m20 head milling

[mikebloco]

What is the most anyone has ever had their head milled? I've heard that you can mill it up to .030" but I can't find a definitive answer on the interwebs

[digger]

0.030" will be fine if you check piston to valve clearance, and piston to head clearance, dial in cam timing with adjustable gear. what are you hoping to achieve besides the obvious result a higher CR? if you arent in some race class where a couple of hp matters then its a probably not worth it.

[mikebloco]

I was just curious. I'm going to check clearance on .039" but I wanted to see if anyone has ever done anything extreme whilst still running a stock thickness head gasket.

[Som]

I had what I measured to be a stock height head (4.925") milled down .028" according to the machine shop. I couldn't measure a point that was thinner than 4.901" after they were done, though -- and that's still .008" lower than the "minimum height" in the Bentley. I was using a nice caliper, but it's not easy to get accurate readings.

I had piston-to-head knocking with the .080" (+.3mm) OEM gasket, though. After doing claying the pistons and taking some measurements, I went with a .092" Cometic MLS gasket. The claying wasn't easy to do as I think I picked up a bad choice for clay -- so it wasn't always easy to tell if I was squishing it. I ended up measuring a clearance of 0.017 on cylinder 6 and gradually more towards the front. The advice I'd received was that the clearance should be .030", but .040" is the safe number. Since I was worried about buying something *too* thick, I went with the .092" (only .012" larger than the .080" for a total clearance of .029").

Put it back together and no knocking.

I'll also point out that the machine shop appeared to deck the head at a bit of an angle, too -- the dimples on the intake side (I believe) were completely gone while there was a tiny bit of the dimples on the exhaust side. It's possible the dimples aren't the same height to begin with, but I'm more inclined to believe the shop decked it with a slight angle.

I'm surprised to hear a 0.030" decking should be fine for piston to head clearance, but I'm sure digger has more experience than I -- I've just done my one head.

Som

[digger]

I had what I measured to be a stock height head (4.925") milled down .028" according to the machine shop. I couldn't measure a point that was thinner than 4.901" after they were done, though -- and that's still .008" lower than the "minimum height" in the Bentley. I was using a nice caliper, but it's not easy to get accurate readings.

I had piston-to-head knocking with the .080" (+.3mm) OEM gasket, though. After doing claying the pistons and taking some measurements, I went with a .092" Cometic MLS gasket. The claying wasn't easy to do as I think I picked up a bad choice for clay -- so it wasn't always easy to tell if I was squishing it. I ended up measuring a clearance of 0.017 on cylinder 6 and gradually more towards the front. The advice I'd received was that the clearance should be .030", but .040" is the safe number. Since I was worried about buying something *too* thick, I went with the .092" (only .012" larger than the .080" for a total clearance of .029").

Put it back together and no knocking.

I'll also point out that the machine shop appeared to deck the head at a bit of an angle, too -- the dimples on the intake side (I believe) were completely gone while there was a tiny bit of the dimples on the exhaust side. It's possible the dimples aren't the same height to begin with, but I'm more inclined to believe the shop decked it with a slight angle.

I'm surprised to hear a 0.030" decking should be fine for piston to head clearance, but I'm sure digger has more experience than I -- I've just done my one head.

Som

i always though everything untouched the piston to head was 0.06" so you have to check, never said its 100% fine but needs to be checked. if the head has been decked alot before then you dont need to take as much off. id target 0.03 to 0.04 for piston to head. you can get a 1.75 or 2.05mm gasket to so some adjsutment

[mikebloco]

I measured the the head gasket on my motor and it was 1mm compressed. I figured it was the stock head gasket. Maybe not though. I'm also running flat top slugs so there might be more clearance from the valves and the piston dish.

Is the OEM head gasket really .3mm/.011" thin?

[4x4_e30]

The dimples are uneven, same thing happened to me. One thing to note about using any kind of calipers, the wider they are spread, the less accurate they become. My Brown and Sharpes read .0015 over at 5 inch spread, Harbor Freights are out by .006.

Also the castings can vary quite a bit, might account for it working for one person but not the other.

[digger]

I measured the the head gasket on my motor and it was 1mm compressed. I figured it was the stock head gasket. Maybe not though. I'm also running flat top slugs so there might be more clearance from the valves and the piston dish.

Is the OEM head gasket really .3mm/.011" thin?

stock gasket is 1.75mm and +0.3mm or 2.05mm. they might compress some but not much

[digger]

The dimples are uneven, same thing happened to me. One thing to note about using any kind of calipers, the wider they are spread, the less accurate they become. My Brown and Sharpes read .0015 over at 5 inch spread, Harbor Freights are out by .006.

Also the castings can vary quite a bit, might account for it working for one person but not the other.

a warped head that gets cleaned up will mean dimples are uneven, with the angled squish band on b25 piston and 885 it makes it tedious to measure

[4x4_e30]

That's assuming I didn't check it for flatness beforehand. The dimples are on the deck surface.. Simple matter of clamping it down, throwing an interapid in the spindle and running over all the surfaces.

[digger]

i have a head with unknown history here nd 3 of the dimples are for all intensive purposes 0.6mm and one 0.3mm. the bottom of dimples appear to be as cast so there will be differences