620 valve seats

[Apollo77]

How good of a seal is required for start up? I've pulled cleaned and lapped and with springs in place they hold a water test for 4-12 seconds over all 8 valves. Is that enough to reassemble, or will I be doing it again soon?

[ggzilla]

Hand lapped valves are good for years. The ones I've done hold water overnight.

 

But if the valves leak after 12 seconds, it is not a good seal.

[Wheezie]

Speaking of valves..

I could use some insight over here on this one: http://community.ratsun.net/topic/62372-my-79-620-ground-up-custom-king-cab-build-ebay-classifide-here-soon/

[Apollo77]

It's not a fast leak, just a droplet or two start to form. I have been slowly working on getting this thing back together and now my motorcycle took a shit turn and now I'm screwed. Don't have the moneys I would need to fix everything. Know what I means

[Wheezie]

Every little bit of leakage is compression lost... "Just sayin'"
May cause back fire too not to metion spitting back into the carb if it is an intake valve leak....

[ggzilla]

Just lap it some more.

[Apollo77]

Ok, it's just such A process doing it by hand

[Apollo77]

Ok, it's just such A process doing it by hand

[datsunaholic]

If lapped properly, they'll hold liquid  indefinitely.  Drips or seepage after 12 seconds isn't a bad seal, it's not a seal at all.

 

But make sure you have them clean.  Even a trace of lapping compound on the seat or face will make it leak.  Wiping with a rag is not enough.

[smoke]

If you're asking if it will run, then yes, just not very good. Like he said, make sure you have it really clean, sounds like you have some crud in between the seat and valve.

[cj8281]

The L16, L18 and the early L20B had brass or bronze valve seats in the intakes.  The intake valves will actually wear into the seat.  Engines with a lot of miles or high amounts of wear can have the intake valves well below the surface of the chamber.  This also moves the other end of the valve up away from the head and reduces spring pressure.  You can see this by the lash adjuster having little or no travel left.  To fix this, the seats need to be removed and new ones installed.  Usually the valve guides are also pretty much gone at this point.  A rebuild is needed. 

 

The later L20B engines had hard intake seats instead of the brass or bronze seats of the earlier heads.

 

On the exhaust seats, the problem is usually pitting that causes them not to seal.  Excessive pitting can lead to burning a valve, if run this way for awhile, it can really screw up the seat and possibly cause cracking.