67rschev

The Porsche factory service manual states that the valve spring installed height spec should be 41.0 + .5 mm . Does this mean that it should be 41 plus or minus .5 mm OR does this mean it should be 41.5 , OR my thought is that they mean it should be between 41.0 to 41.5 mm . All valves were ground and back-cut , and valve seats recieved a 5 angle job and want to make sure the springs are set to proper preload . Thanks

RC924

Thats it. Without consulting FSM and going by your figures:
Stock (new) installed height is 41 mm. Won`t ever be less with stock valves and seats. The permissible increase is +0.5mm. Every time the valves are ground or the seats recut the valves sink and the installed height, which is the distance between the spring seat and retainer, increases accordingly.

There are two factors involved here. The installed height and the spring tension. Even if the installed height is within specs (+0.5) a proficient engine/head builder will shim them all back to the 41mm figure, or at least a tighter tolerance.

Spring seat pressure, or preload, will depend on the individual spring (pairs) pressure. New springs have some variation (-/+ 10%) and used springs usually show quite a wide tolerance. Particularly if the head has been rebuilt due to bent valves/cam belt failure or at a cheap shop where all similar (944, 928) springs are pulled from the same box or bucket.

Best to measure all springs for seat pressure (at installed height) and at open pressure (+ cam lift) and either replace or shim accordingly. However this takes time and/or money and if economics is the priority is usually omitted.

67rschev

I do have access to a valve spring seat pressure checking tool , all though it is specifically made for a Jessel shaft mounted rocker type set up , i think i could rig it up on these heads and make it work . I do have a set of 8 1 mm shims and a set of 9 .5mm shims that came from a different head . Does anyone know what seat pressure should be on these with a stock cam and stock n/a springs ? I'm thinking around 110 to 130 pounds of seat pressure , Any one , any one , Bueler ?

RC924

Was hoping someone in the know would chime in here as these specs don`t appear in the FSM or any official source AFAIK. Have previously searched here and the bird`s site to no avail. Springs are also a 928 part # but still no data.

IMHO doubt seat pressure is any higher than 80 Lbs, if that. As an indication, similar size (not identical) springs, at similar installed height (42.34mm), on a similar flat follower cam, in a 924, have a total seat pressure of 62 Lbs. Inner is 11.5 Lbs @ 1.555" (allowing for seat and retainer) and outer 50.5 Lbs @ 1.667".

These specs are from Integral Cams and correspond with my measurements. Since Integral manufacture 944 cams (for Jon Milledge) they will certainly know the stock specs. Suggest ringing or emailing Steve Gruenwald, a nice helpful guy.

Please post his answer and document the info here for others.

Cheers

blown 944

IIRC they were ~ 120 . I installed my new performance ones at 140.

You have to be careful shimming them b/c they run close to bind already.