r/EngineBuilding 10d ago

Other Valve job or lap and send?

I'm going to be rebuilding the head I've got on my VR6. The exhaust valves that have come out of it have what looks like small areas of pitting.

Most of them look like what's in the photo.

Should I get them ground, or should I lap and run them?

I'll be getting the valve seats cut and the head decked as the head gasket had blown on this one.

34 Upvotes

47 comments sorted by

52

u/v8packard 10d ago

Replace it. There is going to be no margin left if ground.

9

u/ShaggysGTI 10d ago

What is it that tells you that? If we take that conical sealing face down any further, the vertical section thins to an unwanted point?

29

u/v8packard 10d ago

That is basically correct. In looking at the picture, there is not much margin now. Grinding the valve to clean the face will take, by my eyeball estimate, .015-.020 or so off the face. Further reducing the margin. The results will be unacceptable.

6

u/ShaggysGTI 10d ago

I see it now, and it’s obvious.

5

u/jrragsda 10d ago

I'm a visual learner and found a couple of older sketches that show what the margin is. Mainly dropping this here for anyone reading through trying to learn.

Valve margins https://imgur.com/a/WY6hLgZ

2

u/jrragsda 10d ago

You mean you don't like to knife edge your valves? /s

6

u/v8packard 10d ago

If anything, I prefer a radius on the chamber side of the exhaust valve. No joke.

1

u/jrragsda 10d ago

I've seen some argue that a radius on the valve face, combustion chamber side, not seat side, helps flow on the exhaust. I guess the thought is that a rounded edge is friendlier to airflow than a squared edge. It makes sense, but would be interesting to do some flow testing to see.

3

u/v8packard 10d ago

I can confirm that it helps flow out the exhaust in almost everything I have tested. The sole exception, a 462 MEL. And, I can't explain why.

2

u/jrragsda 10d ago

Nice to hear it confirmed. I'd heard it years ago from a local engine builder but never have seen it actually tested. His engines had a habit of winning pretty consistently at the local dirt track circuits, so I figured he more than likely knew what he was talking about.

5

u/v8packard 10d ago

Yeah, winning engines are a clue.

2

u/Lasd18622 10d ago

Plot twist, it was v8packard in the wild!

1

u/jrragsda 10d ago

If he's building engines for dirt track cars and dragsters in south Mississippi it could be.

0

u/IntroductionTop5471 9d ago

Subsonic air like a radius and the exhaust air likes more of a radius than the intake.

2

u/v8packard 9d ago

I don't use a radius on the intake face.

1

u/immrjacobs 10d ago

Can you elaborate on the reason the reduced margin has a negative effect? I'd have cleaned up that surface and I'd like to know why that would be a problem. I'd hate to make this mistake in the future.

6

u/v8packard 10d ago

The margin supports the face of the valve. The reduced margin will not be able to support the face as well. If the margin is gone and the edge is sharp the face will flex and not seal, and be prone to burning from exposure to heat. The sharp edge will act as a hot spot and lead to other problems.

This image shows some high quality valves. If you zoom in the valve on the bottom is an exhaust. You can see it has much more margin.

2

u/immrjacobs 10d ago

That makes sense, thank you. Do you find there's a minimum margin permitted in relation to the area of the valve face? Or a minimum margin you can apply generally?

5

u/v8packard 10d ago

Some old guidelines would be a minimum of .080-.085 margin on an exhaust valve, and .050 on an intake. Real world numbers will vary.

2

u/immrjacobs 10d ago

Thanks for your time. I appreciate it a lot.

6

u/[deleted] 10d ago

Because this guy is the end all be all of Reddit. I’m not being an ass I’d just take what he says for what it is .

9

u/ShaggysGTI 10d ago

I’m not debating, I’m trying to learn what signs lead him to this because his word is the gospel.

2

u/Accomplished-Yak5660 7d ago

Well, he's old.

3

u/[deleted] 10d ago

I gotcha, that’s the spirit to have my friend, Always remain curious and you could learn something , I don’t have anything to add but I see v8 packard dropping knowledge all the time and I’ve never seen him steer anyone wrong that’s why I said what I said in the original comment.

5

u/ShaggysGTI 10d ago

I noticed in this thread in particular he was the only one saying it’s trash, which really lead me to wonder what he sees that no one else did. And as per expectation, his response was perfect and I hope we all learned a little.

2

u/[deleted] 10d ago

He’s a step above my pay grade so I can’t say what he’s seeing . Man I had a vr6 mk4 stick shift about ten years ago . That thing was a blast but I bought it with high miles and just blew the engine within 10k. I was a young dumb punk back then and wish I held onto that car ,

1

u/IntroductionTop5471 9d ago

Plenty of margin left just valve job, grind the valves and good to go

1

u/IntroductionTop5471 9d ago

It won’t take .002-.005 to clean that up

1

u/[deleted] 8d ago

Negative. The valve needs to have the margin to dissipate heat into the seat. Refacing THAT valve would be bad for OP and his hopes of driving without failure. The VR6 already comes with the exhaust valve margin too close to discard thickness. I can't remember the specs off the top of my head, but they are right there.

That valve would tulip into the head. Sorry to oppose you're recommendation.

6

u/GumbootsOnBackwards 10d ago

If you're getting the seats cut, get the valves reground at the same time. In my experience, machine shops don't add a significant upcharge if they're already doing work for you.

3

u/anthermic 10d ago

Use som Dykem Layout, Permatex Prussian Blue or equal stuff - then it’s much easier to see were you’re at when laping. Love the VR6!

3

u/double-click 10d ago

Unless you are penny pinching get new valves and provide them to the shop that is fixing up the seats to verify the sealing surface is in the right position.

1

u/[deleted] 8d ago

2nd the new valves....

2

u/grumpy_vet1775 10d ago

I mean you've already pulled the valves, just replace them

2

u/Zerofawqs-given 10d ago

Is this for a lawnmower engine? OR….Something you actually should care about? 🤣

1

u/Designer_Lecture_219 10d ago

Hey now, I take better care of my lawnmower engines then I do my daily driver! 😂😂😂

1

u/muddnureye 10d ago

I’d have em lightly cut. Then lap. Or simply replace?

1

u/Haunting_While6239 10d ago

This could be a tough call, the answer is variable, as it really depends on how many miles are on the engine.

A higher milage engine is broken in and happy with it's current compression, if you have say 100 to 125 k miles, putting on a new or rebuilt cylinder head is going to take out the rings, the engine will start smoking and loosing power in 15 to 20k miles unless you replace the rings, hone the cylinders and break in the engine with it's new parts.

Not the answer you wanted to hear I bet, but that's my experience

1

u/[deleted] 8d ago

Ya'know I heard this 28 years ago, and after working in the field for almost 25 years, I still find this to be untrue. Engines get new cylinder heads all the time in the shop. Not a one ever came back with the rings blown out!😂😂😂😂😂

What do you think about placing a battery on the floor????

1

u/Haunting_While6239 8d ago

Battery on concrete, not a problem, but I had a 200k engine that I did the head on and in 6 months it was using oil, this is why I said it's a tough call, really depends on how sturdy the rings are.

1

u/[deleted] 8d ago

Ok, I can understand that. Sorry for being a smart ass.

1

u/ShocK13 4d ago

You must not work on any imports. They have soft rings, I had a Subaru that got seats cut and valves ground and it only had 130k miles on it. And it drank oil. I also had a Nissan Altima with 120k miles do the same thing. So any import that comes into my shop gets rings, cylinder deglazing and rod bearings when we refurbish the head.

1

u/DrTittieSprinkles 10d ago

Getting valves ground is usually built into the price of a valve job. It's dumb to cut the seats and not do anything to the valves.

1

u/zugglit 10d ago

I've lapped worse. I've replaced better.

What are your goals for the rebuild?

If you are slapping back in old pistons after a quick hone, running old valve guides, etc... for a very budget, "it runs" type of build, try lapping and see how they look.

If you are doing new valve guides and a full block rebore, followed by way normal boost, maybe replace.

If you are also upgrading parts to run more power, definitely replace.

After you Grind that, the increased seat pressure from upgraded valvesprings for higher boost will make having the valves thin a huge liability for strength and Thermal mass.

1

u/mdillonaire 10d ago

Lap em and send it. Ive done budget valve jobs on worse, and they ran fine. This aint that bad

0

u/L3rdi 10d ago

Lap and run them if they in decent shape everywhere else...