. . . The Skimming Thing

joolie · Post by **joolie** » Sat Apr 28, 2007 11:00 pm

. . . so EVERYONE recomends a head skim when it comes off, but up here (Sweeden) thats not so strait forward, one thing is it will cost a shed load to get it skimmed., everythings more expensive here.

So this ones going to go on without it, and the other thing is the thickest metal rimmed gasket I could find at a decent price (£25 instead of £47) was a 2 notch, it was a 2 notch on the head before, but this is a different cylinder head. Is there anyway to tell its been skimmed before? any teltale signs?

DLM · Post by **DLM** » Sat Apr 28, 2007 11:24 pm

A couple of questions, and a couple of recommendations.

Q1: Is this is a diesel/turbodiesel, or 8v/16v petrol head?

Q2: Have you taken the old head off? Was there definite damage to explain a head gasket failure? (e.g. cracks between cylinders, cracks to pre-combustion chamber(s) on a diesel)

Q3: Did you use new head bolts on the previous head, and carry out the full tightening sequence as per the instructions?

R1: Get the head tested to see if it IS straight - after all, that's the way to decide whether a skim is needed or not. It may be expensive, but otherwise you could spend ma lot more if you put a bent head onto a good block.

R2: If you can obtain an engine that you know is good (perhaps from a Peugot if it's hard to obtain the right Citroen engine in Sweden), why not do an engine swap rather than go through the head gasket torture?

joolie · Post by **joolie** » Sun Apr 29, 2007 12:20 am

Its a TZD.

Swapping the engine is not an option, I dont have the equipment or space (l live in a flat).

FInding the A8A cylinderhead was a nightmare, took more than 9 months, and had to be shipped from the UK anyway! There just aint the cars up here, petrol is most popular, diesel is unheard of almost.

The repair is due to a timing belt blunder, and im trying to do it cheap!

How does one test if the head IS flat or warped. . . maybe theres a garage local I can ask, do they have testing equipment usually?

Post by **docchevron** » Sun Apr 29, 2007 1:07 am

The easiest way to check for flatness is with a good straight edge.

If your satisfied the head is flat, then ok, give it a go, also check the block surface, and clean it up with emery on a block before fitting the head.

A 2notch gasket may be ok, you'll have to check the protrusion of the valves in the the replacement head you have..
If it's on the excessive side, you'll HAVE to get a three notch gasket, or recut the valve seats.
Which would mean re-shimming the tappets.....

joolie · Post by **joolie** » Sun Apr 29, 2007 9:50 am

I cleaned both surfaces with light emery, and checked with a straight edge diagonal and flat on, thats why im wondering why I need it skimmed.

I check the valve protrusion and pistons today.

Is the entire purpose of the gasket, to keep the valves clear of the pistons?

I rekon this is my problem with some of these engine parts, Im not knowing EXACTLY the function of gaskets, and whatnots.

DLM · Post by **DLM** » Sun Apr 29, 2007 10:27 am

Joolie - I'm glad to see the Doc has responded to you as he'll be able to give you better engineering advice. I do know what it's like to live in a flat - I live 10 floors up from my BX!

jeremy · Post by **jeremy** » Sun Apr 29, 2007 12:54 pm

The purpose of the gasket is to take up irregularities in the head and the block and make a good seal. The problem is that this joing forms part of the combustion chamber and is subjected to high pressure and temperature at that point, hot water under moderate pressure around the waterways, high pressure oil where the oil feed to to the head passes through and anso oil drainage.

Its ability to absorb irregularities is really confined to machining marks. However no machining is really absolutely flat and the gasket will compensate for minor problems here - but I do mean minor. (0.25 thou??)

It is necessary to have a gasket of sufficient thickness to ensure there is a clearance between the valves and the pistons. So if the head has been skimmed a thicker gasket will be required.

A head is often skimmed to clean its face up nicely to ensure a good seal. The skim will also show up any inaccuracies (as they will miss the cutter and remain uncut). As you have found cleaning the head face is a long and tedious job and so a professional will have it skimmed to save him work and as a guarantee that the thing is flat. He probably does not possess the kit for checking its properly flat anyway - so by the time its been taken to someone who has the skim makes sense.

Post by **docchevron** » Mon Apr 30, 2007 12:54 am

If your able to get one, I'd play safe and stick and three notch gasket in mate.

I hope the emery paper you used on the head was VERY fine, alloy being soft is prone to "picking up" and even minor marks can cause major problems.

Thats why I've trained the machinist I use to ALWAYS use a new proper grinding wheel on any alloy head I take to him, milling marks ruin the head on XUD's.

I know some machine shops in Bristol that cut heads like that and they say "yeah we leave the milling scores in so it grips the gasket better". Bullsh!t does it, it just means you'll be doing the job again in three months when it fails..

Anyway, if your happy it's flat and free of scoring / milling marks, then that's good enough, sometimes skimming a head is not always the best idea unless you have a REALLY good, known machine shop nearby.

It's worth checking the top of the block for flatness too, It's not unknown to get raised areas around the water jackets or even between the cylinders and around the head bolt holes.

If you're still happy it's flat, ok, go for it. BUT, use NEW head bolts!

And agin I reiterate, if the valve protrusion is even the slightest bit borderline on tolerances, use a three notch gasket, better to be safe than doing the job again!