907 valve stem installed height ?

[s1guy]

Anyone on here know the valve stem installed height for intake and exhaust ? Can,t find it in the Manual. Thanks in advance. 

[drdoom]

Hi Ken,

Perhaps you might expand on which related details of valvetrain you feel should be listed as the stem height is not one I typically expect to deal with.

Cheers

[Steven162]

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

[jonwat]

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. 🙄

[s1guy]

Thanks guys but got the info already. 

[ekwan]

11 hours ago, s1guy said:

Thanks guys but got the info already. 

Mind clarifying precisely which bits of information you were seeking so we'd all be wiser in the future.

[s1guy]

The tip of the valve stem installed height. Some valves depending on who you get them from need to have the tips ground to put you in the correct shim range. 

Edited October 2, 2020 by s1guy

[PrecisionMike]

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 !

[rgh0]

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

 

[PrecisionMike]

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. 

[rgh0]

It would be great to get a copy.  it would inspire me to pull one of my spare heads off the shelf and start measuring it up for the 220 HP 907 engine I want to build for my S1 one day

 

cheers

Rohan

[PrecisionMike]

On 01/10/2020 at 17:31, s1guy said:

Thanks guys but got the info already. 

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

[s1guy]

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. 👍

[PrecisionMike]

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 .

[s1guy]

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. 

[PrecisionMike]

I've worked it all out on mine. I'll write it up and post asap. Thanks

[gusexige]

Pulling up an old thread...........

This is a useful, of not necessary measurement when redoing your head. 

Do I need new seats?

This is the number you need to know. 

The thinnest shims in my head were about .078 or so. 

the measurement from the mounting surface of the cam box[relevant as this is the reference for the cam box, and thus the cam]  is about .980  on this valve, interestingly with the original burnt valve and a new valve

this will allow me to tell the shop a number that they can go to when bedding in the new valve that will, at least in theory, let me get a shim that will work 

Most of the other valves had ~.105+ shims so I am fairly confident that they will be in range. 

[drdoom]

Call out to @PrecisionMike today . . . is the valve springs' installed height known? Is the spring rate known?

Cheers 

[PrecisionMike]

On 05/05/2023 at 23:35, gusexige said:

Pulling up an old thread...........

This is a useful, of not necessary measurement when redoing your head. 

Do I need new seats?

This is the number you need to know. 

The thinnest shims in my head were about .078 or so. 

the measurement from the mounting surface of the cam box[relevant as this is the reference for the cam box, and thus the cam]  is about .980  on this valve, interestingly with the original burnt valve and a new valve

this will allow me to tell the shop a number that they can go to when bedding in the new valve that will, at least in theory, let me get a shim that will work 

Most of the other valves had ~.105+ shims so I am fairly confident that they will be in range. 

Prompted by ddroom, I'll briefly comment here but follow up with a spreadsheet which calculates required shim thickness  and Valve spring pre load when closed and force when opened. 

I got into this heavily because after exhaust seat cut, valve grind and lap I discovered I'd need very thin shims which would not be reliable and indeed were outside the available range. 

The key parameters are

*Valve stem protrusion above the cam carrier / head interface. (To my knowledge Lotus never published that dimension although the valve lengths were for both the 907 and 912 which are different)

*Distance from the carrier/ head interface to the base circle of the cam. (A higher lift cam has a smaller base circle which calls for thicker shims)

*The total thickness of of the pad in the cam follower. (There are four choices in my research, influencing the thickness of the required shim as well as reduced weight and increased stability by virtue of the long skirts)

*Carrier to head sealant cured thickness

A linear tolerance analysis needs to be made in order to determine the required valve stem protrusion referred to in this particular instance but this can only be calculated if the other dimensions are known. 

My spread sheet was a work in progress for me , primarily to determine the spring forces at fully open and closed as I was investigating valve acceleration and inertial mass in order to determine required valve spring rates for my application. It had the side benifit of calculating the required valve shim thickness for each valve.

I will endeavor to post the spread sheet within a few days.

My situation in which I required very thin unavailable shims was resolved by adopting thin pad light weight followers from Lotus Bits. Theses parts are lighter, stronger and long skirt and are beautifully made and rally proven.

The thing that surprised me although obvious when I thought about the detail, was that it doesnt take many valve seat and valve face grinds plus lapping for the valves to suddenly be sticking up another 0.5 to 0.7 mm. And all our 907 / 912/ 910 engines are old now and could be on the third rebuild. 

I hope this helps and I'll do my best to post that spreadsheet.

[ekwan]

If you cannot get the required clearance after valve seat cut & lapping-in, isn't that time for a new valve seat and valve?

[PrecisionMike]

Not necessarily,  although of course that would re instate the clearance. Numerous reface and cuts would recess (or pocket) the valves which would alter the compression ratio. In my case the seats were not pocketing the valves and I wanted to renew all followers anyway for two reasons...the original Lotus cast iron ones can crack and they have surprisingly low engagement in their bores, increasing the likelihood of them rocking particularly with high lift cams. Many people do bring a little off the top of the valve tip to solve the problem but caution must be used to ensure that you dont go through the hardened layer. Lotus publish the valve lengths in the workshop manual so measurement will determine if they tips have been ground. If they haven't,  a few thou * is probably ok but I didnt want to take the chance .

* But what's a few though I hear you ask as indeed I did. I've heard people suggest figures of up to .010 inch but that sounds to much to me but I do stand to be advised. If the situation was marginal a tip grind is the cheapest way out but I would recommend a hardness test after to be sure. 

[drdoom]

I found the matter of valve train hardness not to be an issue through the course of fettling the Twnicam in my classic Elan back in the day. It was fitted with high lift cams and used top hat shims rather than the OEM discs we are familiar with. When there was need to open up clearance I chucked the top hats in a lathe and cut them down incrementally, after which they served trouble free. It would be the same case with shortening the valve stem tips, though by grinding rather than turning, as the valves are not surface hardened AFAIK. Tappets and cams are, of course, hardened for the sake of their operating conditions at the margin, valves crafted with toughness paramount for the same reason, as I see it.

Cheers

[PrecisionMike]

That makes sense and a good transfer of experience based knowledge!