1980 l20b installed valve stem height

[maredlin1@hotmail.com]

I am having a 1980 l20b peanut cylinder head rebuilt, and need to know what the installed valve stem measurement height should be.

Thanks

[datzenmike]

Good question.

 

When the valve seats are replaced and then ground down to fit the valve, the finished product should have the valve flush with the combustion chamber surface. If not ground down enough, the valve will protrude into the combustion chamber and may not cool or flow properly. If it is ground down too much, the valve will sink up into the head and definitely won't flow right. All I can say is that if the valve and the seat is correct, the height falls where ever it falls. I don't think there is a correct height as such and if there were and your valve was too low, sinking the valve up into the head by grinding it more to get it to that height would certainly be wrong.

[datsunaholic]

The stem height isn't given in the factory manual.  However, the spring height is.  But it's given for a tension number, not for actual stem height.  The outer springs at normal "closed" positions are 40mm.  On some engines. you would shim the springs to ensure consistent tension to get that height if valves are more or less recessed.  On an L-series, that would also require changing lash pads to get consistent wipe patterns, which isn't covered in the manual.

[DanielC]

Like Datzenmike said, the depth of the face of the valve is set.  Then different lash pads are used to center the contact  patch on the rocker arm.  If things are correct, you should have about three turns of the valve adjuster from a correctly adjusted valve lash, to the adjuster being bottomed in the head.  The eight adjustments should be fairly even.

The valve stem can be ground, but if it is ground too short, the lash pad will then push on the valve spring retainers, or the top "hat" that the spring pushes on.  This can cause the retainer locks to come out, the spring to disengage from the valve stem, and the valve to drop into the cylinder.   That is not a good situation, at all.

[datsunwizard]

The main goal in the end is to have all the valve stem heights the same, so you're not going have to use different lash pads on each valve to set to wipe pattern. I disagree with what is said above, "The depth of the face of the valve is set". If the height is too high, ( from the seat and/or valve ground too much which, typically is from machinists that don't understand L-series valve train geometry, they grind for the "proper valve seat seal," but don't understand the repercussions of sinking the seat) you replace the seat and/or valve to lower the stem height back to the stock height. The L-series head is very time consuming and expensive to set up correctly and is why most production machine shops don't to a good job on them.

[datzenmike]

So what YOU are saying is... sinking the valve to raise it, or raising it into the combustion chamber to lower it is ok as long as they are all the same height, and you don't have to use different lash pads.  Seriously, where is the sense in this?????

[ggzilla]

It allows the use of all same thickness lash pads when valves of varying lengths are used.

 

Better though to refinish all valve lengths the same, grind/temper to uniform length. Then grind the valve depth all the same (to keep CC and flow equal).

 

A badly warped head which has been milled can upset this ideal.