Valve Lapping

[FuzzyDunlop]

As some of you may know my engine rebuild is in full motion at the moment. I just recieved all the parts I need (minus flywheel bolts) to start assembly!
A member of another forum suggested that before I reassemble my head I should Lap the Valves. Me being open to suggestions and curious to what Lapping is decided, "heck why not".

What is Valve Lapping?
Valve Lapping is basically sanding down the surface of the Valves (which allow fuel and air to flow in, and exhaust to flow out) and the Head (Holds the Camshafts and valves) where they make contact with each other. This is done by applying a Compound Paste (pretty much just oil and a fine grade of sand) to the edges of the valves and spinning them in their seats on the head.

What are the benifits of Lapping?
The two that I am aware of are
1) creates a better seal between the valve and the head, which will prevent gases from escaping during the compression stroke of your engine.
2) the greater contact area allows for more heat dissipation from the valves to the head which will prevent burnt valves.

Here are my results:
Below are two INTAKE valves. The valve on the left has been lapped. The valve on the right has not. The contact surface of the valve has been put into brackets. You can see that the lapped valve has about a %50 increase in surface contact area. Good stuff!

Here is the corresponding seats on the head. The seat on the right is lapped, while the seat on the left is not lapped.

Now those were the INTAKE valves. The EXHAUST valves had much more noticable results.

On the left is a lapped exhaust valve. on the right, unlapped

The left is a lapped seat, the right is not lapped.

From what I can tell, my valve lapping session was a complete success. If anyone intends of ever doing this, it is highly advised to not allow any compound to be left anywhere as you do not want any of this stuff to be left over to possibly attach itself between the piston rings and the cylinder wall. Also do not allow compound to enter into the areas up by the valve seals. Creating friction in these areas is bad bad bad.

Also, here is a short video of me turning two different valves. The first valve is unlapped and the sound while turning is grindy and sticky (you'll hear it :awe The second valve is lapped and the sound while turning is smooth.
http://www.youtube.com/watch?v=M2lnwLJXrZg

 

[FuzzyDunlop]

Also here is a video on griding technique
http://www.youtube.com/watch?v=fhXsH12Rg6s

 

[punjabiplaya]

thumbs up, looks great, makes sense (I've never heard of this specific technique before)

If you've got it all open and rebuilding, why not right?

 

[jlee]

Very nice!

 

[pcgeek11]

Back in the day ( early 1970's ) it was standard practice to lap the valves during an engine rebuild or valve job. Your post suprised me...

 

[FuzzyDunlop]

Back in the day ( early 1970's ) it was standard practice to lap the valves during an engine rebuild or valve job. Your post suprised me...

maybe thats why somebody at Napa called me 'old school'.
I wonder why its gone out of practice.



EDIT: Pulled this from a reputeable source at MR2OC

This is misleading for a few reasons:
1- the example in question lapped valves and seats which had not been freshly cut.
2- seat widths are specified in the FSM, and any proper engine build involves tight precision concerning seat and valve contact widths.
3- the real test of valve seats is called a LEAKDOWN TEST. Lapped valves on uncut seats tend not to pass this. A fresh valve seat and ground valve will.
4- a properly cut seat and valve do not require lapping to seal.
5- valve lapping is still performed, primarily to verify concentricity and contact location on the valve.
The intake valve contact area is low. This can lead to a burnt valve, which will cause leaking.

I can't tell if the exhaust valve contact area is low. This is much more critical.

 

[TakeNoPrisoners]

Seems to me this would give a minimal HP gain at best. Somewhere from 1-3HP.

 

[kornphlake]

Back in the day ( early 1970's ) it was standard practice to lap the valves during an engine rebuild or valve job. Your post suprised me...

My dad used to have 2 jars of Clover valve lapping compound and tool that was basically a rod with a suction cup on either end. It was used for spinning the valves during lapping. I don't know if he ever used the stuff, I know he never lapped the valves on an automobile engine, it may have been used on a lawnmower.

I'm not sure why lapping would be considered bad if you're not rebuilding the head with new valves and valve seats it seems like it would be at least marginally beneficial.

 

[bobdole369]

I remember doing this is 9th grade shop to a lawnmower engine. First motor I ever took from dead (seized) to purring like a kitten. Sold it for a hundred bucks and I picked it up off the side of the road.

 

[JCH13]

Seems to me this would give a minimal HP gain at best. Somewhere from 1-3HP.

Except when a poorly seated valve is causing low compression and leak-down test numbers.... you know, letting the intake charge seep out of the combustion chamber.

 

[PlasmaBomb]

Excellent post, though it did cause me immense confusion as I originally thought that FBB was the OP...

Time for bed...

 

[Ronstang]

My dad used to have 2 jars of Clover valve lapping compound and tool that was basically a rod with a suction cup on either end. It was used for spinning the valves during lapping. I don't know if he ever used the stuff, I know he never lapped the valves on an automobile engine, it may have been used on a lawnmower.

I'm not sure why lapping would be considered bad if you're not rebuilding the head with new valves and valve seats it seems like it would be at least marginally beneficial.

Those suction cup things are a complete waste of time. I had two different sizes at one time. The only problem with them is they don't hold onto the valve very good or very long if they do actually grip....which is not often.

I use a short piece of fuel hose that tightly goes over the valve stem. I then cut a bolt or piece of rod stock off to stick in one end that I then chuck up in a cordless drill. Simply apply lapping compound, put valve in head, push open end of fuel hose over valve stem and proceed to lap on low speed with short bursts raising the valve off the seat every so often and reversing directions occasionally.

In reality you don't need the drill at all and you can just use a longer piece of fuel hose that you roll back and forth between your palms while lifting the valve off the seat every so often. It does not take long at all.

 

[FuzzyDunlop]

Those suction cup things are a complete waste of time. I had two different sizes at one time. The only problem with them is they don't hold onto the valve very good or very long if they do actually grip....which is not often.

I use a short piece of fuel hose that tightly goes over the valve stem. I then cut a bolt or piece of rod stock off to stick in one end that I then chuck up in a cordless drill. Simply apply lapping compound, put valve in head, push open end of fuel hose over valve stem and proceed to lap on low speed with short bursts raising the valve off the seat every so often and reversing directions occasionally.

In reality you don't need the drill at all and you can just use a longer piece of fuel hose that you roll back and forth between your palms while lifting the valve off the seat every so often. It does not take long at all.

yup, that would make sense. fuel hose or vaccum line should also do the trick. I used the suction cup stick thing. didnt have too much problems with it not getting suction. Just have to make sure both the valve base and cup are clean and clear of any compound.

 

[hanoverphist]

im pulling the head off my civic this weekend to replace a bad head gasket. i knew i was going to do some valve work at the same time, but this post has given me more to think about and look at when i have all the pieces laying out.

im hoping the cylinders are still circular too. the rings passed the test, and theres no real clacking from the engine at all. hope i dont find a bunch of other issues while the head is off.

 

[FuzzyDunlop]

READ THIS FIRST: pulled from another 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 _____ 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 ah heck 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.

I don't think sealing is so much the issue. A lapped head should seal pretty well but I think it's likely it won't seal as long and might be more susceptible to blow by and burning valves over time.
I think the performace side of it is actually from the coutour of the valve. For whatever reason from what I have read a properly ground valve flows much better than a lapped valve.
I'd deff be interested in seeing leakdown numbers though.

Get the head 5 angle ground, and the valves ground, then only LIGHTLY lap - like, 10 seconds.
This is the proper and correct technique according to the new standards.

What I did was old school. I will still have a better seal than before, BUT I could have had a better, longer lasting seal with a proper grind.

 

[exdeath]

So when are you going to build your own sand cast and pour your own block? Just about done everything else now :awe:

 

[FuzzyDunlop]

So when are you going to build your own sand cast and pour your own block? Just about done everything else now :awe:

Thats possible?!!!

Researching...

http://www.youtube.com/watch?v=rgL2Jn5mk1A
whoa thats pretty cool!


...maybe after I do my 20V swap...

 

[alkemyst]

maybe thats why somebody at Napa called me 'old school'.
I wonder why its gone out of practice.

tolerances are better as a whole now and most engines are pushed to engineering limits where it nets good results.

Doing things at this level turns a much less bang for buck for the time needed.

If you have the time and are doing it yourself then its still a good move.

However, it's easy to ruin a head too.