Hot Rod magazine , recent issue, has article on an engine problem partially caused by prior ill-advised dealership shop work. Apparently the head was removed to repair gasket leak, then (surprisingly) the top of the block was treated to a rotary grinder gadget to clean off the surface contaminants before replacing head & gasket. Grinding resulted in wavy edges at top of block , rather than what it should be, almost perfectly flat. Hard for me to understand how that could happen at dealership shop? Or is grinding the top of the block using a hand-held power tool a common thing when doing a head gasket job?
You have to use the proper cleaning disc when doing this type of repair.
Tester
Oh, interesting. So the hand-held rotary tool method might be ok to clean the top surface of the block, but have to be sure to use the correct grinding material. Thanks for the info.
It wouldn’t be the first time a factory manual was wrong though. I had my diesel a long time so many references. At any rate when installing and timing the injector pump the manual said to line up the timing marks. My guy at the Inde shop was also an olds mechanic.
Still didn’t run very well so I took it to diesel service in Mendoza, a fuel fledged diesel shop. They offset the timing marks and told me the manual was wrong. It never ran that well since new. When I got to replacing the pumps myself later on, I would always take it there again for timing.
The plastic bristle disc was developed by the aftermarket as a “safe” gasket cleaning method. Several years later manufactures stated to discontinue the use of these abrasive discs for several reasons, one is that they remove metal from the sealing surface. Time is money so some mechanics will continue using them.
Service bulletin below.
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Interesting?
And I’ve been using these discs for over 25 years.
And never had a come-back.
But I also make sure no contaminates enter the engine while and after I do the cleaning.
Tester
I am not a professional mechanic…strictly a DIYer working on my own personal vehicles, so “time is money” is not a concern. In fact, quite the opposite. I am not trying to make a profit, and since it’s my car, I want everything to be top-quality.
I would not be caught dead using any type of powered abrasive tool to clean a gasket mating surface, even one which is less critical, such as the thermostat or water pump mating surface. It goes without saying that proper cleaning of the head gasket mating surface is extremely critical.
When I do a head gasket, which I have done now on three different vehicles, I sand the engine block mating surface by hand. I have a precision sanding block, made of machined aluminum, which is far flatter than a cheap plastic sanding block that you might buy at Ace Hardware. I smear Vaseline around the piston rings to catch any abrasive material which might fall into the cylinders, and then gently sand uniformly to ensure that no area is sanded more than any other area. I start with 800-grit, then 1000-grit, then 1200-grit. This takes a lot of time, but it does result in a smooth, flat surface, which allows the new gasket to seal properly.
Of course, the amount of time required to do this type of work as a DIYer is much higher than what a professional mechanic would spend, or could be paid for. When I did the head gasket on my Daewoo, I probably spent more than 36 hours between taking the car apart, and putting it back together (though I did other work as well, such as changing the transmission fluid and filter). Since I plan to drive the car for many more years, the time and effort was well worth it. A professional mechanic might only be paid 8-10 hours to do this procedure, so he’s got to take shortcuts to allow him to complete the job in that amount of time…even if that means sacrificing overall quality.
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Do you use a precision straight edge and feeler gauge to measure the flatness of the head after doing all that sanding?
Tester
I have only replaced head gaskets twice. Once on the Continental flathead 6 in our 47 Fraser and the second time was on the polyhead 318 in my 61 Dodge Phoenix.
Neither job took me more than an afternoon and I did not try to make anything shiny, just flat and clean. If I could not shine light under a machinist level, it was good enough for me. Both jobs were successful.
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My preferred method is to use aerosol gasket remover. Spray, allow to sit for a bit, and then use single edge razor blades to peel it off. That’s always worked for me but to each their own.
Sounds like that dealership guy may have gotten a bit ham fisted with the grinder; just like some have done with a buffer on paint.
I am talking about sanding the engine block mating surface. If I am doing a head gasket, the head is going to be reconditioned by a professional machine shop, which means cleaning, testing for hairline cracks, resurfacing, valve re-grind and pressure testing, new valve stem seals, etc. I would not attempt a DIY resurfacing to save a few hundred bucks. I want a result which will last for 100,000 miles, not 10,000 miles.
Well, except for the car itself. 
Just my opinion so don’t get all upset, please.
Let me rephrase the question posted by @Tester .
Do you use a precision straight edge and feeler gauge to measure the flatness of the block mating surface after doing all that sanding?
After all this is just as important as the cylinder head mating surface?
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I never send a head into the machine shop unless it NEEDS to go into the machine shop.
I don’t waste money on unnecessary services.
Tester
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