An Engine Rebuild Thread for Beginners

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Snowmark
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Re: An Engine Rebuild Thread for Beginners

Post by Snowmark »

Section 5: Lapping the Valves

From Wikipedia: Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them.

Definition of ‘Lap’ (about the 5th one down after running or cats drinking milk...) - Polish metal, glass or a gem.

As far as I can work out the idea is to ‘polish’ the valve and where it sits in the head (the valve seat....I think) so that the two faces mate together and therefore seal effectively. I’ve also read that a good contact with the two surfaces helps with heat dissipation between the valve and the head which probably improves something.

Tools Required: Power drill, Grinding Paste, Oil
Image

Grinding Paste bought from eBay for approximately £5 I think......it was a while ago. You will see that the container has two ends - coarse and fine.

Firstly, I had to remind myself of the correct numbering sequence for the valves and that the Inlet Valves were the larger ones, exhaust the smaller ones.

Image

I then had to select the correct head for cylinder 1 & 2 - mine are easily distinguished as the left hand head (3&4) has the screw still in that holds the oil cooler pipe work. Then, from my clearly labelled bags, take the valves for cylinder one. Apply some coarse grinding paste to the rim of the valve, dip the shaft in oil and drop it through the valve guide sleeve into the head.

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Where the shaft of the valve emerges then lightly attach the drill chuck and tighten.

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Pulling the drill towards myself so that the valve sat in the valve seat then apply some very slow rotations of the drill. I did two rotations clockwise then reversed the drill and two anti-clockwise. This process was repeated......very slowly until I could feel that the coarseness of the paste had gone - the ‘gritty-ness’ gets less and the drill rotates more smoothly.

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Remove the drill and the valve from the head, wipe up the excess coarse paste and apply a thin layer of fine paste and repeat the process.

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As you can see, this leaves a clear band round the valve where this has ‘polished’ against the head, hopefully improving the contact between the two surfaces.

One point to note is Cleanliness - I was extremely careful to only use my left hand to apply the grinding paste and then my right for handling the valve stems etc. You do not want to get and abrasive paste anywhere it shouldn’t be. I will completely clean the heads and valves before reassembling, however I didn’t see the point in making that process harder than it needs to be (frankly, there’s already enough cleaning to deal with).

Cleaning the valve up shows a clear ring around the valve that I measured.....don’t really know why but had read somewhere that it should be about 2mm.

ImageImage

The picture above shows the Exhaust Valve for cylinder one. The lapped ring was 3mm. The Inlet Valve for cylinder one had a 2.2mm ring - I have absolutely no idea if this is significant or not and shall await incoming wisdom.




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Snowmark
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Re: An Engine Rebuild Thread for Beginners

Post by Snowmark »

T25Convert wrote:Good to hear you are back at it!

Personally I would replace all of the valves (or at least the exhaust valves as a minimum).

They have a very hard life and are prone to failure and aren’t that expensive (around £80 all in).

Saves you some cleaning time too!!

I don’t know if the difference in ring width is pointing towards replacing the exhaust valves (I have read similar advice on other rebuild threads). Anyway, thought I’d continue as I hoped the process would make the decision clearer.......and it did, I think.

On closer inspection of the exhaust valve for Cylinder 1 showed a portion that would not shine

Image

You might just make out the black section that no amount of lapping would shift. I continued on and when it came to the exhaust valve for Cylinder 2 then this didn’t seem to sit as snuggly into the head and didn’t give the same dull ring when lapped

Image

Not the clearest photo but the edge remained shined and unlike the other 3 I had done previously. The combination of this and the mark on the one from Cylinder 1 is leading me towards a new set of exhaust valves.

A few pictures of the heads once lapped.

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Versus one before lapping
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No doubt the eagle eyed (and much more experienced than me) have spotted the cracks between the valve seats - I literally only noticed today when reviewing the pictures before writing this.

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I have read that these are quite common, question is, should I be worried about them? There appears to be a single crack, one on each head for one cylinder on each. Again, advice and consoling pats on the back gratefully received.


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T25Convert
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Re: An Engine Rebuild Thread for Beginners

Post by T25Convert »

I think that the bit that won't shine is burnt - poor seal between valve and head lets hot gas and explosion out, burns valve, worse seal, less contact means more heat in valve, more burn and repeat. Slowly valve gets worse until you loose compression.

Small crack there is normally fine, just see if you can see where it stops.... Does it affect sealing area for the valves?
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Re: An Engine Rebuild Thread for Beginners

Post by itchyfeet »

T25Convert wrote:I think that the bit that won't shine is burnt - poor seal between valve and head lets hot gas and explosion out, burns valve, worse seal, less contact means more heat in valve, more burn and repeat. Slowly valve gets worse until you loose compression

agreed, need another valve.
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Re: An Engine Rebuild Thread for Beginners

Post by 937carrera »

How do you guys suggest that he makes the diagnosis between burnt valve and worn valve seat ?
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Re: An Engine Rebuild Thread for Beginners

Post by Cobra88 »

If the gap is on the valve its burnt valve
If the gap is on the seat burnt seat or am i missing something ?..

I would recut those seats anyway as contacts are much too wide for me

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itchyfeet
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Re: An Engine Rebuild Thread for Beginners

Post by itchyfeet »

Cobra88 wrote:If the gap is on the valve its burnt valve
If the gap is on the seat burnt seat or am i missing something ?..

Rick

Thats my view.

How does a ameteur mechanic recut valve seats?
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Re: An Engine Rebuild Thread for Beginners

Post by 937carrera »

itchyfeet wrote: How does a ameteur mechanic recut valve seats?

They don't, they get them sent away to a machine shop, but they need to make the correct diagnosis first to know whether the issue is the valve seat or the valve itself.

I'm not disagreeing with the view that it is likely that the valve may be burnt, and it may be convenient to simply get a new valve, but the new valve needs to be lapped in and checked too.

For the OP: There are actually three angles cut on the valve and the valve seat, so all three of those need to be correct for best results. And this may be old school but I would not use an electric drill in "pull" mode to lap a valve in. I prefer to use the old wooden stick with sucker on the face of the valve in "push" mode . This also avoids potential damage to the stem of the valve by the chuck. I'm glad you are back on this :)
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Re: An Engine Rebuild Thread for Beginners

Post by tobydog »

itchyfeet wrote: How does a ameteur mechanic recut valve seats?

With difficulty :wink:
Also, check the valve guides for wear.
Last edited by tobydog on 19 Nov 2019, 09:25, edited 1 time in total.
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Re: An Engine Rebuild Thread for Beginners

Post by oilrag »

itchyfeet wrote:
Cobra88 wrote:If the gap is on the valve its burnt valve
If the gap is on the seat burnt seat or am i missing something ?..

Rick

Thats my view.

How does a ameteur mechanic recut valve seats?

An amatuer can buy a machine like my SERDI ,for around £40k or more ,or perhaps a Neway circa 70k,or take it to a local machine shop like mine , and have them done professionally and properly for around £30 per head on a wbx.
I would recommend new valves as a matter of course if having seats recut,a no brainer if you can afford a gasket set then buy the valves.
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Re: An Engine Rebuild Thread for Beginners

Post by 937carrera »

Looking back I see you asked about the seat dimensions: here's the info, note the comment in the Caution box
Attachments
valves.jpg
valves.jpg (55.89 KiB) Viewed 873 times
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Re: An Engine Rebuild Thread for Beginners

Post by sarran1955 »

Hello,

My 2€ worth....

New exhaust valves and guides is a minimum on a VW head overhaul.. :wink:

This is an air-cooled, but it is all the same work...

https://www.youtube.com/watch?v=iGolJzpQVr4" onclick="window.open(this.href);return false;

Part 1 deals with valve guides, part 2 deals with lapping...

As I say in the video, using a 'dob stick' is all very well, but when you have 4 stainless inlet valves to do, there are easier ways..

Too old to struggle.... :roll:

Keep up the good work, a good photo is worth a thousand words.. :)

Cordialement,

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Re: An Engine Rebuild Thread for Beginners

Post by itchyfeet »

I hear what you are all saying but this is a beginners thread.
a long as valve guides are in spec leave them.
valve seats out of spec are not a big issue in my experience.
it may not do 100,000 miles like this but most of us do 2000 a year camping and so thay will be fine. Just replace the bad valve.

keep it realistic of these engines won't get fixed


Heads are replaceable items and these have plenty of life left with a tickle.
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Re: An Engine Rebuild Thread for Beginners

Post by 937carrera »

That's a fair enough comment........ as the reason for this rebuild is because it's a waterleaker, not due to any major running issues
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Re: An Engine Rebuild Thread for Beginners

Post by T25Convert »

itchyfeet wrote:I hear what you are all saying but this is a beginners thread.
a long as valve guides are in spec leave them.
valve seats out of spec are not a big issue in my experience.
it may not do 100,000 miles like this but most of us do 2000 a year camping and so thay will be fine. Just replace the bad valve.

keep it realistic of these engines won't get fixed


Heads are replaceable items and these have plenty of life left with a tickle.

Comes back to what you want from it all really.

My view with mine was it was a bit tired and asthmatic. New valves, rings and bearing shells and it was much better again. Might not have been 100% within spec, but it was a massive improvement over what I had before.

If it were me I'd check the head crack is not too severe, otherwise the current vales and seats become irrelevant anyway as its new head time.

If cracks okay, then check valve guide wear (there is a handy guide in the Haynes to do this crudely), then go with new valves and see if they lap in. You could go for a valve seat re-cut, but you need to find a machine shop you can trust, assuming that oilrag above is not local to you.
RIP - George - 1.9DG '85 AutoSleeper Trident - rusted away

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