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Carrot

Original Poster:

5,930 posts

87 months

[news] 
Tuesday 22nd March 2011 quote quote all
Posted this on GG - was advised to post it here...

Been doing a bit of research into valve lapping in preparation for my engine head job in a couple of weeks.

Lot of forums and debate on this one - there are a lot of people some "allegedly" professional engineers etc, but like "all PH's are in the top 1% of earners and are in finance", some are suspicious...

Anyhow, the debate is generally that in modern engines, it is not advisable to bother lapping the valves, as they are usually machined with such high quality materials these days, that one could potentially cause more harm than good. There are some that say leave it well alone unless there is obviously a problem, and some that say do it anyway, every time you have the head off as it can never do any harm.

Bearing in mind that, in this case, the engine is fairly new (1999), but is a cheap piece of st...

Interesting in hearing thoughts of those who have been in precision engineering / know about it.

tristancliffe

318 posts

98 months

[news] 
Tuesday 22nd March 2011 quote quote all
Personally I would do a little bit of lapping with a fine paste when fitting new valves or new seats. I wouldn't bother if re-using valves with seats (assuming the valves go back in the same place) if there was no problem there beforehand.

Everything is made to a tolerance, and the surface finish of valve and seat might not be perfect for sealing. Lapping, with a suitably fine paste, can't make that worse, so can only help.

duncancallum

774 posts

63 months

[news] 
Tuesday 22nd March 2011 quote quote all
I would lap them in.

Be aware that if its shimmed followers you will have to reset the valve clearences.

Duncan

Pumaracing

1,415 posts

92 months

[news] 
Tuesday 22nd March 2011 quote quote all
Valve lapping is quite a curious engineering process which of necessity I've studied in detail over the years. Prolonged lapping, especially with coarse paste, actually makes the seating surfaces of the valve and head insert concave so the two only make contact on their inner and outer edges. This is obviously very bad for heat dissipation as well as airflow. I can see the effects of heavy lapping very clearly on my valve refacing machine or head seat cutting machine as the grinding wheel or cutter makes initial contact with the concave faces and only touches them along the edges.

You can also easily see the concavity on a valve after prolonged lapping by putting a high quality straight edge across the seat and holding it up to the light. Try it on an old head some time. The mechanism at work here is that the paste on the inner and outer edges of the contact area quickly squeezes out as you start lapping leaving most of the abrasive action taking place along only the centre line of the seat. So lapping can't restore a badly cut or badly worn seat properly as they used to think in't olden days. It might have sufficed for a 30 bhp per litre truck engine from the 1940s but is not what you want for today's high performance machines generating much more heat which needs dissipating properly through surfaces in perfect contact with each other.

However a very light lap with fine paste for just 10 seconds or so to check that the valve and seat are truly concentric and with no high or low spots is a good idea and not a problem. If there isn't an even grey contact area all round both valve and head seat after that then it's probably time for remedial machining rather than further lapping.

For many years now I've used special diamond grit based paste rather than the normal carborundum grit paste you get in little tins with two lids for coarse and fine at each end from car accessory shops. It's horribly expensive but it has a completely different abrasive action which I can't really describe but it's much nicer. Being so hard and sharp, diamond grit abrades the surfaces really fast before the paste has had time to squeeze out and the grit particles don't break down into powder immediately like carborundum does so you don't get the concavity and it takes less time to check that the surfaces are making good contact. However the fine paste from those little tins is perfectly ok for general use. The coarse paste is a definite no no.

I used to have a customer in the early 90s for whom I did the CVH heads for his race car along with many other people's. They generally got a quick refurbish mid season and it took me a while to work out why every time I recut the seat on one of his valves (but no one else's) they were badly concave and only touching the grinding wheel on the inner and outer edges. After speaking to him it turned out that every time I sent a finished head back, despite my own quick lapping to check the seats were perfect he'd stand there for half an hour grinding them in further before assembling everything thinking he was contributing to the general cause and doing some good when in fact he was just buggering up my delicate machining work. After actually showing him what his tinkering had been doing to the concavity of the seats there was one of those "oh sh*t what have I done?" expressions on his face and he left things well alone after that.

In OE engine production valve seats are never lapped which would be horribly time consuming to do on every engine but of course there are constant quality control checks being carried out to make sure the valve and head seat surfaces are being machined to a perfect specification. They also sometimes use a very slightly different angle on the seat in the head and the seat on the valve, maybe half a degree or so, to make the two components "hammer" into full contact after the engine is first started. Not my idea of perfection engineering really. Unfortunately you can't just assume that Joe Bloggs your general engine reconditioner is even capable of cutting proper valve seats which most aren't in my experience so checking them with a quick lap is essential. The much vaunted Serdi machine which is the popular choice these days is a bugger for cutting non concentric seats in the head if there's even a fraction of a thou of valve guide wear. I prefer seat cutting systems with fixed rather than rotating pilots like the Sunnen system.

Max_Torque

7,717 posts

102 months

[news] 
Tuesday 22nd March 2011 quote quote all
Assuming you have some eyes, and they work, then just look at the seat area on the valve and the valve seat, and you can tell how "knackered" the seats are. You need a nice consistent line, approx 2mm thick around the full circuference of the seat and valve. Any excessive piting is bad (leakage path, and also reduced heat transfer from valve to seat (which is the greatest cooling path for the valve).

if your seats look like this:



it's probably worth re-machining them

(if you have hydraulic tappets, you won't need to re-set valve clearances)

Edited by Max_Torque on Tuesday 22 March 15:35

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Carrot

Original Poster:

5,930 posts

87 months

[news] 
Tuesday 22nd March 2011 quote quote all
Wow an amazing amount of information so far - excellent!

They tappets are adjustable from looking at a photo of another matiz engine without the cam cover on, so I don't think they are hydraulic - I have the feeler gague at the ready as I am going to check them anyway before finishing off.

Pumaracing

1,415 posts

92 months

[news] 
Tuesday 22nd March 2011 quote quote all
Any chance Mr Max you could stop posting pics so big that the page widens so much the text scrolls off to the side of what my poxy little 15" monitor can display and I can't read all of it? Page 1 of the "how good are typical cylinder heads" thread is now buggered as is this one. We don't all have 30" flat screen monitors I'm afraid and this website doesn't have an auto adjust width to fit your screen facility sadly.

Max_Torque

7,717 posts

102 months

[news] 
Tuesday 22nd March 2011 quote quote all
hey, i've just got a laptop, with a normal screen wobble

(tip, just use the lower scroll bar to move the page sideways........... ;-)

Pumaracing

1,415 posts

92 months

[news] 
Wednesday 23rd March 2011 quote quote all
Max_Torque said:
hey, i've just got a laptop, with a normal screen :wobble
So you're buggering up the readability of the threads for yourself as well as other people? How about posting a thumbnail or just a link or taking the time to compress the image a bit so it fits on the page and doesn't cause other people an unnecessary problem? Anyway luckily it seems Firefox has an option to right click on images and block everything from that source so I've managed to get rid of them all.

Carrot

Original Poster:

5,930 posts

87 months

[news] 
Wednesday 23rd March 2011 quote quote all
Max_Torque said:
Assuming you have some eyes, and they work, then just look at the seat area on the valve and the valve seat, and you can tell how "knackered" the seats are. You need a nice consistent line, approx 2mm thick around the full circuference of the seat and valve. Any excessive piting is bad (leakage path, and also reduced heat transfer from valve to seat (which is the greatest cooling path for the valve).

if your seats look like this:



it's probably worth re-machining them

(if you have hydraulic tappets, you won't need to re-set valve clearances)

Edited by Max_Torque on Tuesday 22 March 15:35
This is exactly the sort of answer I was looking for - very helpful! thumbup Will have a proper looksee when the head is off.

Pumaracing

1,415 posts

92 months

[news] 
Wednesday 23rd March 2011 quote quote all
Thanks. I checked that and it is set to "true". There may be something else. This website puts Flash advertisements on the pages from time to time and in both threads that have scrolled off screen the adverts are alongside Max's photos which is either a strange coincidence or part of the cause of the problem. I have Flashblock installed so I just get a blank box with a capital F in the middle of it but it does extend off the side of the screen. Ah well.

porka911t

66 posts

90 months

[news] 
Wednesday 23rd March 2011 quote quote all
Cat amongst the pigeons,

If you have your valves and seats cut on a good machine that has not cut 2 million valves you will achieve a perfect vacuum on the inlet and exhaust ports. You can lap all day and these machines will do a perfect job in 15 minutes. Do not waste your time lapping valves.

Imagine the Porsche factory with 100s of guys standing there lapping valves!! I DONT THINK SO.

rev-erend

18,233 posts

169 months

[news] 
Thursday 24th March 2011 quote quote all
porka911t said:
Cat amongst the pigeons,

If you have your valves and seats cut on a good machine that has not cut 2 million valves you will achieve a perfect vacuum on the inlet and exhaust ports. You can lap all day and these machines will do a perfect job in 15 minutes. Do not waste your time lapping valves.

Imagine the Porsche factory with 100s of guys standing there lapping valves!! I DONT THINK SO.
Those guys are too busy repairing RMS. biggrin

Pumaracing

1,415 posts

92 months

[news] 
Friday 25th March 2011 quote quote all
porka911t said:
Cat amongst the pigeons,

If you have your valves and seats cut on a good machine that has not cut 2 million valves you will achieve a perfect vacuum on the inlet and exhaust ports. You can lap all day and these machines will do a perfect job in 15 minutes. Do not waste your time lapping valves.

Imagine the Porsche factory with 100s of guys standing there lapping valves!! I DONT THINK SO.
Drastically wrong. You can still achieve a perfect vacuum as long as the valve and seat make some sort of single line contact along their entire circumference. That doesn't tell you they are both at the same angle and properly matching each other across their full width. I've seen horribly mucked up valve and seat work pull good vacuum readings despite the contact area being a tiny fraction of a mm wide. The seat would then hammer in or burn out in short order.

Lapping or blueing is the only way to check a valve and seat are actually matching each other. Vacuum is a waste of time.

Yuxi

396 posts

74 months

[news] 
Friday 25th March 2011 quote quote all
Valve seat run out (to guide bore) and roundness is still the hardest thing to machine on a cylinder head, even in high volume production. The hardness of the seats means more load is put into the machine spindle bearings and axis bearings, especialy if seats are cut on a horozontal machining centre, which nowadays most are.

However, on most current production engines bolting the head to the block distorts the valve seats way out of cylinder head machining tolerances, mainly due to the deck face of the head bending round the bead in the head gasket and the head bolt loads going through the head.

Every current engine I have worked on has valve/seat angles such that contact is always made at one point, full contact across the width of the seat would mean at least one componet was out of tolerance.
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