valve stem height above head / cam box mating surface

[tmvolumex]

Guy,
Do you happen to know the minimum and maximum allowable valve stem height above the head / cam box mating surface?
I know Fiat sold a sheet metal gauge for reference of this measurement. I am not sure if the gauge indicated minimum, maximum or midpoint for the factory cam and shim combination. The gauge was designed at approximately .850" (21.6mm).
I am interested in the values for factory cams and your cams using the standard available range of shims from Fiat. As you have mentioned on your site, aftermarket cams have varying base circle values, thus the valve stem height values would vary for different manufactures cams.
Thanks in advance,
Tom

[Guy Croft]

Tom, hi

I never got around to making a gauge for that height, being typically British and always 'making do' with what's 'to hand' I've just 'made do' with measuring from valve tip to the spring platform ie: the machined aluminum region around the guide.

Now that valve setup height (or VSH as I call it) measurement tends to be in the region 40.2-40.5mm inlet and 40.5-40.7mm ex on all TC, SOHC and 16v heads, hardly surprising since they all have common valve springs, because of course that height (assuming common placement of the collet groove) sets the installed height of the springs and they all share common spring installed 'poundage'.

(For those who don't know, 'poundage' is engine building slang for the force (in Newtons - N, or pounds force - lbf exerted by the spring when it's in situ and naturally that force rises as the spring is compressed).

The standard shims tend to be in the range inlet 4 to 4.20mm and the ex 4.15 - 4.25 thick. The thinnest you should use is probably 3.25mm (before the cam nose hits the edge of the bucket, causing massive damage) but 3.30 is safer. The thickest is the object of much discussion but I have successfully run with 4.90 shims with GC springs and never dropped a valve due to excessive valve-train mass.

The valve setup height measured as I have indicated with a vernier calliper is affected by the flatnessof the platform and the amount of rock of the valve in the guide so it is pointless really trying to state the measurement to high accuracy, I consider +/- 0.2mm good enough.

The setup height can be adjusted (in particluar increased for cams with small base circle - given that on the TC and SOHC- not the 16v) the base circle must be reduced to gain more than about 10mm lift or the cam will not physically fit in its housing) by grinding the seat , valve or valve tip and certainly the cambox gasket and thickness of the cambox flange are factors too. Grinding the valve seat or valve face drop the valve deeper into the head and to 'blueprinting' the valve/seat combo to take the contact face right out to valve true OD usually drops the valve deeper by 0.5-1mm. On old worn seats and valves it can be alot more. Grinding the valve tip of course reduces the VSH.

Regarding gasket and cambox it is perfectly possible to leave out the gasket and use a good grade of auto silicon sealant instead (I recommend Wurth) but it is vital to get it in the right places and not block the oil feed galleries. Machining the cambox is a practice I avoid at all costs because it is irreversible and can cause problems on later builds if the cams are changed - eg: back to standard. It used to be done a lot by firms who offered 'reprofiled' 8V TC and SOHC cams (that were all useless I have to say, yielding a fractional increase in top end if you were lucky with a massive reduction in the breadth of the torque curve.

The factory gauge was designed to tell you when the valve tip was too high and thus the head could not be shimmed up in any straightforward way. You have to remember that an OE dealer would be bound to overhaul the engine to an approved standard laid out in the OE manual and grinding the valves too deep would lead not only to shiming problems but a reduction of the stated compression ratio - the chamber volume increases.

I hope that is helpful to a degree,

GC

[tmvolumex]

Guy,
Thanks for the very complete response and set up data.
I did a little research, looking through a Fiat 131 shop manual, and found that the manual specifies:

Thickness of tappet cap plates:
basic dimension......... 0.1575" plus or minus 0.0004" or 4.0 plus or minus 0.01mm.

From this statement one could assume that Fiat gauge number A96218 (Go / No Go gauge to check the distance of the valve stem to head / cam box mounting surface) is designed with a .1575 shim in mind ???? I am not sure about this and Ill have to see if I have a head that has not had a recut on the seats to determine if that is for sure.
The shop manual also calls out the shim availability range of .1280 to .1850 in or 3.25 to 4.70 mm. Here in the States, shim availability range goes as high as .1930in or 4.9mm.
On the last engine that I built, I installed a 40-80 Alquati cam on the intake side and retained the stock cam on the exhaust side. I was in a hurry to build the engine and did not check the set up height. Upon assembly, I wound up with near minimum thickness shims on the exhaust cam and near maximum thickness shims on the intake side. It all worked, but just barely. I am paying more attention on my current engine build and am shooting for a more even shim size distribution. Because I plan on playing with different cam types on this engine, the valve set up height is even more complicated. I want the head to be compatible with different cams without having to disassemble the engine and modify the seats or valves.
Sounds like I need a VSH vs cam base circle compatibility chart.
Thanks,
Tom

[Guy Croft]

Tom, hi

IMHO you're making the job way too complicated. Just dry build it with cam in housing and valve with bucket and thin shim, see how much room there is in there.

G

[tmvolumex]

Guy,
Yes, I know it sounds like I am building a mountain out of a mole hill.
The cylinder head is a a machine shop, getting new exhaust seats installed and the shop called me to say:
"The gauge that you supplied indicates the VSH height is too high, what do you want us to do?". Since I dont have the head in my possesion and I want to be able to use Alquati cams and a stock ones, it is a little more complicated than usual.
Guess Ill have to take a trip to the machine shop with both cam types, cam boxes etc and sort it out.
Thanks,
Tom

[Guy Croft]

Tom, hi

OK. The simple rule of thumb with billet cams in this case is for each 1mm more lift than std (let us assume std is about 9.5mm nominal) then the base circle radius will be 1mm smaller or to put it another way - 1mm further from the valve. So fitted to a head with valve and seat combo with contemporary VSH of about 40.5mm, a 10.5mm lift billet comp cam is going to leave a gap of 1mm which, considering the std shim is about 4.20 would need a way heavy and thick OE type shim - about 5.20mm or thicker.

So if you are doing seat inserts for a comp cam like your Alquati you can do one of two things:

1) Instal and machine the seat insert so that the valve tip stands at 40.5 as before - which is easy enough to do if bigger valves are being fitted - and use a top hat shim or leave out the cambox gasket (given that I am not a great fan of machining the cambox mating face)

- or -

2) Instal the seat insert deeper into the head with the 45 deg contact face deeper too, so the valve stands 41.5mm VSH and then use an OE type shim of about 4.20mm.

Installing deeper in case 2) would require machining the bore for the insert deeper into the head rather than just machining a few thou deeper to clean up the mating face.

The same end result as 2) can also be achieved using a less tall insert, though if using ferrous seat inserts or sintered ones I would go for some extra depth to assure a good holding fit and boring out 1mm deeper will cause no problems. With copper alloy inserts which have an even higher coeff of expansion than aluminum you can get away with very short inserts with reduced interference fit, eg: 2-4thou" with Colsibro or Trojan cf 4-6thou" with cast-iron alloys. Some sintered inserts (eg: those made by KS) can be fitted at low interference too.

If you need to you can of course grind the valve tip quite a bit provided you don't take off so much that the bucket hits the spring cap (and I would not worry in the least about losing any stellite coating on the tip, the stem may well be 21-4n stainless anyway and it's not like the engine has to do 20,000 miles a year)

If you go route 2) and later decide to revert to a std cam all you have to do is add a top hat shim.

As far as 2) goes, with very high lift cams it can be tricky to get the installed position of the seat absolutely right and in practical terms it can actually go in with the top face of the seat exactly where it is in std form. With your setup then all you have to do is grind the 45 deg contact face deeper to suit your actual base circle dia and then blend into the surrounding chamber region with a 20 or 30 deg top grind.* What I do is instal the insert as a parallel item with the top face standing higher than the chamber and then machine it down in-situ. This is much preferable to fully-finishing the insert prior to fitting but the final result depends on the patience and skill of the machinist.

It is a good idea to instal as described above * and do a prelim 45 grind, pop the valve in and check the VSH. If it's not wihtin shimming range (ignoring top hats) then grind the contact face deeper. If the operator is using something like Serdi with digital height setting etc this should be a routine he is intimately aquainted with, because of course the most work done on Serdi and other expensive seat machining systems is diesel, where the placement of the valves is absolutely critical.

The att dwg shows, typically, how I set out big valve conversions and you can see the whole thing is designed to give the VSH I need - 41.5-42mm for GC 3A cams.

Hope that helps some,

G

[tmvolumex]

Guy,
Your response was great, thanks. I especially like your rule of thumb.

Guy wrote:
"The simple rule of thumb with billet cams in this case is for each 1mm more lift than std (let us assume std is about 9.5mm nominal) then the base circle will be 1mm smaller or to put it another way - 1mm further from the valve. So fitted to a head with valve and seat combo with contemporary VSH of about 40.5mm, a 10.5mm lift billet comp cam is going to leave a gap of 1mm which, considering the std shim is about 4.20 would need a way heavy and thick OE type shim - about 5.20mm or thicker."

Last night I checked the base circle on several cams; Fiat, Alquati, Seat, Sierra, Abarth and found, as you rule of thumb indicates, if the lift is around stock, then so is the base circle. In, fact I did not find any measurable difference in the base circle in all the cams I checked. The base circle on the 40 - 80 Alquati billet cam that I want to swap is the same as stock. This is great as I should not have any problem swapping cams if the initial VSH is correct. I am not sure why I had such a difference in VSH on the last engine I built, but it must have been an issue with the valves or the seats not the cam. Since the machine shop working on my current head indicates that the aftermarket big exhaust valves (38mm diameter) are long, I will have to take some material off the top of the stem. It doesn’t look like the valve retainer set up will allow much to be taken off, looks like less than 1mm.
Thanks again for the help,
Tom

[Guy Croft]

Good - oh Tom!

G