How important is lapping the valves?

[jaredmac11]

I am replacing the valve stem seals in my M42. I have an opportunity to lap the valves, but this would be my first time doing so. I am fearful that I can do more harm than good by potentially getting some of the lapping grit/compound in the seal or on the stem.

Is this a pretty valuable step?

also, should I lube the valve stems with assembly lube or is oil fine?


this is a crosspost from m42club.com. I sometimes forget how dead that forum is... it can be a week until I get a response, despite the great content that does go through there.

[digger]

IMO you only use lapping as a final check that the seats were cut or ground properly. if the seats are well used then lapping is not the method to fix them.

You have identified one issue with lapping seats is the grit needs to be thoroughly removed afterwards which is not always as easy as it sounds. i know some are of the opinion that there is no need to lap if the seats are cut properly and this requires the valve guides to be in excellent condition.

if the head is not terribly old and guides are in spec and valve surfaces look good i would leave them, other wise check condition of guides and potentially freshen them then get a new VJ if needed

[TobyB]

Agreed-
but if you're doing something shady like replacing a valve with another used valve,
or you've lost touch with which valve goes in which hole and you don't wanna spring for
a valve job,
then a bit of Crocus goes a long way.

Also agreed that that stuff will mess up your guides and stems in a hurry.

Do use oil- and the little condom to keep the keeper grooves from cutting the seal.

hth

t

[jaredmac11]

Great input, I think I have the idea.

I am removing the valves to remove the carbon build up. I was either going to dip them or use a soft bristle on the grinding wheel to clean them up (Ill take recommendations, I figured brass wire brush would be safe?)

Im keeping order of the valves, but I always wondered, if you remove the valves should you lap them? Even if you keep the correct order, wouldn't they lose their mating surface seal just because the rotational orientation isnt the same?

[roguetoaster]

Valves should rotate fairly freely, so orientation isn't a problem. This is also the idea behind lapping the valves IIRC, so you can have a good seat/seal all around the perimeter.

[TobyB]

Yeah, if you keep them where they were, they'll be fine. Some cars even have bearings under the exhaust springs to encourage them to rotate.

As to the carbon, chemical soak followed by a pretty aggressive wire wheel, usually.
Carbon's really hard- it's also really brittle, so you can often scrape it off with a matknife
if you can get under an edge of it.

t