Fiat 8v Maximum Port Size

[mack124]

Hello Guy,
Thanks for the help on choosing a head. You said "first check to see which head has more metal to reface around the inlet valve insert, the best m8 and m6 threads, the best coolant gallery condition, best plug threads - some simple practical considerations." I've checked all and the two heads come in fairly equal. I think it would be better for me to go for the 1756 head with the big ports (I'll probably do less damage and the port maintain a good size all the way through.

This leads me to my next question. I assume I don't want to go any bigger? I think i read some where that 36 mm is getting close to maximum size. (especialy with std cams)

What sort of size do I aim for for in the exhaust ports?

Finaly (yeh right!) I mention before the 1756's inlet valves were 0.5 mm bigger. However, the edge of the valve is quite a bit sharper ie. the radius on the very outer edge is almost non-existant. Will this valve have a tendency to give problems or when I regrind them (and they need it) do I redo the outer radius? and if so would I maybe be better to use the slightly smaller valve that has a bit more meat? I would send a photo but I seem to have missplaced my camera at the annual Fiat Hill Climb in Bathurst Australia.

Bye for now
Thanks
Gill

[Guy Croft]

Gill, hi

I'd go std size port at entry tapering progressively to 34mm at narrowest point horizontally on the inlet port. Smooth the short side radius only, without enlarging the radius such as it is in cast form, smooth the port floor and roof.

On the ex side relieve either side of the guide boss about 1mm off and blend into the rest of the port profile, and tidy up the short side radius, make that rad a bit bigger because it is intrusive in std trim. If you're doing the porting with the guides out you can make the ex guide boss more aerodynamic too. The

Use the larger 124 valves and regrind them all to 45 deg, then using a pillar drill and file with 80&120 grit carborundun paper (just by hand) put a small rad - 0.5mm - on the sharp outer edge of the inlets, grind the seats right out to true valve diameter and give the seat inserts 20 deg top grind, 45 x 1.5mm contact face, 70 deg grind in throat to bring contact face back to stated dimension.
Leave the all seat insert angle edges sharp on the insert. After you have lapped the inlets do a 30 deg back grind on the inlet valves to pull the contact face back to true contact width. You can do all the seat work with Neway cutters if you don't have seat grinding tools, (see links) but you'll need their 8mm expanding pilot for centring on old guides.

The chamber itself needs no more than a 'polish' up to clean it.

GC