Lapping valves... for or against?

[Carrot]

Been doing a bit of research into valve lapping in preparation for my engine head job in a couple of weeks.

Lot of forums and debate on this one - there are a lot of people some "allegedly" professional engineers etc, but like "all PH's are in the top 1% of earners and are in finance", some are suspicious...

Anyhow, the debate is generally that in modern engines, it is not advisable to bother lapping the valves, as they are usually machined with such high quality materials these days, that one could potentially cause more harm than good. There are some that say leave it well alone unless there is obviously a problem, and some that say do it anyway, every time you have the head off as it can never do any harm.

Bearing in mind that, in this case, the engine is fairly new (1999), but is a cheap piece of st...

Interesting in hearing thoughts of those who have been in precision engineering / know about it.

[ollie854]

When a Ford Focus at work burnt a valve out we lapped all the valves instead of just doing the one.

[Gaspode]

With the head off and the valves in place, invert the head and fill the combustion chambers with paraffin and see if any leak past the valves. If they do, then lap them in very gently and carefully.

[warmfuzzies]

They should be cut with a slightly different angle on the seats to the valves, then lapped really gently and blued to check the seat.
The smaller the surface area of contact, the higher the force to keep it shut and the better it will be.

In my experience of building a few engines.

K

[WhoseGeneration]

Manufacturer's workshop manual should provide what you need to know.

[hondafanatic]

It's probably just me, but it feels ages since this kind of thread has appeared on PH.

I've no idea what the OP is on about, but I'm interested enough in cars and engines that I'm off to google the subject and eagerly learn something new that I don't think could descend into a bun fight with Internet 'meme's' or require the words 'oxo', 'nuke' or the ironic use of 'HTH'.

I will allow 'mapped' and 'cool story bro' references as I think I've asked for those and rightly deserve the onslaught.

[paranoid airbag]

hondafanatic said:

I've no idea what the OP is on about, but I'm interested enough in cars and engines that I'm off to google the subject and eagerly learn something new that I don't think could descend into a bun fight with Internet 'meme's' or require the words 'oxo', 'nuke' or the ironic use of 'HTH'.

ditto

[Mars]

Yes to lapping but only if you've had cause to disturb a particular valve or there's damage.

[Huntsman]

I've rebuilt a dozen or so engines, but all of them have been older stuff like MG and Healey and Jag etc.

I didn't know there is debate over doing it on modern motors, interesting to see what other opinions come along.

[ronaldo342]

The modern day valve seats they are referring to are 5,3 or 2 angle seats, these do not require lapping so long as you have the correct seat cutters/ grinders, the benefit of these seats are to promote better gas flow in high performance engines and also better seating as the valve beds in, it also cuts or pushes any carbon or other foreign objects away from the seat unlike a lapped in valve seat that could trap a foreign object under the wider seat, some other engines work on the seat a valve having a 1 degree difference with the valve sealing around the top of the seat a light lap and blue on these to check.
check the manual or speak to someone in the know on your engine as if you you wish to just lap them in there may be a maximum seat width and also the valve face to cylinder head face height.hope this helps

[shalmaneser]

How old is the engine? Why are you replacing the valves?

I'd be surprised if anyone can come up with a good reason not to lap in the valves, especially if the valve guides have been replaced or there was any valve/piston contact....

EDIT: Just seen it's a '99.

Definitely lap the valves in.

[sparkybean]

The proof if in the pudding, shirley? Do a compression test before and after and see if it helps at all.

Bit st if it makes it worse though...

[Carrot]

Head is coming off to replace the valve stem seals, so there is no known valve damage - more of a case of "While it's off..." - was suggested on another thread so looked into it.

Engine runs a bit rough anyway - but then it is a 3 cylinder bag of ste so not expecting much

Compression tester was ordered yesterday, not doing this job for a few weeks now so coming to comp test it and see if there is a noticeable difference between all 3. I have the original service manual, it does not specifically say that you should not lap the valves...

[Carrot]

hondafanatic said:

I will allow 'mapped' and 'cool story bro' references as I think I've asked for those and rightly deserve the onslaught.

I do like a good 'cool story bro'

[Gaspode]

Carrot said:

Head is coming off to replace the valve stem seals, so there is no known valve damage - more of a case of "While it's off..." - was suggested on another thread so looked into it.

Engine runs a bit rough anyway - but then it is a 3 cylinder bag of ste so not expecting much

Compression tester was ordered yesterday, not doing this job for a few weeks now so coming to comp test it and see if there is a noticeable difference between all 3. I have the original service manual, it does not specifically say that you should not lap the valves...

Comp test won't tell you whether the seats are leaking, it could be the rings. Do you really have to take the head off just to replace the stem seals? Normal practice would be to do it from above. You can either adapt an old spark plug to fit an airline to, or the old dodge was simply to poke loads of string into the combustion chamber to stop the valve falling out.

[Carrot]

Gaspode said:

Carrot said:

Head is coming off to replace the valve stem seals, so there is no known valve damage - more of a case of "While it's off..." - was suggested on another thread so looked into it.

Engine runs a bit rough anyway - but then it is a 3 cylinder bag of ste so not expecting much

Compression tester was ordered yesterday, not doing this job for a few weeks now so coming to comp test it and see if there is a noticeable difference between all 3. I have the original service manual, it does not specifically say that you should not lap the valves...

Comp test won't tell you whether the seats are leaking, it could be the rings. Do you really have to take the head off just to replace the stem seals? Normal practice would be to do it from above. You can either adapt an old spark plug to fit an airline to, or the old dodge was simply to poke loads of string into the combustion chamber to stop the valve falling out.

Very sure it's Not the rings, just get a puff of blue smoke when starting. Under load and warmed up its fine.

I have seen the methods to do this in place - but bear in mind the whole point of this car is to learn how to do this kind of thing, so the head is coming off as more of a learning exercise. I got this car because if I fk it up, its worth more in scrap than it was to buy it

[The Black Flash]

Try posting in the Engines and Drivetrains forum, you'll get good info there.

[rigga]

Depends if the valves are coated or not,not sure on modern car applications,but bike valves that generally see higher rpm's are nowadays coated and if lapped in this process removed the coating and the lifespan is greatly reduced,just replaced some and as advised just fitted them without lapping .... checked to see there was no visable seat damage first though,if there was i would have used the od valves to lightly clean it up.

[z20letzaf]

I have a z20let and have lapped the valves in and its left me with what appears to be rings around the valve and the seat itself is this OK or is it a new head job

[Krikkit]

z20letzaf said:

I have a z20let and have lapped the valves in and its left me with what appears to be rings around the valve and the seat itself is this OK or is it a new head job

Quoted for posterity

What do you think valve lapping does?