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Old 25-05-2008   #1
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Pistons?

i was wondering if the pistons are mean't to hit the valves on the cylinder head? because i'm in the middle of changing my head gasket and noticed ring imprints on the top of my pistons? want to know before i go buying the wrong stuff for a different problem.

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Shannon
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Old 25-05-2008   #2
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Re: Pistons?

1108 (at least) has small cut outs for the valves. But, tell us more. What engine, what problems, pics?
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Old 25-05-2008   #3
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Re: Pistons?

Yea it's an 1108, must just be the cut outs. i'll go take some pics now. Also if there is a lack of compression is it always the head gasket? because i took the whole engine down and there was no HG fail but there was serious lack of compression. could it be the spark plugs?

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Old 25-05-2008   #4
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Re: Pistons?

Could be valves. Unlikely to be spark plugs unless the thread is completely shot. For future reference, always get a compression test done before pulling a head. Anyway, makes sense to skim the head, get the seats re cut, new valve guide oil seals, re-shim the valves.

If the compresssion is low after that, it can only be the rings/pistons.
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Old 25-05-2008   #5
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Re: Pistons?

I did the compression test before hand, thats why i thought it was the HG but it doesn't seem to bad. i'm trying to get the head skimmed sometime this week all going well.

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Old 25-05-2008   #6
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Re: Pistons?












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Old 25-05-2008   #7
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Re: Pistons?

If there's been contact, you'll always see a bright ring. Can't tell a lot from the head (except no rot) and gasket.

Generally if the HG has gone you'll see low comp on one cylinder or two adjoining ones.
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Old 25-05-2008   #8
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Re: Pistons?

What actually were the results on all of the cylinders then?

Did you run the test with all the plugs out, throttle open, and a warm engine?

I had low compression on all my cylinder. For me the lands were broken on the pistons. Thought thats not very common, it could be the rings.

How many miles has the engine done?

Kristian
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Old 25-05-2008   #9
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Re: Pistons?

There are no witness marks on the valves to suggest contact with the pistons.

How are the bores?

On the engine we stripped to become my race engine, the bores were barrel shaped. Quite tight at TDC on each piston but very wobbly when in the middle of its travel, and this was an engine with no known overheating problems.

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Old 25-05-2008   #10
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Re: Pistons?

the throttle wasn't open but the engine was well warmed and all the plugs were out. the results i got were very different:

140 psi
105 psi
124 psi
115 psi

but i found recently that the timing was off on the car. Who ever owned the car before me fitted a timing belt and the timings was off as i found today when inspecting it. When taking the head off, i only lined up the crankshaft pully toi TDC but when i checked earlier the camshaft pulley wasn't on TDC when the crankshaft was. so maybe this could be to do with off timing? i'm not sure

the bores are fine. doesn't seem to be any scrape or scratches for the pistons.

Thanks

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Old 26-05-2008   #11
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Re: Pistons?

Timing shouldn't affect the compression readings. Difficult call now it's in bits.

I think (depending on how broke I was/how much time I had) I'd go through the head carefully, the usual stick everything from one cylinder in one freezer bag rountine, check for guide/valve wear, etc. and then skim, cut in valves, re-shim. That way at least you know you have a good head.

Unless it has done say 80k miles plus, in which case it's unlikely not to need a rebore.
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Old 26-05-2008   #12
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Re: Pistons?

If the timing is a bit out, could it not leave the inlet or exhaust valves open longer on the compression stoke, giving a low reading? Of course, if it was this far out, it would have run like a bag of nails as well.

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Old 26-05-2008   #13
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Re: Pistons?

Maybe (that's a new experiment for you, Kris!), but you'd still expect them to be out the same amount, wouldn't you?
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Old 26-05-2008   #14
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Re: Pistons?

It would have been handy to see the results of a wet versus dry test, but I think that one and three are close enough, its two and four where the problems lie.

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