Head machined too far?

#1

Took a spare head I’d purchased down to a machine shop down here to deck/reinstall guides/cut valves and seats.

After the initial inspection, they said the head would need 0.013" taken off. I had measured with my cheapie dial gauge as best I could, and I’d measured 4.925" (exactly the stock height), and the dimples looked pretty substantial, so I figured I would be fine (4.912" being above the “minimum” 4.909" height). I measured the height at only a couple spots.

Picked up the head yesterday and he says they took off 0.028" (!!!) and that their machine is more accurate than the initial assessment they made with the feeler gauge. This, of course, puts me at 4.897" based on my original (and documented stock) measurement. That said, the dimples on the intake side are gone and the exhaust side has a bit of the 2 dimples left.

Measuring the intake side with the digital caliper they had there, and in only a couple places, we got 4.915" – which doesn’t seem to jive with 4.925" (stock height) minus 0.028" (what they say they took off).

I remeasured when I got home using my dial caliper again and I got a few measurements across the intake side – ranging from 4.897" to 4.910". One problem is that the arms on my caliper aren’t particularly long, so it’s hard to ensure both arms are flat against both surfaces of the head. I noticed even being slightly off on the arms was enough to throw the measurement off quite a bit. My caliper is made of plastic – so I’m not sure if plastic bending could be an issue, too. Either way, most of my measurements were low.

Thing is… I had already purchased a BMW head gasket to do the reinstall.

So, I’m trying to figure out if:

a) I should buy a nicer caliper and remeasure – maybe some stock head heights were slightly taller? Bentley says it’s +/- 0.004", so even best case this seems unlikely,
b) assume the worst and just buy a thicker head gasket now,
c) if the extra 12/1000" of an inch below “minimum height” is enough to cause valve/piston interference – particularly when considering…
d) if this far a drop is enough for cam timing to be affected

Any help here would be appreciated. Obviously I’d prefer to just hear “it’s not that big a deal, you should still have clearance, just use the stock HG and put it back together”. :slight_smile:

Som

#2

Going with (a) right now. Caliper should arrive tomorrow.

Otherwise, either I get word that the height will still work, or I need to call ECS and see if they’ll swap my head for the +0.3mm version. :-/

Got less than a month til the Sonoma race and this is my first head rebuild/swap.

Som

#3

Put clay on top of the highest point on #1 and #6 pistons. Bolt your head to the block (without cam installed) using an old head gasket and turn the engine over twice. Remove the head and measure the thinnest area of clay on the pistons. Slice a clean edge on the clay with a knife and measure withe the bottom of a caliper. Absolute min is .030"!

On head thickness, 4.909" almost always works unless you have a heavily decked block. Align your caliper along the head bolt holes to accurately measure. The thicker gasket is good for about .011" additional clearance.

#4

Thank you!

Clarification question on the clay process. How hard do I need to bolt down the head to get an accurate measurement? Do I need to use all the bolts and use the tightening sequence to keep the pressure even or am I not going to be tightening the bolts hard enough for that to matter?

If you’re sayin “4.909 almost always works”, is it safe to say that 4.897" will almost certainly not work? I’m having a hard time reconciling the various data points, but they did say the machine indicated it took off 0.028" and that it was likely the most accurate measurement.

I’ll try the clay measurement (I’m sure the wife won’t mind a trip to Michael’s), but I’m wondering if there’s even any hope that with 0.028" removed that I could still find more than 0.030" after the clay measurement.

Som

#5

Got a Mitutoyo digital caliper and measured a bunch of spots (reading goes to .0005"). My numbers are all over the place, but I seemed to get roughly the same numbers even when I came back and remeasured the same spots a second/third time. Here’s what I got:

[table]
[tr]
[td]Location[/td]
[td]Intake[/td]
[td]Exhaust[/td]
[/tr]
[tr]
[td]Before 1 (front of head)[/td]
[td]-[/td]
[td]4.9070"[/td]
[/tr]
[tr]
[td]Between 1/2[/td]
[td]4.9080"[/td]
[td]4.9100"[/td]
[/tr]
[tr]
[td]Between 2/3[/td]
[td]4.9125"[/td]
[td]4.913"[/td]
[/tr]
[tr]
[td]Between 3/4[/td]
[td]4.9025"[/td]
[td]4.9145"[/td]
[/tr]
[tr]
[td]Between 4/5[/td]
[td]-[/td]
[td]4.9140"[/td]
[/tr]
[tr]
[td]Between 5/6[/td]
[td]4.9070"[/td]
[td]4.9100"[/td]
[/tr]
[tr]
[td]After 6 (back of head)[/td]
[td]4.9010"[/td]
[td]4.9025"[/td]
[/tr]
[/table]

Pretty much all my numbers on intake side are below the 4.909" minimum – and seemingly by a decent amount (up to 0.008"). This matches up with the intake side having the dimples gone. I can’t really reconcile why I had the higher measurements in the middle cylinders on both sides like that – figure that’d have to be a pretty big f-up by the machine shop if it was wrong, so I’m inclined to believe I’m not great with the caliper.

Exhaust side appears to be at the bare minimum – guessing they decked the head at a slight angle. Fair to assume this is within general tolerances?

Unless someone can say for sure that going this far under the minimum head height is “generally not a problem”, I’m gonna start the process of ordering the +0.3mm head gasket. Which sucks, because the only place I’ve seen it online was ECS (HG was ~$75), but it would be about $50-60 to overnight/Sat ship it so I can resume work next weekend (it takes them a few days to get it, and with Memorial Day, they wouldn’t be able to order it until Tuesday).

This is gonna get tight. Hope I don’t run into any other major issues putting this back together. :unsure:

Som

#6

Got some great help from Rich yesterday on the head. Based on how much was taken off the head, the main concern was around whether a head that was that warped to begin with would have its cam journals still aligned. The test for this was to ensure the new cam slid into the head without significant force – which it did, very smoothly.

The level to which the head was machined, though, does mean I will likely need the thicker head gasket. Fortunately, the local dealership can have one by Tuesday. Have to pay full price for it ($94), but that’s cheaper than the $70-ish dollars I can get it for online + overnight shipping would cost.

So, in the meantime, I started with fitting the new HD rockers I got with the existing eccentric/hardware. And that’s where my next problem has come in – the eccentrics don’t fit (easily).

The old rocker/eccentrics all slid in/out with ease. I was able to mix/match 6 rocker/eccentrics to get them to fit, but even that “fit” required a little pushing/squeezing to get them in. I verified that I’d be able to rotate the eccentrics pretty easily once they were in, so I’m comfortable with those.

The remaining 6, though, just won’t fit. Measuring again with my fancy new caliper, I find that my eccentrics are consistently 0.197" wide. This was off the used head I’d purchased (off a '91). I measured the eccentrics on the head ('87) that was in the race car before – 0.198" wide. The two are close enough that I’m guessing that’s the normal eccentric width.

The gaps on the HD rockers that didn’t fit range from 0.192" to 0.195". While I could possibly squeeze the eccentrics in, obviously there’d be 2 concerns to that approach – stretching the rocker arm’s arms, and not being able to rotate the eccentric once it was in.

The tldr question is – if the eccentrics don’t fit in the rockers, should I a) grind down the rockers, b) grind down the eccentrics, or c) ask IE for new rockers?

I’m guessing either a) or b) is an acceptable solution, but just wanted to see if it’s common to need to do this or if I just got a bad batch of rockers.

Thanks,

Som

#7

Som, the eccentric fit issue is common with the IE rockers. I lightly grind then polish the slots until the eccentrics fit properly.

#8

A dremel with a sanding disk would probably work well.

#9

Awesome. Thanks guys.

Som

#10

And one more question while I’m in the middle of this head rebuilding. I’ll preface this post by saying there’s nothing of monumental significance to my observations and I already know my likely course of action. Read on if minutiae is your thing. If not, you’ve been warned. :slight_smile:

When I was removing the rocker shafts, I noticed a few things.

  1. I was banging up the rocker shaft plug by using a 3/8" extension for a drift. This caused the shaft to mushroom ever so slightly which made the shaft removal a pain in the butt. Using a 12mm socket at the end of the extension solved this problem. The shaft plug, while bang up, doesn’t appear to be punctured in any way. I’m fairly certain it’ll continue serving its purpose of keeping oil in the shaft.

  2. The ends of each of the rocker shafts were different. The intake shaft looked to have a regular flat-ended plug. The exhaust shaft looked to have little “cups” that were pressed into the ends. These rocker shafts came off an engine that came from a '91 car. Looking at the rocker shafts in my '87 head, the plugs are both the “flat end” kind.

At first, it thought the shaft was just missing the plugs altogether, so I purchased 4 new plugs – 1 to replace the intake plug I had beat up, and 2 to put on the exhaust shaft. This leads me to…

  1. The shaft plugs I purchased (using a P/N found on realoem) are significantly smaller (roughly 2mm) than the existing plugs on the shafts. Then I noticed that there are actually 2 different P/Ns – one is the plug (11331266400) the other is a screw plug (11331274923). Of course, I bought the regular plug. I’m guessing I would need the screw type plugs.

Here’s an image that clarifies:

So my question is – is the “cupped” rocker shaft plug something common? I don’t really know the history of this engine. While the head height appears to suggest it had never been decked (thus, I’m guessing it had never been removed), the rockers had the cylinder and intake/exhaust designations carved into them – suggesting the shafts had been removed and maybe replaced.

Also, when are the regular plugs used vs the screw plugs? I’m guessing it’s not something they completely switched over to from one year to the next as the two engines I have are from the far ends of the build timeline – 12/86 to whatever the earliest possible '91 engine would be.

Anyways, I’ll be leaving the banged up shaft plug the way it is and will be installed both shafts as is. Just thought it was weird and worth sharing. :slight_smile:

Som

#11

I’m pretty sure I have a +3 head gasket still in the wrapper I want to get rid of.

#12

Bummer!! I had the nearby dealership order me one. Picking it up in a couple hours.

Thanks, though!

Who knows… if I botch this maybe I’ll hit you up if I can’t reuse this gasket. :slight_smile:

Som