clearnace between head and piston and you probably improve the sqiush clearance. what cam you using this might casue problem if a bigger cam? always check above

i heard you cant get stock 1.75mm gaskets anymore from OEM or goetze only 2.05mm (+0.3) but there may be old stock around. can get custom whatever you like.

my guess is going to the divots and a 2.05mm gasket go hand in hand but might be conservative meaning might get away with nomoinal gasket in some cases

[AlphaE]

Thanks Digger. im using a stock i cam. the block is untouched as far as i know. i got a elring 1.75 full upper gasket kit .

how would i actually check the clearance. does it have to be special clay? or can i use silly putty or something like that? So i use the old (already compressed) HG, torque head w old bolts to like ???? 20Lbs???? set up TB and turn by hand. pull head. cut clay/silly putty, measure. Right??

i gotta mic them divots. they are crankin DEEP!!! way deeper than the +.3 (.012) fatter gasket. there is now way the slightly fatter than stock gasket makes up for shavin down to the divots..

Jlevei, does that min thickness you buzz it down to to bump comp get you to the bottom of the divots?? according to all data the min thickness is only .012 off. you know how much you cut off?

thanks for the input guys..