Lotus Elan and Lotus Elan Plus 2 Forums

Approximately how far recessed should the valves be from the head surface - looking at the gap between the edge of the valves and the surface of the head near the scale in the picture.
This is a spare head, at original thickness, with the stock valves, particularly the exhaust, the valve would need to be recessed about .2 in. (a lot!) from present to have the shim at nominal (about .110). The spring pockets have been recessed (by previous owner) so I can't just go by the nominal pocket to valve top distance.
I can purchase the longer valves but curious on valve depth to head surface.



Jack

jk952 wrote:Approximately how far recessed should the valves be from the head surface - looking at the gap between the edge of the valves and the surface of the head near the scale in the picture.
This is a spare head, at original thickness, with the stock valves, particularly the exhaust, the valve would need to be recessed about .2 in. (a lot!) from present to have the shim at nominal (about .110). The spring pockets have been recessed (by previous owner) so I can't just go by the nominal pocket to valve top distance.
I can purchase the longer valves but curious on valve depth to head surface.

Spare head.jpg

Jack

in case you are assuming valves should not protrude below head surface, on my heads the valves hang out some : the pockets on the pistons are there for a purpose I guess...

Assuming an original thickness head and stock valves and cams and followers then like you observe to get the correct distance of the valve tip from the cam you end up with the valve edge below the surface of the head face by a small amount.

If your using original components then you will probably need to shim the valve springs if the spring pockets have been previously deepened for some reason.

cheers
Rohan

Yes that is what I was wondering - if normal for them to be at, or close to surface.
The large bucket shim with standard valve confused me as unrelated to spring / pocket depth.
Thanks to both of you.
Jack

Potentially on this head the valves were set above the head face to maintain the standard spring height with the spring pockets having been deepened at some previous time. This deepening used to be done to accommodate higher lift valve springs before modern valve springs were developed that could accommodate the higher lift with the standard pocket depth.

The cam centre line should be used as the reference when building up a head to get the correct valve and spring package assembly location to suit the cam lift with a suitable cam base circle and using a suitable follower and shim thickness. Lots of variables and if anything is non standard there are usually a number of ways to accommodate the variation ... some better than others.

cheers
Rohan

regarding the photo, valve protrusion below the head depends only on the seat relative to the head face for a given valve (e.g. cutting the seat more would move the valve up the head so that it protrudes less) ; spring pocket is a different question, independently addressed to the extent that the given head can accommodate an eventual spring pocket deepening without breaking through (then shimming spring pockets to obtain a desired closing force will not modify the valve protrusion).

And as Rohan mentioned, when one wants to maximize all parameters (valve lift, valve diameter, spring length etc) for a head skimmed numerous times then the various tolerances along the valve stem (including spring pocket strength) get linked by a closed equation where if you want to gain somewhere you decrease the margin somewhere else. But I would think that regarding the valves themselves, difficulties may occur when they are opened rather than closed.

Picture of NEW QED Sprint Head, Sprint valves.
Maybe of help.