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907 valve stem installed height ?


s1guy

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1 hour ago, Steven162 said:

It's fitted correctly when the circlip touches the head Shirley

Are you not thinking about the guide? The question is asking about the stem. 🙄

Cheers,

John W

http://jonwatkins.co.uk

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  • 11 months later...

Installed valve Spring height usually refers to the distance between the valve spring seat and the seat at the retainer. It is of interest since it affects the spring preload with valve fully closed and the peak load when fully open. 

Installed stem height refers to the protrusion of the valve stem tip beyond some known datumn. I use the mating surface at the cam tower to cylinder head. The greater the distance,the smaller the shim . As correctly pointed out it is possible to run out of shim range. 

I've just had one seat installed and re cut to correct exactly that, reducing stem height by 0.50 mm. The stem tip can be ground but only by a few thou as the thickness of the hardened surface could easily be ruined.

To my knowledge the lotus manual does not specify either Installed valve spring height or installed stem height. If any one has these values it would be very interesting to know.

Further, given a constant valve stem length, the parameters discussed affect compression ratio as for example a recessed seat or excessively ground seat on the valve itself results in pocketing the valve which increases combustion chamber volume.

There are a few choices of cam follower offering reduced pad thickness which offer a solution to the shim range issue. I've seen pad thickness offered in steel long skirt followers of 4.00 , 5.55 and 6.25 mm. These are primarily or usually considered with the use of high lift cams which usually have reduced base circle diameters where otherwise very thick shims would be required.

The devil is always in the detail !

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The thinner followers are typically needed when using high lift cams that have both a smaller base circle to keep the lobe tip radius acceptable but also to enable a longer valve stem and larger valve tip height and long valve spring that can accommodate the higher lift.  

Of course the thinner follower can also be used to hide bodges with valve seat over cutting or reground cams.

I deal with this when building competition twin cams for my Elan and have a design spreadsheet to manage all the variables. One day I will set up a similar design spread sheet for the 9xx engines when I finally get around to building a big engine for my S1 Esprit :) but so far the original engine keeps working.

cheers

Rohan

 

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On 17/09/2021 at 11:20, rgh0 said:

The thinner followers are typically needed when using high lift cams that have both a smaller base circle to keep the lobe tip radius acceptable but also to enable a longer valve stem and larger valve tip height and long valve spring that can accommodate the higher lift.  

Of course the thinner follower can also be used to hide bodges with valve seat over cutting or reground cams.

I deal with this when building competition twin cams for my Elan and have a design spreadsheet to manage all the variables. One day I will set up a similar design spread sheet for the 9xx engines when I finally get around to building a big engine for my S1 Esprit :) but so far the original engine keeps working.

cheers

Rohan

 

Hi Rohan, I do have such a spread sheet . It's a little clumsy and I do have intent to make it more robust. I hope to doing this fairly soon and post it for consideration. 

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  • 5 months later...
11 hours ago, PrecisionMike said:

Hi. Did you ever post the info you were after and got. I'd be very interested. Thankyou

Yes. Needed the valve stem height because i was installing new valves. Had to have the exhaust taken down 20thou and the intake 10thou to put me in the correct shim range. 👍

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1 hour ago, s1guy said:

Yes. Needed the valve stem height because i was installing new valves. Had to have the exhaust taken down 20thou and the intake 10thou to put me in the correct shim range. 👍

Thanks for your reply. Did you ever find a specification for valve spring installed height or valve protrusion? On mine (912 na) my exhaust valves consistently protrude 0.5 mm more than the inlet. I solved shim range issue by using new thin pad steel long skirt followers .

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16 hours ago, PrecisionMike said:

Thanks for your reply. Did you ever find a specification for valve spring installed height or valve protrusion? On mine (912 na) my exhaust valves consistently protrude 0.5 mm more than the inlet. I solved shim range issue by using new thin pad steel long skirt followers .

Cylinder shop figured that out. Unfortunately I didn't get the specs. 

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