zogster

Hey all, Happy New Year! Hope your 2018 is a good one.

So, it looks as if I need to replace my head gasket again (did a simple block test with the blue liquid bubbly thing... turned green-yellow which I understand is a pretty reliable indicator of exhaust gases leaking where they shouldn't)

I've done it before so the procedure itself is no big deal, but I have a couple of questions:

• How do I check whether the head and/or block need skimming? (Have skimmed the head each time before)
• The head has been skimmed at least twice before - at what point can no more metal be safely removed?
• Related to above, at what point does a thicker head gasket become necessary?

I think I've replaced the head gasket maybe three times in the past 8 years or so, which seems a lot. Car is an everyday driver though not driven every day, or particularly hard. Probably averages 5,000 miles per year, a little less right now. Am I just a bit unlucky? Or possibly not doing a very good job at replacing the gasket each time so that it's failing sooner than it should?

I'd just like to make sure that I do the job properly this time, and minimise the chance of having to do it again sooner than really necessary

Tom M'Guinn

Here is a page from the tech spec book that shows the limits for unevenness and machining. To check it, use a precision straight edge in various directions across the head (and deck) and see if a feeler gauge (or flashlight light) can slip through anywhere. If you've had the head skimmed each time you replaced the gasket, chances are you are out of spec or very close, so a thicker HG is probably a good idea. No real way to skim the block without a full rebuild, but worth checking just to know. Three HG's in 8 years is a lot. What is your state of tune and mods, and how much boost and octane do you run? If anything other than stock, a wideband and knock counter go a long way in preserving head gaskets. My guess is you are getting some knock under boost, which is leading to premature HG failure. People then start to change head gaskets and skim the head while chasing the problem, creating a viscous circle where you are actually increasing the compression ratio and the motor's tendency to knock.

I've never had much luck with those exhaust in coolant testers. What are you other symptoms? Surest sign of a bad gasket is a car that pressurizes the coolant tank under boost -- if you drive hard and it spits out coolant, yet drive easy and have no overflow, then chances are strong the HG is bad.

zogster

Standard tune, no mods, and running on regular unleaded UK fuel (I say regular, but probably 90% of the time I fill up with Shell V-Power)

I did the test because I'd been losing coolant and couldn't find a leak, simple as that. I did think I might have had a leak somewhere near the turbo, but the few spots of coolant that I saw that I suspected might have come from around there could also have come from the overflow tube I think. Not enough data...

Tom M'Guinn

Pull the overflow hose up and put it in a plastic soda bottle so you can monitor if anything is coming out. If the overflow volume corresponds to boosting, then it's probably a bad HG. Stock engine shouldn't be going through HGs that quickly, however. After skimming the head several times, your compression ratio is higher than stock, so I'd use the highest octane available every time. In the meantime, get a pressure tester and see if you have an external coolant leak. They are super easy to use and without doubt the very best way to know if/where you have leaks. This one works well on the 951:
https://www.tooltopia.com/otc-tools-...CABEgKvVPD_BwE

zogster

Yup, good idea, I really should have a pressure test kit by now. Will investigate further before I replace anything, but i'll be pleasantly surprised if it isn't the HG

Tom M'Guinn

zogster

I never knew I was a buddhist! Though as a Dudeist, I'm already part-way there of course.