Stem Heights for GC IIIa Cams

[robert kenney]

Guy, Hi
I am in the middle of setting up the valve stem heights on the Fiat T/C head. I will be installing the GC IIIa cams I purchased from you and have a few questions regarding stem height recommendations that would work best with the reduced base circle of the IIIa cams. I would like to use standard range (thickness) shims and avoid special cam box gaskets. I have a fairly large range to work with and should be able to hit the numbers.

The valves I am using are 42.5mm and 37mm head OD with the intake modified per GC details. I am also using the GC damped springs and will be setting them up @ the largest installed height I can fit which will be well above the minimum you recommend on the supplied documentation

The head porting turned out very nice. I ended up dusting of my porting tools and doing it my self. Ended up @ 36mm dia. at the port face to match the manifold and 35-35.2mm dia. parallel to the bowl. Also turned the bronze guides at the port end to minimize obstruction and retain maximum support for life.

Thanks Again
Robert Kenney

[robert kenney]

Guy,
I may have answered the question my self but please check my numbers,
Stock Fiat T/C stem height (tip to spring seat) 40.5 - 41.0 mm
Stock cam minor lobe diameter rough- 1.025"
GC IIIa cam minor lobe diameter rough- .930
Differance in lobe minor diameters/2 --- .048" (1.2 mm)


New stem height 41.5- 42.2 mm ???

Thanks...... Robert Kenney

[Guy Croft]

Hi Rob

when I do heads the valves go deeper into the head, the inlets tend to go deeper and of course you can put either valve as deep or shallow in the head as you want - certainly deeper helps with cams with smaller base circle like the IIIA (about 1mm as you state).
On the inlet side I would give the seats a 30 deg top grind (or cut) and then redo the contact face at 45 deg to 1.75-2mm wide and put a 30 deg back grind on the inlet valves. On the ex side you can do a 20 or 30 deg top grind.

The other ways to cope with the reduced base circle are:

1. Cam box machining (which has to be done with great care, alignment of the machining in both planes must be spot-on or the cam won't run true)
2. Top hat shims (these are perfectly safe with GC triple springs to well over 9k)
3. Leave out cam box gasket and use high grade silicon sealant, taking care not to get it in the oilways.

All shimming ops are complicated when non-standard cams are used on TC and SOHC engines, especially if the valve tio heights have been changed, folks who send heads to me tend to forget this. Re-shimming a GC spec head is no easy job.

GC

[robert kenney]

Guy , Thanks for the reply.

I guess I will be ok at the 42mm range +/-. I will have to mock and see where I end up. Then I may need to adjust as you have recommended.

I will carefully check the cam box plains on the head for flatness and geometry. The boxes them selves are fine.

Robert Kenney

[Guy Croft]

Yes, it's very important to mock up. I have attached a table I fill in for every head I do. The VSH (valve setup height) controls the installed height of the spring too, thus by recording VSH and the distance tip-underside of spring cap etc you can calculate how much lift the setup will tolerate - not easy to measure in-situ on most heads.

Incidentally you can't expect more than +/- 0.2mm of accuracy with VSH because the spring platforms are never very flat adn there is a bit of movement of the valve stem in the guide too..

GC

[robert kenney]

Guy,
I will post the dimensions after I finish the first round of machining.

Many Thanks....Robert

[robert kenney]

Guy Hi,
Every thing set up perfectly using un modified boxes and gaskets

Springs installed @ 38mm on exhaust and 37.5mm on the intakes with .5mm thick spring seat shims (surface ground OEM 1mm washers)

Stem heights were adjusted to leave .015 clearance on old shims to allow easier set up on exhaust and intakes running clearances.

I will be able to use 4.15mm +/- shims on both sides

Cam box surfaces need a kiss to remove a bit of a non-plainer condition (.003").

Thanks for your advice
Robert Kenney