M20 head gasket question?

[Kuhl 951]

I'm in the middle of a HG changeout due to a failure on the 91 325 iX (M20) in stalled traffic. I checked tonight to see if the head warped and it looks like I'm right at the limit of .009 to.011 inch with a machinist straight edge. Spec says the max. machining is .012. My question is do I have to use the special .3mm HG for machined heads? The set I bought is the standard gasket and it came in a set that I have already opened and can't return. If I machine the head and use the standard gasket will the difference just be having to buy higher octane gas or will there be driveabilty issues? I read about heat straightening but really don't know much about it. The option of a replacement head isn't something I want to spend right now. Any ideas or help would be greatly appreciated.

Thanks,
Steve

[thejlevie]

With that much warp I'd have the head lightly machined. I think you'd be okay with a standard gasket if it is only a light skim cut. The compression ratio might go up enough that you'd need to use one grade higher gas (and you'd gain a bit of power).

[Chris's318i]

Anytime I rebuild an engine (only 350 Chevy's) I have the heads milled. I never have used special gaskets. I usually have .010 taken off in most cases. This amount will clean up any nicks and most warp and definitley bring you within tolerance. I would go with a regular head gasket, get the valves ground and new seals. Just my $.02 worth.

[Kuhl 951]

Anytime I rebuild an engine (only 350 Chevy's) I have the heads milled. I never have used special gaskets. I usually have .010 taken off in most cases. This amount will clean up any nicks and most warp and definitley bring you within tolerance. I would go with a regular head gasket, get the valves ground and new seals. Just my $.02 worth.

I know what you mean but SBC are a pushrod design and are more forgiving. The OHC design requires straight bearing supports for the cam, warpage is not a good thing.

I've found a shop that does heat straightening first before the a skim cut. The problem with a warp that's that large is no matter how much you machine the bottom the cam is now in a slight bind also which will kill it over time. The valves and rockers are all good so here's wahat I'm having done.

1. Take assembled head to shop;
2. They disassemble, check for cracks and hot tank it;
3. They will do the heat straightening in a 400 deg. oven with shims for 3
hours and return it's flatness;
4. Machine the lightest cut possible (.001-.003) to clean it up;
5. Line bore cam if necessary;
6. Grind valves and seats, new valve guides/seals, cam seals.
7. Reassemble and pressure test.

So basically I'm doing a better valve job rather than just machining out the warp. All the above work comes to $400 which is better than a used head (tested) @ $795 or a completely rebuilt head at $1395 from Bavarian. It's more than I wanted to spend but milling that much (.011) isn't a good idea when it's only .001 away from the max. spec recommended. This way I'll be sure the cam stays aligned and the loss of head thickness with a standard gasket won't retard the spark any which is a serious problem on machined OHC heads. I'm just glad I read up some over the last couple of days on heat straightening of aluminum heads. The actual heat method is only $80 which I think is a killer deal.

Again thanks for all the help, I do appreciate it!

[Stück]

You can get a good used head for $200 or less here on forums. Kids wrap these cars around objects daily. There are plenty of them available.

As for your head... get it tanked, checked for cracks, resurfaced and then reinstall it. Use the normal thickness HG. You have a stock CR of 8.8:1. The bump to 8.804:1 whatever won't bother it. After you factor in removing carbon deposits in the chambers while you are there you will likely lower your dynamic CR beyond what it was prefailure.

[Kuhl 951]

You can get a good used head for $200 or less here on forums. Kids wrap these cars around objects daily. There are plenty of them available.

As for your head... get it tanked, checked for cracks, resurfaced and then reinstall it. Use the normal thickness HG. You have a stock CR of 8.8:1. The bump to 8.804:1 whatever won't bother it. After you factor in removing carbon deposits in the chambers while you are there you will likely lower your dynamic CR beyond what it was prefailure.

What you say is certainly true but for less than $200 more I eliminate shipping and the uncertainty of an unknown replacement head having it's own issues. Call me an a$$ but my perspective is that kids that wrap their cars around objects don't know how to drive yet and I really don't need or want any parts that have been beat like a junk yard dog. This way I know what I'm getting and can restore the head to near spec compression with the assurance that the cam is aligned properly. When I bought the car last year all cylinders were 149+ so I know the bottom end is still sound. I might find when I get to the machine shop that I can eliminate the valve job and save another $175. To me it's all a matter of not doing this project ever again. Getting the car off the Oakland Bay Bridge in rush hour traffic is what sent the heat into terminal shock What started the whole thing was the metal nipple from the throttle body heater hose broke off at the thermostat housing and pumped the system almost dry in no time. Breaking down on that bridge makes the 6' oclock news so my son coasted as best he could to the first breakdown area he could find. Anyway it gives me a chance to replace the rest of those hard access cooling hoses I skipped last Summer.