885 Head thickness

[korn]

So I just got the head back from the machine shop; it is showing a head thickness of 124.30mm; Bentley tells me a minimum head thickness of 124.70 is required; Would this head be ok to use with oem gasket and cam?

I can no longer see the "dimples" in the bottom. Here is a pic of the number in the corner...

[SkiFree]

Not without claying it at a minimum.

[korn]

I'll have to look into that next. The thicker gasket gives me +.30; which will be close to Bentley's minimum... That's pretty much my only option that I can find...

[Som]

I just ran into this same issue. Machine shop checked the deck of a head that hadn't been previously shaved with a straight edge and feeler gauges. They called me and said I'd need to remove 0.013" -- which, would have been fine. I gave them the go ahead to do the rest of the work on it (new guides, valve/seat grinding).

When I got the head, they said they had to take off .028" -- something they only found out when they got it on the machine to deck the head. Unfortunately, that put me well below the Bentley's minimum height.

I put the head together with a +0.012" (+0.3mm) gasket, started it up, and it knocked. Not piston/valve knocking, but piston/head knocking. I took it apart, clayed it, but the claying wasn't super easy to be accurate with (I probably picked shitty clay, and it was my first time doing it). I measured a minimum clearance of about 0.017". I was told a good range was 0.030"-0.040", so that explained my knocking.

To make up the difference, I bought a Cometic MLS head gasket (~$300). They can make those to custom thicknesses. I went with 0.092" (stock is 0.068", the +0.3mm is 0.080"). Part number would be C4393-092. The 0.092" brought me to 0.029" of clearance, which was close enough for me.

The downside of this option is that I've decreased the compression in the cylinders. Even though I've technically made up the shaved height with head gasket material, the head gasket volume added is a perfect cylinder where the volume they shaved off the head was domed -- so a net loss of compression.

Wouldn't have been my first option, but I wasn't specific enough with the machine shop about how much they could take off. They should have known, but whatever. Since I'd spent $500 on the head work, my choice was either to buy a thicker head gasket, or basically chuck the head and spend another $500 on a new head. *shrug*

Som

[korn]

Good info. Thanks. I'd like to get it right the first time... (Wouldn't we all); do you know how far below minimum you were?

[korn]

OK... Now that I am a head milling GURU... (this is my first head job); Here's my plan: someone correct me if I am off...

1) ordered a +.3 gasket
2) pull off my current head (cracked near valve stud)
3) Clay the pistons to determine if there is any P/V interference
I'm looking for a minimum of .4mm

I believe I should be OK, but we will see. I'm not sure if the block has ever been milled (unlikely at 162K). I am currently at 124.30mm on the head, vs. Bentley's minimum of 124.68... The +.3 HG will put me within .1mm of their min...

So; my question is; If this works out, I should see slightly higher compression but slightly retarded timing. Running a stock cam, will I need to get into an adjustable cam gear?

[redlightpete]

You need to clay the actual head you're going to use. Use your old, compressed gasket, and you should have 0.3 mm more clearance when you assemble with your thicker gasket.

[Som]

As far as the head height, I measured several times, at several different points on the head. Despite having a reasonably accurate caliper (Mitutoyo), I couldn't get absolutely consistent results. A slight difference in angle and I'd get differences of several thousandths of an inch.

My pre-machine shop measurement was done with a cheapie plastic caliper, but -- I actually measured the exact stock height of 4.925" with it. When I got the head back, I couldn't measure any height below 4.900" -- but this didn't really jive with the machine shop saying they'd shaved 0.028" (which would have meant I was at 4.897"). On top of that, 2 of my dimples were showing (barely), while 2 on the other side were gone. Not sure how accurate the dimples are, so it's possible either the dimples were different depths or the shop decked the head at an angle. :-/

As far as claying goes, that part was even less accurate for me, though possibly because I used some crappy clay. I wasn't able to peel my clay off the piston without it stretching/falling apart, so I had to measure it on the piston using the depth gauge end of the caliper. It wasn't particularly easy to tell when I had the caliper pressed up *just* to the top of the clay and not actually dipping into it or compressing it. I'd look for recommendations on the type of clay to use and consider using it sparingly to avoid getting it squeeze out to the cylinder walls.

Also, it's not piston-to-valve clearance that you should be worried about -- apparently there's a decent amount of clearance between the piston/valves already (relatively speaking). It's piston-to-head clearance you'll be concerned with. It doesn't hurt to get all 3 clearances, though. You'll basically be laying clay down on the intake cutout, the exhaust cutout, and the crest of the bump/dome/ridge on the piston -- which is where it gets closest to the head. Bolt the head on, hand rotate the crank twice and then measure the clay depth. It's this point that needs to be 0.030"/~.76mm (0.040"/~1mm to be safe).

Just to be clear, I'd go through these posts and make sure you're clear on measurements called out as inches vs. mm. I got used to the inch measurements, so that's what I've been giving my measurements in. It's .040", not .4mm (unless you meant .4mm about something else?).

Agree with Pete -- make sure to use the head you're going to put on when doing the clay measurements or it's pointless.

Regarding the cam gear, from what I read on the forums, the retarded timing just serves to move the power band earlier in the RPM range. Not sure if I ever saw definitive testing to prove/disprove this, though.

Regarding the compression, I wouldn't be so sure about the compression increasing. While it's true that you'll be running a lower head height than stock, the 0.012"/0.3mm you're adding in the head gasket is a full cylinder's worth of volume (pi x r^2 x height), where the volume you removed from the head was domed, and therefore less volume than a full cylinder. If you manage to avoid piston-to-head interference with the 0.3mm gasket (I wasn't, but my head might have been even thinner than yours), then you *might* be running a slightly higher compression, but it wouldn't be by much.

And for the record, I'm no master at this myself. Rich on the Spec E30 forum was a tremendous help to me in giving me a bunch of this information. Just trying to pass it on as it was explained to me. :)

Som

[korn]

Thanks SOM. That helps. My confusion now lies in the piston to head clearance issue; and why would that be closer than the valve... I'm sure that will become more evident when I clay it.

Do you have the link to the thread you started on the other forum? Reading through that should help.

[Som]

The thread is here, but most of Rich's help was over the phone.

http://spece30.com/forum/16-general-discussion/80915-head-machined-too-far

The piston/head clearance is an issue because the head is domed and the piston bulges up above the plane of the block's top. To be clear, I don't know if every engine will see the piston/head clearance be tighter than the piston/valve clearance -- just that it's how it is on this one.

Som