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Gtir Motorsport club » General Discusion » Gtir related Discussion » Valve Grinding/lapping

Valve Grinding/lapping

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1 Valve Grinding/lapping on 1st March 2016, 9:30 pm

burrows1980

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Whats peoples views on the above?

Ive always ground the valves with course paste then used the fine, but I decided to google it tonight for some reason and come across some horror storys about doing it!  By grinding the valve it can cause the surface to become concave due to the grinding paste moving to the outside of the valve but leaving a line in the center, hence concave.

I removed all my valves last night, the seats in the head look fine, inlet valves not a mark as you would expect but the exhaust valves have took a battering.  I cant afford to buy 8 new valves but thinking of just a light lap with the fine compound.  I did a comp test a few years ago before starting the project and all was fine, so hopefully only need a light lap.

Obviously Ive gotta be careful of too much grinding will alter the shim gaps and prove equally as expensive to replace them than gettin new valves....

I read this on Piston Head Forum:

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.

2 Re: Valve Grinding/lapping on 2nd March 2016, 12:34 am

jay1


iv always used fine paste to lap in valves, never had any issues doing it that way

3 Re: Valve Grinding/lapping on 2nd March 2016, 7:47 am

Mr B

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gtir technician
gtir technician
Main thing is not over do it, fine paste with short lapping session good if seats good enough.
If looks unlikely recover exhaust easily with lapping then getting them all recut by an above average engine machine shop for way better finished result.
Have a try on an exhaust valve & see how goes, if looks hard get nice finish consider machine shop. Precision shims in oz do shims if nissan too slow/expensive, bob has selection too, also the shim blade discs in old shim caps work well if done with thought/care & allows precise clearances which far better than running excessive clearance.

4 Re: Valve Grinding/lapping on 2nd March 2016, 6:44 pm

burrows1980

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Cheers chaps, will have a look on Friday to see how bad they are. Funnily enough I was looking at precision shims last night from a post you put up a while ago, worth bearing in mind.

When I started the bodywork I bought a Sealey RE92/31 joggler/punch tool as it has the 6mm punch for the shims!! Thought ahead all them years ago Laughing

5 Re: Valve Grinding/lapping on 9th March 2016, 1:21 pm

burrows1980

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Probably the best condition valves and seats ive ever lapped in! Really good condition, didnt take long at all.

Cleaned the carbon off the valves in the lathe first, careful not to bend or mark the stem:

https://www.youtube.com/watch?v=-6Cf9dhckcA

Then a quick lap, although this one took a bit longer than the rest:

https://www.youtube.com/watch?v=zGMH3J91so8

Just need to check the shim clearances now.

6 Re: Valve Grinding/lapping on 9th March 2016, 5:07 pm

nomad

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Admin
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Handy having a lathe .... ive done it with a corded drill in a vice ....top work anyway Wink


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