Lapping vs Cutting Valves - Comments?

OK-944 · 02-09-2010, 10:31 AM

Here's another rebuild question - I've been doing some research into valve renewal, and have discovered some opinions about valve lapping that are somewhat negative. Sounds like lapping can leave some cupping of the contact surfaces, while cutting tends to offer greater evenness of contact.

I actually lapped some valves quite a few years back, and its really pretty straight forward as a DIY procedure...while the prospect of re-cutting sounds more like it should be left to a good machine shop.

My gut tells me that I can lap the valves if I'm careful...but that I still might not expect quite as long a service life from the head as if I send it off for re-cutting.

Any comments would be most appreciated. Thanks!

cvriv.charles · 02-09-2010, 12:37 PM

Anything done by a professional precision machine shop is probably going to be better then a quick corner cutting DIY procedure. Cutting will probably give the best surface contact at the right angle. Lapping will give good contac surface too but the two surfaces will conform to each other.

HondaDustR · 02-09-2010, 12:51 PM

I always thought lapping was a quick re-seating job for valves and seats already in good shape, or that have been freshly cut and need to be ground to match the valves precisely. Don't know, but if your guides, valves, and/or seats are worn out, it's probably not going to help a whole lot and you should get some valve work done at a shop.

wild man · 02-09-2010, 03:18 PM

They are both completely different procedures, with regard to the final rezult. Cutting rezults in a 1 degree difference between the mating surfaces, wile lapping rezults in a 0 degree diiference between the mating surfaces.

924RACR · 02-10-2010, 04:53 AM

I apparently lean the other way than everyone here. I'd definitely go for lapping by hand.

It's what racers do (like me!) and remember, nothing is done on a racecar unless it makes the car go faster...

wild man · 02-10-2010, 05:01 AM

Quote:

Originally Posted by 924RACR

and remember, nothing is done on a racecar unless it makes the car go faster...

Is that really true? Since I am not a racer, I never contemplated that thought. It's not that I wouldn't want to be one, it's the funding issues that keep me off of the track. But hell, it's not even 9 am here yet, and I have already learned something.

OK-944 · 02-10-2010, 05:22 AM

Thanks everyone...its great to get all of this input!

My sense is that I should carefully evaluate the existing contact angles and conditions of contact faces - and if all looks ok, I'll go ahead and do a little (gentle) lapping. If all is not ok, I'll send the head off for an R and R.

One more question: isn't there a test a "water test" to test for valve leakage? Then again, how accurate could this be in the absence of engine pressure?

wild man · 02-10-2010, 05:28 AM

I prefer to call it a "leak-down" test. That is because the water will leak "down" (by the force of gravity) past the valves, if it fails the test. The test does work rather well, if given enough time to complete (I give it at least an hour).

cvriv.charles · 02-10-2010, 05:52 AM

Eh I would check the valves to see if they are worn first. If not then lap them. I would even lap new valves. Think of lapping as "fine tuning" or "fine fitting"

.

You can do as wildman said and wait fr the water to leak through but in my opinion I wouldnt even bother seeing that gas/ air is thinner then water. where the water wont leak through gas/ air will. After your lapping. Turn the head upside down so the valve faces are facing up. Pour water into each of those cavities. Blast the other side of the valve faces with compressed air. The stem side through the intake and exhaust ports.. See how much air passes through. You'll see bubbles.

I wouldnt lap worn valves.

HondaDustR · 09:02 AM

Quote:

Originally Posted by cvriv.charles

Eh I would check the valves to see if they are worn first. If not then lap them. I would even lap new valves. Think of lapping as "fine tuning" or "fine fitting"

. .

Couldn't agree more. It really rejuvinates the seal of used but good valvetrain pretty well and can remove minor pitting with patience. Just be very anal about keeping the grinding paste off the valve stem. Also, do not use the permatex water based paste that comes in a tube at auto stores. Too thin and too coarse. Get some good stuff like this Clover Lapping Compound - 120 Grit & 280 Grit - 2 oz

Less than perfect seats and valves need the coarse paste so you're not lapping for 20 years, but finish with the fine grit if you do. You're looking for a smooth, even strip around the seat and valve that's free of surface defects. If there's deep pitting, the valve seat contact width or valve margin is out of spec, or you cannot obtain an even contact that seals, then you'll need to have the seats cut and most likely new valves installed. Carb cleaner or WD-40 works better for checking the seal since it will creep through smaller gaps than water.

This obviously will not work well with worn out guides, since they provide the stability needed for a precisely matched grind. Well, worn out guides suck even if you could get a good seal since the valve will just ratlle around and wear the seat out funny again. Check your guides, especially the exhaust. New guides require the valve seats to be re-cut.

Compression = power!

924RACR · 02-10-2010, 11:22 AM

Quote:

Originally Posted by wild man

Is that really true? Since I am not a racer, I never contemplated that thought. It's not that I wouldn't want to be one, it's the funding issues that keep me off of the track. But hell, it's not even 9 am here yet, and I have already learned something.

Heh heh... I don't know a single racer that doesn't struggle to make the finances happen. It's just that some do it with 10-100 times the money that others do...