Should I lap the valves?

Received small block heads from an engine shop today. Had them insert new valve guides and do a three angle vale job. My question: Should I lap the valves? The engine shop did not return the valves marked for specific cylinders. This is a stock 273 ´67 year buildup.

Ragnar in Iceland

It's always a good idea to lap the valves too match each seat.

Definitely do that to see where the valves seat on the seats
I do it on all the heads I have worked on, no matter who does the work
BTW,it is up to the engine assembler to ensure every part of the motor is done correctly, correct

Pour something down the ports and see if they are leaking.

We always vacuum tested the heads after a valve job. If they held a good vacuum pressure, we sent them out. If they didn't, then we lapped the valves. With that said, I always lap them in heads I'm doing now, just because.

Agreed, with the head on its side with ex ports on top then with the intake ports on top pour some solvent in the ports & see if it gets wet at the valve heads. I read/saw something (on the dark web) so it has to be correct! that lapping is old school/obsolete & the article went on to say that the contact area will change drastically once it get hot so it is ineffective but I would think that yes heat is going to alter things but I'd want a perfect seal as I can get in the beginning for a good baseline. It does give a good pattern on the height/width of the contact area.

X 2

If I was assembling new valves and/or heads I always lapped the valves. If the heads were sent out for work, I always inspected them and would leak-check as Magnum and RR suggested.


Tim

Thanks for you input guys.

I´ll do some lap dancing today.

Ragnar in Iceland

I lap them not to get them to seal, a good valve job should get them to seal, but you lap them as a poor mans way of checking to make sure they are touching all the way around and the sealing surface is more than just a sharp edge. Checking with fluid alone does not tell you if it has a good contact area, espercially important on the exhaust for heat transfer. Could also be sealing against the wrong side of the margin, I like to see them seal more against the outside edge of the valve face than the inside but just checking with fluid you will never know. Also if the machine shop assembles them they could have left a layer of oil or grease or something that seals them instead of the metal metal contact (I had a shop in texas do this one time and had to have them re-do a slant six head when I was 17 and they argued with me that they did not have to seal with just metal to metal, never used them again). If you have to grind them a bunch with lapping compound to get them to seal you can end up with a curved sealing surface, I have done this in a pinch on junk yard builds but it aint the right way for a good engine. Just use lapping for checking the machine shops work not grinding.

My shop teacher taught that lapping doesn't guarantee a good seal because the valves expand a little when in operation and don't contact the seat the same as when lapped cold.

So a machine shop installed new guides, then cut the seats based on the new guides but never checked to make sure the valves will seal on the freshly cut seats? Sounds odd to me.

This. And food for thought - many machine shops have went to 3 angle cutters to do their "production" valve jobs instead of using stones, to save time dressing the seats in the heads. While the cutters are great to remove material for larger valves, IMO they are not intended to be used as the sole method of dressing seats. IMO it is absolutely necessary to lap in the valves on heads that have had cutters used for the seats, to smooth out the inherent chatter bumps from the cutters.

I use blueing on every valve seat I cut and lap them in. Then it’s time for a good cleaning.

just good general machining practice 101. ^^^^

NO IT'S NOT. After you've done the valve seats you smear some gooky blue stuff on the seats then with the valve you pop the valve closed and open one or two times. There should be a clean line all around the seat and the valve face. That's how you tell.

If your machinery is any good at all every seat will show sealed. You can also do the leak test. In my experience with a Neway manual seat cutter and an old Sioux valve grinder I never had to redo a seat. I did, however, find a number of new economy valves that needed touchup on their faces.

The only time to use lapping compound on the valves is when you are trying to see if the existing seats seal all the way around, and then you only do enough to lightly mark the seats. That's when you are trying to avoid reworking the heads.

Your valve guy whether high end or ma-and-pa shop SHOULD HAVE CHECKED all the valves for seating before they were reassembled.

The valves do heat up more than the seats and the expansion is different because of the differing temps. So the surfaces don't match in service. On the other hand, if you don't start actually making grooves in the seat I guess it won't hurt.

R.

your statement is generally correct, however, since the valves rotate, i use the "apply very light pressure, while twisting the valve with your thumb" process to see what the contact looks like. then, if not the whole way around the seat or valve, apply a little amount of compound to get a perfect pattern [one i'm satisfied with], then as a final test fill the chamber with solvent for a leak test. i do very few heads, and only have very old equipment at my disposal. this works for me, it appears your method is the "correct" way, and should be the followed practice if one does way more work in this area than i do. no disrespect intended toward you [or others] in any way. please accept my apologies if offended.