Perhaps this belongs in "Technical"? Perhaps not.
I see, in various threads about head gaskets, that continual reference is made to "skimming" (or grinding) cylinder heads when gaskets are replaced. Why? This is hardly often necessary. Has it become simply "the routine"?
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HGF is sometimes caused by a warped cylinder head, perhaps resulting from overheating if the coolant level has been allowed to fall. Sometimes too bits of gasket or sealant adhere very tightly to the mating faces of head and block, and a light skim will clean them off.
However the great unacknowledged benefit is a slight increase in the compression ratio of the engine. Once in proper tune, it should deliver a couple of extra horses.
:o}
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modern gaskets are multi layered and ultra thin and nearly everything is made of alloy
so the days of a quick sand down of the block face a skim of seedless jam and the old type headgasket fitted are unfortunately long gone
skim or waste your money is the new motto
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modern gaskets are multi layered and ultra thin and nearly everything is made of alloy
I don't know what relehavce that has, unless you're saying it's difficult to get the mating surfaces clean.
Head bolts exert enormous force, and will easily bend a head to match the block, giving a tight seal (unless the head's cracked, or has a "step" in it, in which case it's unfit for further use anyway).
so the days of a quick sand down of the block face a skim of seedless jam and the old type headgasket fitted are unfortunately long gone
I have never, ever, ever - on any engine, motorbike or car - used jointing compound, nor (except on machines with solid copper head gaskets) re-used an old one. It's *very* poor procedure indeed. I'm surprised you did this, even in "the old days".
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if you are working on a rattle corsa engine these run so hot that the heads bend like a banana if the h/g goes
obviously i know nothing but just keep a spare engine in stock just in case i get my leg lifted at the sales
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i6.photobucket.com/albums/y246/smartiesx3/10-02-10...g
i6.photobucket.com/albums/y246/smartiesx3/10-02-10...g
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>>> I have never, ever, ever - on any engine, motorbike or car - used jointing compound, nor (except on machines with solid copper head gaskets) re-used an old one. It's *very* poor procedure indeed. I'm surprised you did this, even in "the old days". <<<
Depends on the engine. On the aircooled Suzuki triples, the cylinder head is a single casting across three individual, separately mounted barrels. The barrels bolt to the cases, and the head mounts on short studs screwed into the barrels. Head bolt torque is 26 lb-ft.
Even though the individual head gaskets are a compressible aluminium sandwich with a flame ring, the combination of relatively high revs and the fact the studs DON'T link crankcase to head allow the head joints to move slightly relative to the barrels, even with the inherent smoothness of a 120-degree 2-stroke triple.
If you don't spray the gaskets with Permatex Copper or give them a thin trace of red Hylomar, you get a compression leak very quickly..
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I asked the local mobile mechanic about this when I had HGF on my mk3 Cavalier at 184,000 miles. He said he always proposed doing a skim to the customer in HGF cases on the basis that it only added about 50 quid to the bill, but he'd had a couple of cases where a non skimmed head had started going again 10,000 miles or so after repair.
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Erm
a no of years ago I cooked the 1100 cc Polo, hot Aug day heavy traffic etc etc
So I pulled the head and faithfully following the Haynes manual, fitted a new gasket
It did not do very few miles before coolant loss.
A mate said, you did get that head skimmed, didnt you?
erm no I muttered/confessed
So this time I had it skimmed, and repeated the process, again doing it myself
Voila a result.
Make of that what you will
Basically I was told that if the aluminium head had got a "run of heating".... it WOULD be warped.
Always
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in the old days ie: bmc minis , anglias etc that had a cast iron head , they had to get pritty warm to be damaged , with the advent of alloy heads , and a prime example was the hillman imp , the alloy does distort when overheated , and in most/a lot of cases does need skimming,
this has nothing to do with removing bits of old gaskets?
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I agree, Alloy head...skim it. Not worth the labour of putting it all back together without.
Ted
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We used to give the head a couple of light passes on the skimmer as a matter of course.
If the stone just touched it all the way across, you knew the face was pretty true.
Cast iron heads didn't tend to warp, but some alloy ones did.
If the combustion chamber was in the head, you had plenty of 'meat' on the head face to work with on the skimmer.
If the valves were flush with the head face, it didn't take many passes on the skimmer before you were starting to grind away the valve seats.
One head was so badly warped, we skimmed off as much as we dared and then refitted it with two headgaskets.
Should have scrapped it, but the customer had no money and was begging us to get him going again as cheaply as possible.
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This reminds me of a BMC Austin 1100 head job my mate and I did a long time ago.
We were "Doing Up" this car and along with various other jobs, we were replacing the head gasket, so off we went to the local reconditioning shop, who we knew quite well and asked them to skim the head.
A day or so later, we picked it up and duly proceeded to refit it along with a new gasket. All cooling pipes fitted and we pour some coolant into the radiator only to see it weeping out from the block/head joint.
So, puzzled why this should be, we took it off again to examine the head gasket which we assume was faulty. We fitted another new one, bolted it all back up again and yes you guessed it, water seeping out yet again.Now getting slightly pee'd off with the job.
Head off again and back to the engineering shop to see if they had any ideas. After an examination by the boss, he called over the young lad who had done the grinding. Did you check to see if the grinder was grinding flat he says.
Turns out the apprentice had mounted the head on the machine and somehow not got the head flat, so in effect had ground a wedge off the head so when we torqued the head down, the back face went down first, leaving the front edge to allow water to seep out.
Another lesson learnt !
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Another lesson learnt !
Ay chihuahua! Did anyone measure the compression ratio after the further necessary flat skim?
'Faster than a Lotus Cortina this one mate.'
:o}
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...turns out the apprentice had mounted the head on the machine and somehow not got the head flat...
The machine we had was an engineering bench with a rotating stone mounted in it.
The adjustment was by moving the stone in its mounting using a finely threaded wheel.
I only ever used it under supervision, but I think the adjustment was only up and down, so with that particular machine it was possible to take too much off, but it shouldn't have been possible to take off a wedge.
One thing you had to do was adjust a guide to make sure the stone passed the full width of the head.
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I skim heads every day at my business in sussex and believe me the majority of what comes through my door deffinitely needs to be skimmed,there are just as many heads with corrosion problems causing hgf as there are failing through simply overheating for other reasons, the 16 valve vauxhall heads always blow between number four cylinder and the water jacket,fiat puntos are nearly always corroded in the same place on one of the end cylinders and k series rover heads nearly always have rings pressed into the face by the fire lip in the gasket when they get so hot they go soft, very rarely do i skim a head and think blimey that didnt need doing ,probably less than 1 in 10.
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When I had a Morris Minor MM 1952 side valve for 11 years in Zambia the head gasket failed about once every nine months. I usually found out in town when I tried the starter and there was thump as the piston hit the water in the affected cylinder. I always managed to start the engine by leaning on the starting handle until all of the water was pushed into the sump and then turning the handle like an idiot. The compressions were quite low and it wasn't too difficult. Eventually the engine would begin to fire on one cylinder, and then two, and I could start home. Three and then four cylinders cut in soon afterwards and we were sailing. Once home I parked up, and then next morning I would loosen the sump plug until all of the water had run out and then tighten it back up. Engine oil was then in very short supply and about £45.00 per gallon, so we didn't waste it. I always kept a spare gasket, so I was always quickly back on the road.
I did have the head skimmed once, but it didn't make much difference.
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