I just picked up a valve lapping tool and some paste.

Any tips..things to look for when doing this and when done, besides removing ALL of the paste?

how long and how hard do you "spin" them in general"?

From what i gather they are lapped until a consistent shade/color/finish is present along the valve seat . This would indicate the valve seat and valve fitting correctly and the mating surface is flush.

Also some of my valves seats (in the head), exhaust in particular have some very very small dimples in the seat. Almost as if something got under the valves now and then over time as they closed and was squished. Ill try and capture a pic of it but it is very hard to see.

My guess is the lapping should remove these minor imperfections??.


...and yes my valves will be matched to their original ports.

 

I was going to do this myself as well, but I also have plans to smooth the intake and exhaust runners a bit, and I WASN'T going to do that. The machine shop has to press out the old guides and put in new ones anyway.

I think they say that when new guides are put in, then you need to recut or replace the seats. I will have to find that section in the manuals.

 

By lapping tool, you mean suction cup on a stick? I know there are other tools for this job, but it really a hand operation.

Here's how I was taught and it hasn't failed me yet;

Use just enough compound to do the job.
Place the lapping tool between both palms, and gently spin it back and forth. Let the compound do the work, no pressure.
Lift tool, turn 1/4 turn or so and repeat as above.
Do this several times, pull valve up (or out), wipe away compound and check for a nice even ring around both seat and valve (as you have described)
Stop and do the next.

The dimples you see on the exhaust seats are pits from the heat, and should lap away unless they are really deep.

 

Yup, thats the hi-tech tool Jim. Thanks for your input. Ill let you know about the pitting

 

Another way of doing it, the way we do aircraft engines is as follows:

You need two things, an electric drill and a rubber house that just barely will fit on the stem of the valve. Put just enough compound on the valve, slid the valve into the head and from the top of the head, put the 6" or so piece of hose onto the valve stem. Chuck the hose into the drill and slowly spin the valve while pulling back gently on the drill. Every few seconds ease up and let the valve get a new bite on the compound.

 

A thin even satin grey line is the result you want usually on both valve and seat. I used to use a specialist for this if the seats were not in good condition - he had all the tools for facing seats and valves to clean up, do the multi angle stuff, advise if exhaust valve head was too thin after facing. Saves loads of time this way, especially with hardened seats. After that it was only a minute or two each valve, as described above. If guides are replaced its mandatory.
jp 83 Euro S AT

 

Tony,

The aforementioned techniques are pretty sound, but, man, do you really want to do this yourself? The machine work to do valves, guides and a light (just to ensure flatness) deck job on a pair of 4v heads is only about $500 (though your cost may vary if the shop doesn't know water-cooled P-cars). A very small price to pay (all things being relative).

Don't get me wrong, hand-lapping can work ok, and if the budget is really tight and it's the only car you have, you gotta do whatcha gotta do.

But you'll find the shop-cut heads light years more accurate, esp since they set the critically important valve height for you(if they're any good), which is a royal PITA without the shop-air operated tools and, more importantly, jigs and dial indicator set-ups. And they have the set-ups to install new guides straight.

I would also beg you to get those heads decked (after being checked for true of course), and they will probably need at least get a coupla .001" shave just to clean them up after all this time in car. This is monstrously important to getting a consistent seal on the head on a blown app - which, IIRC, is where you're going.

Not to mention the fact that the valve angles will be consistent from hole to hole - which means that your engine will breathe more consistently cylinder to cylinder - this becomes more critical in a force-inducted app. where inconsistent cylinder fill can lead to even greater output power discrepancies from cylinder to cylinder- which then unbalances input loads on the crank - which is an issue.

But, hand/drill lapping can work- I just can't say I would recommend it if you can afford to have it done it right.

Good luck either way,

Greg

 

Tony,

I heard that the latest theory on hand lapping is don't do it. If you lap at room temperature, you can make them perfect, but when hot they don't mate anymore.

When the seat and valve are seperately precision ground, and not hand lapped, then they will seat perfectly at any temperature.

I think Greg's advise is the way to go. Have it done by a reputable shop. And don't hand lap them.

 

Tony - did you end up hand lapping your valves? I am looking at doing this finally, but what Greg and others have said makes sense about machine services.

 

When I asked a machine shop to lap the valves they said that it doesn't do much, and you are better off with resurfacing the valve seat and the valve face.
Just my 2 cents.

Klim

 

Lapping isn't going to remove pits in the seat. If the seats are pitted bite the bullet and get a valve job. Pitted seats are suggestive of a head that has some miles on it. Have you checked valve guide to stem wear?

 

I know your asking Tony FB, but I will answer for my heads. They have 75k miles on them, and the guides seems just fine with a meter on them. I was thinking just some seat and valve cleanup would be good.

I was actually thinking some wear on the guides would be good for the higher temps in supercharging. Maybe even coat the stems and faces. Hmmm....

 

Brendan--

"Some wear on the guides" is NOT real good for SC applications. It's more room for intake pressure/air to find its way into the crankcase, possibly overloading the vapor recovery system. It also increases the chances that a valve will not seat perfectly on center, although this probably doesn't apply as much to the 928 with the bucket tappets. Proper fitment of the guides to the valves means you have less room for oil to get sucked in, so less coking on the intake valve tulip.


On the lapping: test for a pattern with some blue paste like dykem, in several different alignments/rotational positions of the valve. You need to have good contact in all positions before you ever start lapping the finish. Your multi-angle valve job will look great at this point, your contact width will be perfect on both the seat and the valve. Then and only then should you do a little lapping to polish the mating faces.

The lapping process uses a little bit of abrasive powder floated in oil. Use just a thin film on the valve face to start with, and remove the old/replace with new as necessary during the process. The better/smoother the valve grinding was, the less lapping will be required. You are lapping to improve the quality of the metal finish where the valve lands on the seat, and no more. Don't lap for a groove, or think that minor scratches or pits will magically be removed by lapping. You are trying to take the 1200 finsih from the fine stones and make them mirror-fine. The goal: best heat transfer and seal.

Again, use the minimum amount of paste at each step, make sure that none goes down into the brass guides, and clean the head thorougly with detergent and water when you are done. You'll also want to number the valves as you finish each one, so it can go back to the position it was lapped to.

 

Thanks for the reply Bob.

When I said wear, I was speaking to 75k miles worth of wear on the guides - not the wobbly wear people would see on, say, a 100k mile 993 head. So I was getting at the fact that there would be no oil coking, especially with new valve stem seals. But with higher temps expanding the valve stems more, per A. Graham Bell's book "Forced induction"... performance tuning something or other, he says that more valve stem to guide clearances can help prevent any galling.

I have to take all this stuff out and really look at it again.

 

Some of the best engine builders today still lap valves as Bob just described. Some do not. It is different from lapping a lawn mower's valves, and of course there are no lead deposits nowadays to help seating. You can do this yourself, and it is cheap insurance of a perfect seal. That is why it is still done.